1519 lines
55 KiB
Plaintext
1519 lines
55 KiB
Plaintext
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.\" Format this file with:
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.\" pic file | tbl | troff -ms
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.\"
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.\" \*s stands for $, and avoids problems when this file is checked in.
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.ds s $
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.de D(
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.DS
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.de Id
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.ND \\$4
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..
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.Id $Id: rcs.ms,v 5.4 1995/06/01 16:23:43 eggert Exp $
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.RP
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.TL
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RCS\*-A System for Version Control
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.sp
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.AU
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Walter F. Tichy
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.AI
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Department of Computer Sciences
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Purdue University
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West Lafayette, Indiana 47907
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.sp
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.AB
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An important problem in program development and maintenance is version control,
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i.e., the task of keeping a software system consisting of many versions and
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configurations well organized.
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The Revision Control System (RCS)
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is a software tool that assists with that task.
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RCS manages revisions of text documents, in particular source programs,
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documentation, and test data.
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It automates the storing, retrieval, logging and identification of revisions,
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and it provides selection mechanisms for composing configurations.
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This paper introduces basic version control concepts and
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discusses the practice of version control
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using RCS.
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For conserving space, RCS stores deltas, i.e., differences between
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successive revisions. Several delta storage methods are discussed.
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Usage statistics show that RCS's delta storage method is
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space and time efficient.
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The paper concludes with a detailed survey of version control tools.
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.sp
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\fBKeywords\fR: configuration management, history management,
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version control, revisions, deltas.
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.AE
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.FS
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An earlier version of this paper was published in
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.I "Software\*-Practice & Experience"
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.B 15 ,
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7 (July 1985), 637-654.
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.FE
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.nr VS 18p
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.LP
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.NH
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Introduction
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.PP
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Version control is the task of keeping software
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systems consisting of many versions and configurations well organized.
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The Revision Control System (RCS) is a set of UNIX
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commands that assist with that task.
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.PP
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RCS' primary function is to manage \fIrevision groups\fR.
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A revision group is a set of text documents, called \fIrevisions\fR,
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that evolved from each other. A new revision is
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created by manually editing an existing one.
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RCS organizes the revisions into an ancestral tree. The initial revision
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is the root of the tree, and the tree edges indicate
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from which revision a given one evolved.
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Besides managing individual revision groups, RCS provides
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flexible selection functions for composing configurations.
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RCS may be combined with MAKE\u1\d,
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resulting in a powerful package for version control.
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.PP
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RCS also offers facilities for
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merging updates with customer modifications,
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for distributed software development, and
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for automatic identification.
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Identification is the `stamping'
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of revisions and configurations with unique markers.
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These markers are akin to serial numbers,
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telling software maintainers unambiguously which configuration
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is before them.
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.PP
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RCS is designed for both production and experimental
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environments.
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In production environments,
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access controls detect update conflicts and prevent overlapping changes.
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In experimental environments, where strong controls are
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counterproductive, it is possible to loosen the controls.
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.PP
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Although RCS was originally intended for programs, it is useful for any
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text that is revised frequently and whose previous revisions must be
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preserved. RCS has been applied successfully to store the source
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text for drawings, VLSI layouts, documentation, specifications,
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test data, form letters and articles.
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.PP
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This paper discusses the practice of
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version control using RCS.
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It also introduces basic version control concepts,
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useful for clarifying current practice and designing similar systems.
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Revision groups of individual components are treated in the next three sections,
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and the extensions to configurations follow.
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Because of its size, a survey of version control tools
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appears at the end of the paper.
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.NH
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Getting started with RCS
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.PP
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Suppose a text file \fIf.c\fR is to be placed under control of RCS.
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Invoking the check-in command
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.D(
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ci f.c
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.D)
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creates a new revision group with the contents of
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\fIf.c\fR as the initial
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revision (numbered 1.1)
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and stores the group into the file \fIf.c,v\fR.
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Unless told otherwise, the command deletes \fIf.c\fR.
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It also asks for a description of the group.
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The description should state the common purpose of all revisions in the group,
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and becomes part of the group's documentation.
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All later check-in commands will ask for a log entry,
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which should summarize the changes made.
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(The first revision is assigned a default log message,
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which just records the fact that it is the initial revision.)
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.PP
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Files ending in \fI,v\fR
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are called \fIRCS files\fR (\fIv\fR stands for \fIv\fRersions);
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the others are called working files.
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To get back the working file \fIf.c\fR in the previous example,
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execute the check-out command:
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.D(
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co f.c
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.D)
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.R
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This command extracts the latest revision from
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the revision group \fIf.c,v\fR and writes
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it into \fIf.c\fR.
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The file \fIf.c\fR can now be edited and, when finished,
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checked back in with \fIci\fR:
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.D(
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ci f.c
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.D)
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\fICi\fR assigns number 1.2 to
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the new revision.
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If \fIci\fR complains with the message
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.D(
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ci error: no lock set by <login>
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.D)
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then the system administrator has decided to configure RCS for a
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production environment by enabling the `strict locking feature'.
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If this feature is enabled, all RCS files are initialized
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such that check-in operations require a lock on the previous revision
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(the one from which the current one evolved).
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Locking prevents overlapping modifications if several people work on the same file.
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If locking is required, the revision should
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have been locked during the check-out by using
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the option \fI\-l\fR:
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.D(
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co \-l f.c
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.D)
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Of course it is too late now for the check-out with locking, because
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\fIf.c\fR has already been changed; checking out the file again
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would overwrite the modifications.
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(To prevent accidental overwrites, \fIco\fR senses the presence
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of a working file and asks whether the user really intended to overwrite it.
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The overwriting check-out is sometimes useful for
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backing up to the previous revision.)
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To be able to proceed with the check-in in the present case, first execute
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.D(
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rcs \-l f.c
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.D)
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This command retroactively locks the latest revision, unless someone
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else locked it in the meantime. In this case, the two programmers
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involved have to negotiate whose
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modifications should take precedence.
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.PP
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If an RCS file is private, i.e., if only the owner of the file is expected
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to deposit revisions into it, the strict locking feature is unnecessary and
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may be disabled.
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If strict locking is disabled,
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the owner of the RCS file need not have a lock for check-in.
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For safety reasons, all others
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still do. Turning strict locking off and on is done with the commands:
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.D(
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rcs \-U f.c \fRand\fP rcs \-L f.c
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.D)
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These commands enable or disable the strict locking feature for each RCS file
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individually.
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The system administrator only decides whether strict locking is
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enabled initially.
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.PP
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To reduce the clutter in a working directory, all RCS files can be moved
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to a subdirectory with the name \fIRCS\fR.
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RCS commands look first into that directory for RCS files.
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All the commands presented above work
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with the \fIRCS\fR subdirectory without change.\(dg
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.FS \(dg
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Pairs of RCS and working files can actually be specified in 3 ways:
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a) both are given, b) only the working file is given, c) only the
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RCS file is given.
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If a pair is given, both files may have arbitrary path prefixes;
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RCS commands pair them up intelligently.
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.FE
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.PP
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It may be undesirable that \fIci\fR deletes the working file.
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For instance, sometimes one would like to save the current revision,
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but continue editing.
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Invoking
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.D(
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ci \-l f.c
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.D)
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checks in \fIf.c\fR as usual, but performs an additional
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check-out with locking afterwards. Thus, the working file does
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not disappear after the check-in.
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Similarly, the option
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\fI\-u\fR does a check-in followed by a check-out without
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locking. This option is useful if the file is needed for compilation after the check-in.
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Both options update the identification markers in the working file
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(see below).
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.PP
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Besides the operations \fIci\fR and \fIco\fR, RCS provides the following
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commands:
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.sp 0
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.nr VS 12p
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.vs 12p
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.TS
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tab(%);
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li l.
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ident%extract identification markers
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rcs%change RCS file attributes
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rcsclean%remove unchanged working files (optional)
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rcsdiff%compare revisions
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rcsfreeze%record a configuration (optional)
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rcsmerge%merge revisions
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rlog%read log messages and other information in RCS files
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.TE
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A synopsis of these commands appears in the Appendix.
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.NH 2
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Automatic Identification
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.PP
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RCS can stamp source and object code with special identification strings,
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similar to product and serial numbers.
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To obtain such identification, place the marker
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.D(
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\*sId\*s
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.D)
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into the text of a revision, for instance inside a comment.
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The check-out operation will replace this marker with a string of the form
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.D(
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\*sId: filename revisionnumber date time author state locker \*s
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.D)
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This string need never be touched, because \fIco\fR keeps it
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up to date automatically.
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To propagate the marker into object code, simply put
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it into a literal character string. In C, this is done as follows:
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.D(
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static char rcsid[] = \&"\*sId\*s\&";
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.D)
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The command \fIident\fR extracts such markers from any file, in particular from
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object code.
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\fIIdent\fR helps to find out
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which revisions of which modules were used in a given program.
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It returns a complete and unambiguous component list,
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from which a copy of the program can be reconstructed.
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This facility is invaluable for program maintenance.
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.PP
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There are several additional identification markers, one for each component
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of \*sId\*s.
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The marker
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.D(
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\*sLog\*s
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.D)
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has a similar function. It accumulates
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the log messages that are requested during check-in.
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Thus, one can maintain the complete history of a revision directly inside it,
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by enclosing it in a comment.
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Figure 1 is an edited version of a log contained in revision 4.1 of
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the file \fIci.c\fR. The log appears at the beginning of the file,
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and makes it easy to determine what the recent modifications were.
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.sp
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.nr VS 12p
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.vs 12p
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.ne 18
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.nf
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.in +0.5i
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/*
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.in +\w'/'u
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* \*sLog: ci.c,v \*s
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* Revision 4.1 1983/05/10 17:03:06 wft
|
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* Added option \-d and \-w, and updated assignment of date, etc. to new delta.
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* Added handling of default branches.
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*
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* Revision 3.9 1983/02/15 15:25:44 wft
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* Added call to fastcopy() to copy remainder of RCS file.
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|
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*
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* Revision 3.8 1983/01/14 15:34:05 wft
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* Added ignoring of interrupts while new RCS file is renamed;
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* avoids deletion of RCS files by interrupts.
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*
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* Revision 3.7 1982/12/10 16:09:20 wft
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* Corrected checking of return code from diff.
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* An RCS file now inherits its mode during the first ci from the working file,
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* except that write permission is removed.
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*/
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.in 0
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.ce 1
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Figure 1. Log entries produced by the marker \*sLog\*s.
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.fi
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.nr VS 18p
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.vs 18p
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|
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.sp 0
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|
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.LP
|
||
|
|
Since revisions are stored in the form of differences,
|
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|
|
each log message is
|
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|
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physically stored once,
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|
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independent of the number of revisions present.
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Thus, the \*sLog\*s marker incurs negligible space overhead.
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.NH
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|
|
The RCS Revision Tree
|
||
|
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.PP
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|
RCS arranges revisions in an ancestral tree.
|
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|
The \fIci\fR command builds this tree; the auxiliary command \fIrcs\fR
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|
|
prunes it.
|
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|
The tree has a root revision, normally numbered 1.1, and successive revisions
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|
|
are numbered 1.2, 1.3, etc. The first field of a revision number
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|
|
is called the \fIrelease number\fR and the second one
|
||
|
|
the \fIlevel number\fR. Unless given explicitly,
|
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|
the \fIci\fR command assigns a new revision number
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||
|
|
by incrementing the level number of the previous revision.
|
||
|
|
The release number must be incremented explicitly, using the
|
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|
|
\fI\-r\fR option of \fIci\fR.
|
||
|
|
Assuming there are revisions 1.1, 1.2, and 1.3 in the RCS file f.c,v, the command
|
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|
|
.D(
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|
|
ci \-r2.1 f.c \fRor\fP ci \-r2 f.c
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.D)
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|
assigns the number 2.1 to the new revision.
|
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|
|
Later check-ins without the \fI\-r\fR option will assign the numbers 2.2, 2.3,
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and so on.
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The release number should be incremented only at major transition points
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in the development, for instance when a new release of a software product has
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been completed.
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.NH 2
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When are branches needed?
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|
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.PP
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A young revision tree is slender:
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It consists of only one branch, called the trunk.
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As the tree ages, side branches may form.
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Branches are needed in the following 4 situations.
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|
.IP "\fITemporary fixes\fR"
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|
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.sp 0
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|
Suppose a tree has 5 revisions grouped in 2 releases,
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as illustrated in Figure 2.
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|
Revision 1.3, the last one of release 1, is in operation at customer sites,
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|
while release 2 is in active development.
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.ne 4
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|
|
.PS 4i
|
||
|
|
.ps -2
|
||
|
|
box "1.1"
|
||
|
|
arrow
|
||
|
|
box "1.2"
|
||
|
|
arrow
|
||
|
|
box "1.3"
|
||
|
|
arrow
|
||
|
|
box "2.1"
|
||
|
|
arrow
|
||
|
|
box "2.2"
|
||
|
|
arrow dashed
|
||
|
|
.ps +2
|
||
|
|
.PE
|
||
|
|
.ce 1
|
||
|
|
Figure 2. A slender revision tree.
|
||
|
|
.sp 0
|
||
|
|
Now imagine a customer requesting a fix of
|
||
|
|
a problem in revision 1.3, although actual development has moved on
|
||
|
|
to release 2. RCS does not permit an extra
|
||
|
|
revision to be spliced in between 1.3 and 2.1, since that would not reflect
|
||
|
|
the actual development history. Instead, create a branch
|
||
|
|
at revision 1.3, and check in the fix on that branch.
|
||
|
|
The first branch starting at 1.3 has number 1.3.1, and
|
||
|
|
the revisions on that branch are numbered 1.3.1.1, 1.3.1.2, etc.
|
||
|
|
The double numbering is needed to allow for another
|
||
|
|
branch at 1.3, say 1.3.2.
|
||
|
|
Revisions on the second branch would be numbered
|
||
|
|
1.3.2.1, 1.3.2.2, and so on.
|
||
|
|
The following steps create
|
||
|
|
branch 1.3.1 and add revision 1.3.1.1:
|
||
|
|
.sp 0
|
||
|
|
.I
|
||
|
|
.nr VS 12p
|
||
|
|
.vs 12p
|
||
|
|
.TS
|
||
|
|
tab(%);
|
||
|
|
l l l.
|
||
|
|
%co \-r1.3 f.c% \*- check out revision 1.3
|
||
|
|
%edit f.c% \*- change it
|
||
|
|
%ci \-r1.3.1 f.c% \*- check it in on branch 1.3.1
|
||
|
|
.TE
|
||
|
|
.nr VS 18p
|
||
|
|
.vs 18p
|
||
|
|
.R
|
||
|
|
This sequence of commands transforms the tree of Figure 2 into
|
||
|
|
the one in Figure 3.
|
||
|
|
Note that it may be necessary to incorporate the differences
|
||
|
|
between 1.3 and 1.3.1.1
|
||
|
|
into a revision at level 2. The operation \fIrcsmerge\fR automates this
|
||
|
|
process (see the Appendix).
|
||
|
|
.ne 7
|
||
|
|
.PS 4i
|
||
|
|
.ps -2
|
||
|
|
box "1.1"
|
||
|
|
arrow
|
||
|
|
box "1.2"
|
||
|
|
arrow
|
||
|
|
R13: box "1.3"
|
||
|
|
arrow
|
||
|
|
R21: box "2.1"
|
||
|
|
arrow
|
||
|
|
R22: box "2.2"
|
||
|
|
arrow dashed
|
||
|
|
line invis down from R21.s
|
||
|
|
RB1: box "1.3.1.1"
|
||
|
|
arrow dashed right from RB1.e
|
||
|
|
arrow from R13.s to RB1.w
|
||
|
|
.ps +2
|
||
|
|
.PE
|
||
|
|
.ce 1
|
||
|
|
Figure 3. A revision tree with one side branch
|
||
|
|
.sp
|
||
|
|
.IP "\fIDistributed development and customer modifications\fR"
|
||
|
|
.sp 0
|
||
|
|
Assume a situation as in Figure 2, where revision 1.3 is in operation
|
||
|
|
at several customer sites,
|
||
|
|
while release 2 is in development.
|
||
|
|
Customer sites should use RCS to store the distributed software.
|
||
|
|
However, customer modifications should not be placed on the same branch
|
||
|
|
as the distributed source; instead, they should be placed on a side branch.
|
||
|
|
When the next software distribution arrives,
|
||
|
|
it should be appended to the trunk of
|
||
|
|
the customer's RCS file, and the customer
|
||
|
|
can then merge the local modifications back into the new release.
|
||
|
|
In the above example, a
|
||
|
|
customer's RCS file would contain the following tree, assuming
|
||
|
|
that the customer has received revision 1.3, added his local modifications
|
||
|
|
as revision 1.3.1.1, then received revision 2.4, and merged
|
||
|
|
2.4 and 1.3.1.1, resulting in 2.4.1.1.
|
||
|
|
.ne 7
|
||
|
|
.PS 4i
|
||
|
|
.ps -2
|
||
|
|
R13: box "1.3"
|
||
|
|
line invis
|
||
|
|
R21: box invis
|
||
|
|
line invis
|
||
|
|
R22: box invis
|
||
|
|
line invis
|
||
|
|
R24: box "2.4"
|
||
|
|
line invis
|
||
|
|
R25: box invis
|
||
|
|
line invis
|
||
|
|
arrow from R13.e to R24.w
|
||
|
|
line invis down from R21.s
|
||
|
|
RB1: box "1.3.1.1"
|
||
|
|
arrow from R13.s to RB1.w
|
||
|
|
right
|
||
|
|
line invis down from R25.s
|
||
|
|
RB2: box "2.4.1.1"
|
||
|
|
arrow from R24.s to RB2.w
|
||
|
|
.ps +2
|
||
|
|
.PE
|
||
|
|
.ce 1
|
||
|
|
Figure 4. A customer's revision tree with local modifications.
|
||
|
|
.sp 1
|
||
|
|
This approach is actually practiced in the CSNET project,
|
||
|
|
where several universities and a company cooperate
|
||
|
|
in developing a national computer network.
|
||
|
|
.IP "\fIParallel development\fR"
|
||
|
|
.sp 0
|
||
|
|
Sometimes it is desirable to explore an alternate design or
|
||
|
|
a different implementation technique in parallel with the
|
||
|
|
main line development. Such development
|
||
|
|
should be carried out on a side branch.
|
||
|
|
The experimental changes may later be moved into the main line, or abandoned.
|
||
|
|
.IP "\fIConflicting updates\fR"
|
||
|
|
.sp 0
|
||
|
|
A common occurrence is that one programmer
|
||
|
|
has checked out a revision, but cannot complete the assignment
|
||
|
|
for some reason. In the meantime, another person
|
||
|
|
must perform another modification
|
||
|
|
immediately. In that case, the second person should check-out the same revision,
|
||
|
|
modify it, and check it in on a side branch, for later merging.
|
||
|
|
.PP
|
||
|
|
Every node in a revision tree consists of the following attributes:
|
||
|
|
a revision number, a check-in date and time, the author's identification,
|
||
|
|
a log entry, a state and the actual text. All these attributes
|
||
|
|
are determined at the time the revision is checked in.
|
||
|
|
The state attribute indicates the status of a revision.
|
||
|
|
It is set automatically to `experimental' during check-in.
|
||
|
|
A revision can later be promoted to a higher status, for example
|
||
|
|
`stable' or `released'. The set of states is user-defined.
|
||
|
|
.NH 2
|
||
|
|
Revisions are represented as deltas
|
||
|
|
.PP
|
||
|
|
For conserving space, RCS stores revisions in the form
|
||
|
|
of deltas, i.e., as differences between revisions.
|
||
|
|
The user interface completely hides this fact.
|
||
|
|
.PP
|
||
|
|
A delta is a sequence of edit commands that transforms one string
|
||
|
|
into another. The deltas employed by RCS are line-based, which means
|
||
|
|
that the only edit commands allowed are insertion and deletion of lines.
|
||
|
|
If a single character in a line is changed, the
|
||
|
|
edit scripts consider the entire line changed.
|
||
|
|
The program \fIdiff\fR\u2\d
|
||
|
|
produces a small, line-based delta between pairs of text files.
|
||
|
|
A character-based edit script would take much longer to compute,
|
||
|
|
and would not be significantly shorter.
|
||
|
|
.PP
|
||
|
|
Using deltas is a classical space-time tradeoff: deltas reduce the
|
||
|
|
space consumed, but increase access time.
|
||
|
|
However, a version control tool should impose as little delay
|
||
|
|
as possible on programmers.
|
||
|
|
Excessive delays discourage the use of version controls,
|
||
|
|
or induce programmers to take shortcuts that compromise system integrity.
|
||
|
|
To gain reasonably fast access time for both editing and compiling,
|
||
|
|
RCS arranges deltas in the following way.
|
||
|
|
The most recent revision on the trunk is stored intact.
|
||
|
|
All other revisions on the trunk are stored as reverse deltas.
|
||
|
|
A reverse delta describes how to go backward in the development history:
|
||
|
|
it produces the desired revision if applied to the successor of that revision.
|
||
|
|
This implementation has the advantage
|
||
|
|
that extraction of the latest revision is a simple and fast copy
|
||
|
|
operation.
|
||
|
|
Adding a new revision to the trunk is also fast: \fIci\fR simply
|
||
|
|
adds the new revision intact, replaces the previous
|
||
|
|
revision with a reverse delta, and keeps the rest of the old deltas.
|
||
|
|
Thus, \fIci\fR requires the computation
|
||
|
|
of only one new delta.
|
||
|
|
.PP
|
||
|
|
Branches need special treatment. The naive solution would be to
|
||
|
|
store complete copies for the tips of all branches.
|
||
|
|
Clearly, this approach would cost too much space. Instead,
|
||
|
|
RCS uses \fIforward\fR deltas for branches. Regenerating a revision
|
||
|
|
on a side branch proceeds as follows. First, extract the latest revision
|
||
|
|
on the trunk; secondly, apply reverse deltas until the fork revision for
|
||
|
|
the branch is obtained; thirdly, apply forward deltas until the desired
|
||
|
|
branch revision is reached. Figure 5 illustrates a tree with
|
||
|
|
one side branch. Triangles pointing to the left and right represent
|
||
|
|
reverse and forward deltas, respectively.
|
||
|
|
.ne 8
|
||
|
|
.PS 4i
|
||
|
|
.ps -2
|
||
|
|
define BD X [line invis $1 right .5;
|
||
|
|
line up .3 then left .5 down .3 then right .5 down .3 then up .3] X
|
||
|
|
|
||
|
|
define FD X [line invis $1 right .5;
|
||
|
|
line left .5 down .3 then up .6 then right .5 down .3;] X
|
||
|
|
|
||
|
|
right
|
||
|
|
D11: BD(" 1.1")
|
||
|
|
arrow right from D11.e
|
||
|
|
D12: BD(" 1.2")
|
||
|
|
arrow right from D12.e
|
||
|
|
D13: BD(" 1.3")
|
||
|
|
arrow right from D13.e
|
||
|
|
D21: BD(" 2.1")
|
||
|
|
arrow right from D21.e
|
||
|
|
D22: box "2.2"
|
||
|
|
line invis down from D21.s
|
||
|
|
F1: FD("1.3.1.1 ")
|
||
|
|
arrow from D13.se to F1.w
|
||
|
|
arrow from F1.e right
|
||
|
|
right
|
||
|
|
F2: FD("1.3.1.2 ")
|
||
|
|
.ps +2
|
||
|
|
.PE
|
||
|
|
.ce 1
|
||
|
|
Figure 5. A revision tree with reverse and forward deltas.
|
||
|
|
.sp 0
|
||
|
|
.PP
|
||
|
|
Although implementing fast check-out for the latest trunk revision,
|
||
|
|
this arrangement has the disadvantage that generation of other revisions
|
||
|
|
takes time proportional to the number of deltas applied. For example,
|
||
|
|
regenerating the branch tip in Figure 5 requires application of five
|
||
|
|
deltas (including the initial one). Since usage statistics show that
|
||
|
|
the latest trunk revision is the one that is retrieved in 95 per cent
|
||
|
|
of all cases (see the section on usage statistics), biasing check-out time
|
||
|
|
in favor of that revision results in significant savings.
|
||
|
|
However, careful implementation of the delta application process is
|
||
|
|
necessary to provide low retrieval overhead for other revisions, in
|
||
|
|
particular for branch tips.
|
||
|
|
.PP
|
||
|
|
There are several techniques for delta application.
|
||
|
|
The naive one is to pass each delta to a general-purpose text editor.
|
||
|
|
A prototype of RCS invoked the UNIX editor \fIed\fR both
|
||
|
|
for applying deltas and for expanding the identification markers.
|
||
|
|
Although easy to implement, performance was poor, owing to the
|
||
|
|
high start-up costs and excess generality of \fIed\fR. An intermediate
|
||
|
|
version of RCS used a special-purpose, stream-oriented editor.
|
||
|
|
This technique reduced the cost of applying a delta to the cost of
|
||
|
|
checking out the latest trunk revision. The reason for this behavior
|
||
|
|
is that each delta application involves a complete pass over
|
||
|
|
the preceding revision.
|
||
|
|
.PP
|
||
|
|
However, there is a much better algorithm. Note that the deltas are
|
||
|
|
line oriented and that most of the work of a stream editor involves
|
||
|
|
copying unchanged lines from one revision to the next. A faster
|
||
|
|
algorithm avoids unnecessary copying of character strings by using
|
||
|
|
a \fIpiece table\fR.
|
||
|
|
A piece table is a one-dimensional array, specifying how a given
|
||
|
|
revision is `pieced together' from lines in the RCS file.
|
||
|
|
Suppose piece table \fIPT\dr\u\fR represents revision \fIr\fR.
|
||
|
|
Then \fIPT\dr\u[i]\fR contains the starting position of line \fIi\fR
|
||
|
|
of revision \fIr\fR.
|
||
|
|
Application of the next delta transforms piece table \fIPT\dr\u\fR
|
||
|
|
into \fIPT\dr+1\u\fR. For instance, a delete command removes a
|
||
|
|
series of entries from the piece table. An insertion command inserts
|
||
|
|
new entries, moving the entries following the insertion point further down the
|
||
|
|
array. The inserted entries point to the text lines in the delta.
|
||
|
|
Thus, no I/O is involved except for reading the delta itself. When all
|
||
|
|
deltas have been applied to the piece table, a sequential pass
|
||
|
|
through the table looks up each line in the RCS file and copies it to
|
||
|
|
the output file, updating identification markers at the same time.
|
||
|
|
Of course, the RCS file must permit random access, since the copied
|
||
|
|
lines are scattered throughout that file. Figure 6 illustrates an
|
||
|
|
RCS file with two revisions and the corresponding piece tables.
|
||
|
|
.ne 13
|
||
|
|
.sp 6
|
||
|
|
.ce 1
|
||
|
|
\fIFigure 6 is not available.\fP
|
||
|
|
.sp 5
|
||
|
|
.ce 1
|
||
|
|
Figure 6. An RCS file and its piece tables
|
||
|
|
.sp 0
|
||
|
|
.PP
|
||
|
|
The piece table approach has the property that the time for applying a single
|
||
|
|
delta is roughly determined by the size of the delta, and not by the
|
||
|
|
size of the revision. For example, if a delta is
|
||
|
|
10 per cent of the size of a revision, then applying it takes only
|
||
|
|
10 per cent of the time to generate the latest trunk revision. (The stream
|
||
|
|
editor would take 100 per cent.)
|
||
|
|
.PP
|
||
|
|
There is an important alternative for representing deltas that affects
|
||
|
|
performance. SCCS\u3\d,
|
||
|
|
a precursor of RCS, uses \fIinterleaved\fR deltas.
|
||
|
|
A file containing interleaved deltas is partitioned into blocks of lines.
|
||
|
|
Each block has a header that specifies to which revision(s) the block
|
||
|
|
belongs. The blocks are sorted out in such a way that a single
|
||
|
|
pass over the file can pick up all the lines belonging to a given
|
||
|
|
revision. Thus, the regeneration time for all revisions is the same:
|
||
|
|
all headers must be inspected, and the associated blocks either copied
|
||
|
|
or skipped. As the number of revisions increases, the cost of retrieving
|
||
|
|
any revision is much higher than the cost of checking out the
|
||
|
|
latest trunk revision with reverse deltas. A detailed comparison
|
||
|
|
of SCCS's interleaved deltas and RCS's reverse deltas can be found
|
||
|
|
in Reference 4.
|
||
|
|
This reference considers the version of RCS with the
|
||
|
|
stream editor only. The piece table method improves performance
|
||
|
|
further, so that RCS is always faster than SCCS, except if 10
|
||
|
|
or more deltas are applied.
|
||
|
|
.PP
|
||
|
|
Additional speed-up for both delta methods can be obtained by caching
|
||
|
|
the most recently generated revision, as has been implemented in DSEE.\u5\d
|
||
|
|
With caching, access time to frequently used revisions can approach normal file
|
||
|
|
access time, at the cost of some additional space.
|
||
|
|
.NH
|
||
|
|
Locking: A Controversial Issue
|
||
|
|
.PP
|
||
|
|
The locking mechanism for RCS was difficult to design.
|
||
|
|
The problem and its solution are first presented in their `pure' form,
|
||
|
|
followed by a discussion of the complications
|
||
|
|
caused by `real-world' considerations.
|
||
|
|
.PP
|
||
|
|
RCS must prevent two or more persons from depositing competing changes of the
|
||
|
|
same revision.
|
||
|
|
Suppose two programmers check out revision 2.4 and
|
||
|
|
modify it. Programmer A checks in a revision before programmer B\&.
|
||
|
|
Unfortunately, programmer B has not seen A's
|
||
|
|
changes, so the effect is that A's changes are covered up by B's deposit.
|
||
|
|
A's changes are not lost since all revisions
|
||
|
|
are saved, but they are confined to a single revision.\(dd
|
||
|
|
.FS \(dd
|
||
|
|
Note that this problem is entirely different from the atomicity problem.
|
||
|
|
Atomicity means that
|
||
|
|
concurrent update operations on the same RCS file cannot be permitted,
|
||
|
|
because that may result in inconsistent data.
|
||
|
|
Atomic updates are essential (and implemented in RCS),
|
||
|
|
but do not solve the conflict discussed here.
|
||
|
|
.FE
|
||
|
|
.PP
|
||
|
|
This conflict is prevented in RCS by locking.
|
||
|
|
Whenever someone intends to edit a revision (as opposed
|
||
|
|
to reading or compiling it), the revision should be checked out
|
||
|
|
and locked,
|
||
|
|
using the \fI\-l\fR option on \fIco\fR. On subsequent check-in,
|
||
|
|
\fIci\fR tests the lock and then removes it.
|
||
|
|
At most one programmer at a time may
|
||
|
|
lock a particular revision, and only this programmer may check in
|
||
|
|
the succeeding revision.
|
||
|
|
Thus, while a revision is locked, it is the exclusive responsibility
|
||
|
|
of the locker.
|
||
|
|
.PP
|
||
|
|
An important maxim for software tools like RCS is that they must
|
||
|
|
not stand in the way of making progress with a project.
|
||
|
|
This consideration leads to several weakenings of the locking mechanism.
|
||
|
|
First of all, even if a revision is locked, it can
|
||
|
|
still be checked out. This is necessary if other people
|
||
|
|
wish to compile or inspect the locked revision
|
||
|
|
while the next one is in preparation. The only operations they
|
||
|
|
cannot do are to lock the revision or to check in the succeeding one. Secondly,
|
||
|
|
check-in operations on other branches in the RCS file are still possible; the
|
||
|
|
locking of one revision does not affect any other revision.
|
||
|
|
Thirdly, revisions are occasionally locked for a long period of time
|
||
|
|
because a programmer is absent or otherwise unable to complete
|
||
|
|
the assignment. If another programmer has to make a pressing change,
|
||
|
|
there are the following three alternatives for making progress:
|
||
|
|
a) find out who is holding the lock and ask that person to release it;
|
||
|
|
b) check out the locked revision, modify it, check it
|
||
|
|
in on a branch, and merge the changes later;
|
||
|
|
c) break the lock. Breaking a lock leaves a highly visible
|
||
|
|
trace, namely an electronic mail message that is sent automatically to the
|
||
|
|
holder of the lock, recording the breaker and a commentary requested from him.
|
||
|
|
Thus, breaking locks is tolerated under certain circumstances,
|
||
|
|
but will not go unnoticed.
|
||
|
|
Experience has shown that the automatic mail message attaches a high enough
|
||
|
|
stigma to lock breaking,
|
||
|
|
such that programmers break locks only in real emergencies,
|
||
|
|
or when a co-worker resigns and leaves locked revisions behind.
|
||
|
|
.PP
|
||
|
|
If an RCS file is private, i.e., when a programmer owns an RCS file
|
||
|
|
and does not expect anyone else to perform check-in operations,
|
||
|
|
locking is an unnecessary nuisance.
|
||
|
|
In this case,
|
||
|
|
the `strict locking feature' discussed earlier may be disabled,
|
||
|
|
provided that file protection
|
||
|
|
is set such that only the owner may write the RCS file.
|
||
|
|
This has the effect that only the owner can check-in revisions,
|
||
|
|
and that no lock is needed for doing so.
|
||
|
|
.PP
|
||
|
|
As added protection,
|
||
|
|
each RCS file contains an access list that specifies the users
|
||
|
|
who may execute update operations. If an access list is empty,
|
||
|
|
only normal UNIX file protection applies. Thus, the access list is
|
||
|
|
useful for restricting the set of people who would otherwise have update
|
||
|
|
permission. Just as with locking, the access list
|
||
|
|
has no effect on read-only operations such as \fIco\fR. This approach
|
||
|
|
is consistent with the UNIX philosophy of openness, which contributes
|
||
|
|
to a productive software development environment.
|
||
|
|
.NH
|
||
|
|
Configuration Management
|
||
|
|
.PP
|
||
|
|
The preceding sections described how RCS deals with revisions of individual
|
||
|
|
components; this section discusses how to handle configurations.
|
||
|
|
A configuration is a set of revisions, where each revision comes
|
||
|
|
from a different revision group, and the revisions are selected
|
||
|
|
according to a certain criterion.
|
||
|
|
For example,
|
||
|
|
in order to build a functioning compiler, the `right'
|
||
|
|
revisions from the scanner, the parser, the optimizer
|
||
|
|
and the code generator must be combined.
|
||
|
|
RCS, in conjunction with MAKE,
|
||
|
|
provides a number of facilities to effect a smooth selection.
|
||
|
|
.NH 2
|
||
|
|
RCS Selection Functions
|
||
|
|
.PP
|
||
|
|
.IP "\fIDefault selection\fR"
|
||
|
|
.sp 0
|
||
|
|
During development, the usual selection criterion is to choose
|
||
|
|
the latest revision of all components. The \fIco\fR command
|
||
|
|
makes this selection by default. For example, the command
|
||
|
|
.D(
|
||
|
|
co *,v
|
||
|
|
.D)
|
||
|
|
retrieves the latest revision on the default branch of each RCS file
|
||
|
|
in the current directory.
|
||
|
|
The default branch is usually the trunk, but may be
|
||
|
|
set to be a side branch.
|
||
|
|
Side branches as defaults are needed in distributed software development,
|
||
|
|
as discussed in the section on the RCS revision tree.
|
||
|
|
.sp
|
||
|
|
.IP "\fIRelease based selection\fR"
|
||
|
|
.sp 0
|
||
|
|
Specifying a release or branch number selects the latest revision in
|
||
|
|
that release or branch.
|
||
|
|
For instance,
|
||
|
|
.D(
|
||
|
|
co \-r2 *,v
|
||
|
|
.D)
|
||
|
|
retrieves the latest revision with release number 2 from each RCS file.
|
||
|
|
This selection is convenient if a release has been completed and
|
||
|
|
development has moved on to the next release.
|
||
|
|
.sp
|
||
|
|
.IP "\fIState and author based selection\fR"
|
||
|
|
.sp 0
|
||
|
|
If the highest level number within a given release number
|
||
|
|
is not the desired one,
|
||
|
|
the state attribute can help. For example,
|
||
|
|
.D(
|
||
|
|
co \-r2 \-sReleased *,v
|
||
|
|
.D)
|
||
|
|
retrieves the latest revision with release number 2 whose state attribute
|
||
|
|
is `Released'.
|
||
|
|
Of course, the state attribute has to be set appropriately, using the
|
||
|
|
\fIci\fR or \fIrcs\fR commands.
|
||
|
|
Another alternative is to select a revision by its author,
|
||
|
|
using the \fI\-w\fR option.
|
||
|
|
.sp
|
||
|
|
.IP "\fIDate based selection\fR"
|
||
|
|
.sp 0
|
||
|
|
Revisions may also be selected by date.
|
||
|
|
Suppose a release of an entire system was
|
||
|
|
completed and current on March 4, at 1:00 p.m. local time. Then the command
|
||
|
|
.D(
|
||
|
|
co \-d'March 4, 1:00 pm LT' *,v
|
||
|
|
.D)
|
||
|
|
checks out all the components of that release, independent of the numbering.
|
||
|
|
The \fI\-d\fR option specifies a `cutoff date', i.e.,
|
||
|
|
the revision selected has a check-in date that
|
||
|
|
is closest to, but not after the date given.
|
||
|
|
.IP "\fIName based selection\fR"
|
||
|
|
.sp 0
|
||
|
|
The most powerful selection function is based on assigning symbolic
|
||
|
|
names to revisions and branches.
|
||
|
|
In large systems, a single release number or date is not sufficient
|
||
|
|
to collect the appropriate revisions from all groups.
|
||
|
|
For example, suppose one wishes to combine release 2
|
||
|
|
of one subsystem and release 15 of another.
|
||
|
|
Most likely, the creation dates of those releases differ also.
|
||
|
|
Thus, a single revision number or date passed to the \fIco\fR command
|
||
|
|
will not suffice to select the right revisions.
|
||
|
|
Symbolic revision numbers solve this problem.
|
||
|
|
Each RCS file may contain a set of symbolic names that are mapped
|
||
|
|
to numeric revision numbers. For example, assume
|
||
|
|
the symbol \fIV3\fR is bound to release number 2 in file \fIs,v\fR, and to
|
||
|
|
revision number 15.9 in \fIt,v\fR.
|
||
|
|
Then the single command
|
||
|
|
.D(
|
||
|
|
co \-rV3 s,v t,v
|
||
|
|
.D)
|
||
|
|
retrieves the latest revision of release 2 from \fIs,v\fR,
|
||
|
|
and revision 15.9 from \fIt,v\fR.
|
||
|
|
In a large system with many modules, checking out all
|
||
|
|
revisions with one command greatly simplifies configuration management.
|
||
|
|
.PP
|
||
|
|
Judicious use of symbolic revision numbers helps with organizing
|
||
|
|
large configurations.
|
||
|
|
A special command, \fIrcsfreeze\fR,
|
||
|
|
assigns a symbolic revision number to a selected revision
|
||
|
|
in every RCS file.
|
||
|
|
\fIRcsfreeze\fR effectively freezes a configuration.
|
||
|
|
The assigned symbolic revision number selects all components
|
||
|
|
of the configuration.
|
||
|
|
If necessary, symbolic numbers
|
||
|
|
may even be intermixed with numeric ones. Thus, \fIV3.5\fR in the
|
||
|
|
above example
|
||
|
|
would select revision 2.5 in \fIs,v\fR and branch 15.9.5 in \fIt,v\fR.
|
||
|
|
.PP
|
||
|
|
The options \fI\-r\fR, \fI\-s\fR, \fI\-w\fR and \fI\-d\fR
|
||
|
|
may be combined. If a branch is given, the latest revision
|
||
|
|
on that branch satisfying all conditions is retrieved;
|
||
|
|
otherwise, the default branch is used.
|
||
|
|
.NH 2
|
||
|
|
Combining MAKE and RCS
|
||
|
|
.PP
|
||
|
|
MAKE\u1\d
|
||
|
|
is a program that processes configurations.
|
||
|
|
It is driven by configuration specifications
|
||
|
|
recorded in a special file, called a `Makefile'.
|
||
|
|
MAKE avoids redundant processing steps
|
||
|
|
by comparing creation dates of source and processed objects.
|
||
|
|
For example, when instructed to compile all
|
||
|
|
modules of a given system, it only recompiles
|
||
|
|
those source modules that were changed
|
||
|
|
since they were processed last.
|
||
|
|
.PP
|
||
|
|
MAKE has been extended with an auto-checkout feature for RCS.*
|
||
|
|
.FS *
|
||
|
|
This auto-checkout extension is available only in some versions of MAKE,
|
||
|
|
e.g. GNU MAKE.
|
||
|
|
.FE
|
||
|
|
When a certain file to be processed is not present,
|
||
|
|
MAKE attempts a check-out operation.
|
||
|
|
If successful, MAKE performs the required processing, and then deletes
|
||
|
|
the checked out file to conserve space.
|
||
|
|
The selection parameters discussed above can be passed to MAKE
|
||
|
|
either as parameters, or directly embedded in the Makefile.
|
||
|
|
MAKE has also been extended to search the subdirectory named \fIRCS\fR
|
||
|
|
for needed files, rather than just the current working directory.
|
||
|
|
However, if a working file is present, MAKE totally ignores the corresponding
|
||
|
|
RCS file and uses the working file.
|
||
|
|
(In newer versions of MAKE distributed by AT&T and others,
|
||
|
|
auto-checkout can be
|
||
|
|
achieved with the rule DEFAULT, instead of a special extension of MAKE.
|
||
|
|
However, a file checked out by the rule DEFAULT
|
||
|
|
will not be deleted after processing. \fIRcsclean\fR can be
|
||
|
|
used for that purpose.)
|
||
|
|
.PP
|
||
|
|
With auto-checkout, RCS/MAKE can effect a selection rule
|
||
|
|
especially tuned for multi-person software development and maintenance.
|
||
|
|
In these situations,
|
||
|
|
programmers should obtain configurations that consist of
|
||
|
|
the revisions they have personally checked out plus the latest
|
||
|
|
checked in revision of all other revision groups.
|
||
|
|
This schema can be set up as follows.
|
||
|
|
.PP
|
||
|
|
Each programmer chooses a working directory
|
||
|
|
and places into it a symbolic link, named \fIRCS\fR,
|
||
|
|
to the directory containing the relevant RCS files.
|
||
|
|
The symbolic link makes sure that \fIco\fR and \fIci\fR
|
||
|
|
operations need only specify the working files, and that
|
||
|
|
the Makefile need not be changed.
|
||
|
|
The programmer then checks out the needed files and modifies them.
|
||
|
|
If MAKE is invoked,
|
||
|
|
it composes configurations by selecting those
|
||
|
|
revisions that are checked out, and the rest from the
|
||
|
|
subdirectory \fIRCS\fR.
|
||
|
|
The latter selection may be controlled by a symbolic
|
||
|
|
revision number or any of the other selection criteria.
|
||
|
|
If there are several programmers editing in separate working directories,
|
||
|
|
they are insulated from each other's changes until checking in their
|
||
|
|
modifications.
|
||
|
|
.PP
|
||
|
|
Similarly, a maintainer can recreate an older configuration
|
||
|
|
by starting to work in an empty working directory.
|
||
|
|
During the initial MAKE invocation, all revisions are selected from RCS files.
|
||
|
|
As the maintainer checks out files and modifies them,
|
||
|
|
a new configuration is gradually built up.
|
||
|
|
Every time MAKE is invoked, it substitutes the modified revisions
|
||
|
|
into the configuration being manipulated.
|
||
|
|
.PP
|
||
|
|
A final application of RCS is to use it for storing Makefiles.
|
||
|
|
Revision groups of Makefiles represent
|
||
|
|
multiple versions of configurations.
|
||
|
|
Whenever a configuration is baselined or distributed,
|
||
|
|
the best approach is to unambiguously fix
|
||
|
|
the configuration with a symbolic revision number by calling
|
||
|
|
\fIrcsfreeze\fR,
|
||
|
|
to embed that symbol into the Makefile, and to
|
||
|
|
check in the Makefile (using the same symbolic revision number).
|
||
|
|
With this approach, old configurations
|
||
|
|
can be regenerated easily and reliably.
|
||
|
|
.NH
|
||
|
|
Usage Statistics
|
||
|
|
.PP
|
||
|
|
The following usage statistics were collected on two DEC VAX-11/780
|
||
|
|
computers of the Purdue Computer Science Department. Both machines
|
||
|
|
are mainly used for research purposes. Thus, the data
|
||
|
|
reflect an environment in which the majority of projects
|
||
|
|
involve prototyping and advanced software development,
|
||
|
|
but relatively little long-term maintenance.
|
||
|
|
.PP
|
||
|
|
For the first experiment,
|
||
|
|
the \fIci\fR and \fIco\fR operations were instrumented
|
||
|
|
to log the number of backward and forward deltas applied.
|
||
|
|
The data were collected during a 13 month period
|
||
|
|
from Dec. 1982 to Dec. 1983.
|
||
|
|
Table I summarizes the results.
|
||
|
|
.sp 0
|
||
|
|
.nr VS 12p
|
||
|
|
.vs 12p
|
||
|
|
.TS
|
||
|
|
center,box,tab(#);
|
||
|
|
c|c|c|c|c s|c s
|
||
|
|
c|c|c|c|c s|c s
|
||
|
|
l|n|n|n|n n|n n.
|
||
|
|
Operation#Total#Total deltas#Mean deltas#Operations#Branch
|
||
|
|
#operations #applied#applied#with >1 delta#operations
|
||
|
|
_
|
||
|
|
co # 7867# 9320#1.18#509#(6%)#203#(3%)
|
||
|
|
ci # 3468# 2207#0.64# 85#(2%)# 75#(2%)
|
||
|
|
ci & co#11335#11527#1.02#594#(5%)#278#(2%)
|
||
|
|
.TE
|
||
|
|
.ce 1
|
||
|
|
Table I. Statistics for \fIco\fR and \fIci\fR operations.
|
||
|
|
.nr VS 18p
|
||
|
|
.vs 18p
|
||
|
|
.PP
|
||
|
|
The first two lines show statistics for check-out and check-in;
|
||
|
|
the third line shows the combination.
|
||
|
|
Recall that \fIci\fR performs an implicit check-out to obtain
|
||
|
|
a revision for computing the delta.
|
||
|
|
In all measures presented, the most recent revision (stored intact)
|
||
|
|
counts as one delta. The number of deltas applied represents
|
||
|
|
the number of passes necessary, where the first `pass' is a copying step.
|
||
|
|
.PP
|
||
|
|
Note that the check-out operation is executed more than
|
||
|
|
twice as frequently as the check-in operation.
|
||
|
|
The fourth column gives the mean number of deltas
|
||
|
|
applied in all three cases.
|
||
|
|
For \fIci\fR, the mean number of deltas applied is less
|
||
|
|
than one.
|
||
|
|
The reasons are that the initial check-in requires no delta at all, and that
|
||
|
|
the only time \fIci\fR requires more than one delta is for branches.
|
||
|
|
Column 5 shows the actual number of operations that applied more than one
|
||
|
|
delta.
|
||
|
|
The last column indicates that branches were not used often.
|
||
|
|
.PP
|
||
|
|
The last three columns demonstrate that the most recent trunk revision
|
||
|
|
is by far the most frequently accessed.
|
||
|
|
For RCS, check-out of
|
||
|
|
this revision is a simple copy operation, which is the absolute minimum
|
||
|
|
given the copy-semantics of \fIco\fR.
|
||
|
|
Access to older revisions and branches
|
||
|
|
is more common in non-academic environments,
|
||
|
|
yet even if access to older deltas were an order
|
||
|
|
of magnitude more frequent,
|
||
|
|
the combined average number of deltas applied would still be below 1.2.
|
||
|
|
Since RCS is faster than SCCS until up to 10 delta applications,
|
||
|
|
reverse deltas are clearly the method of choice.
|
||
|
|
.PP
|
||
|
|
The second experiment, conducted in March of 1984,
|
||
|
|
involved surveying the existing RCS files
|
||
|
|
on our two machines. The goal was to determine the mean number of
|
||
|
|
revisions per RCS file, as well as the space consumed by them.
|
||
|
|
Table II shows the results. (Tables I and II were produced at different
|
||
|
|
times and are unrelated.)
|
||
|
|
.sp 0
|
||
|
|
.nr VS 12p
|
||
|
|
.vs 12p
|
||
|
|
.TS
|
||
|
|
center,box,tab(#);
|
||
|
|
c | c | c | c | c | c | c
|
||
|
|
c | c | c | c | c | c | c
|
||
|
|
l | n | n | n | n | n | n.
|
||
|
|
#Total RCS#Total#Mean#Mean size of#Mean size of#Overhead
|
||
|
|
#files#revisions#revisions#RCS files#revisions
|
||
|
|
_
|
||
|
|
All files #8033#11133#1.39#6156#5585#1.10
|
||
|
|
Files with#1477# 4578#3.10#8074#6041#1.34
|
||
|
|
\(>= 2 deltas
|
||
|
|
.TE
|
||
|
|
.ce 1
|
||
|
|
Table II. Statistics for RCS files.
|
||
|
|
.nr VS 18p
|
||
|
|
.vs 18p
|
||
|
|
.PP
|
||
|
|
The mean number of revisions per RCS file is 1.39.
|
||
|
|
Columns 5 and 6 show the mean sizes (in bytes) of an RCS file
|
||
|
|
and of the latest revision of each RCS file, respectively.
|
||
|
|
The `overhead' column contains the ratio of the mean sizes.
|
||
|
|
Assuming that all revisions in an RCS file are approximately the same size,
|
||
|
|
this ratio gives a measure of the space consumed by the extra revisions.
|
||
|
|
.PP
|
||
|
|
In our sample, over 80 per cent of the RCS files contained only a single revision.
|
||
|
|
The reason is that our
|
||
|
|
systems programmers routinely check in all source files
|
||
|
|
on the distribution tapes, even though they may never touch them again.
|
||
|
|
To get a better indication of how much space savings are possible
|
||
|
|
with deltas, all measures with those files
|
||
|
|
that contained 2 or more revisions were recomputed. Only for those files
|
||
|
|
is RCS necessary.
|
||
|
|
As shown in the second line, the average number of revisions for those files is
|
||
|
|
3.10, with an overhead of 1.34. This means that the extra 2.10 deltas
|
||
|
|
require 34 per cent extra space, or
|
||
|
|
16 per cent per extra revision.
|
||
|
|
Rochkind\u3\d
|
||
|
|
measured the space consumed by SCCS, and
|
||
|
|
reported an average of 5 revisions per group
|
||
|
|
and an overhead of 1.37 (or about 9 per cent per extra revision).
|
||
|
|
In a later paper, Glasser\u6\d
|
||
|
|
observed an average of 7 revisions per group in a single, large project,
|
||
|
|
but provided no overhead figure.
|
||
|
|
In his paper on DSEE\u5\d,
|
||
|
|
Leblang reported that delta storage combined with blank compression
|
||
|
|
results in an overhead of a mere 1\-2 per cent per revision.
|
||
|
|
Since leading blanks accounted for about 20 per cent of the surveyed Pascal
|
||
|
|
programs, a revision group with 5\-10 members was smaller
|
||
|
|
than a single cleartext copy.
|
||
|
|
.PP
|
||
|
|
The above observations demonstrate clearly that the space needed
|
||
|
|
for extra revisions is small. With delta storage, the luxury of
|
||
|
|
keeping multiple revisions online is certainly affordable.
|
||
|
|
In fact, introducing a system with delta storage may reduce
|
||
|
|
storage requirements, because programmers often save back-up copies
|
||
|
|
anyway. Since back-up copies are stored much more efficiently with deltas,
|
||
|
|
introducing a system such as RCS may
|
||
|
|
actually free a considerable amount of space.
|
||
|
|
.NH
|
||
|
|
Survey of Version Control Tools
|
||
|
|
.PP
|
||
|
|
The need to keep back-up copies of software arose when
|
||
|
|
programs and data were no longer stored on paper media, but were entered
|
||
|
|
from terminals and stored on disk.
|
||
|
|
Back-up copies are desirable for reliability, and many modern editors
|
||
|
|
automatically save a back-up copy for every file touched.
|
||
|
|
This strategy
|
||
|
|
is valuable for short-term back-ups, but not suitable for long-term
|
||
|
|
version control, since an existing back-up copy is overwritten whenever the
|
||
|
|
corresponding file is edited.
|
||
|
|
.PP
|
||
|
|
Tape archives are suitable for long-term, offline storage.
|
||
|
|
If all changed files are dumped on a back-up tape once per day, old revisions
|
||
|
|
remain accessible. However, tape archives are unsatisfactory
|
||
|
|
for version control in several ways. First, backing up the file
|
||
|
|
system every 24 hours does not capture intermediate revisions.
|
||
|
|
Secondly, the old revisions are not online,
|
||
|
|
and accessing them is tedious and time-consuming.
|
||
|
|
In particular, it is impractical to
|
||
|
|
compare several old revisions of a group,
|
||
|
|
because that may require mounting and searching several tapes.
|
||
|
|
Tape archives are important fail-safe tools in the
|
||
|
|
event of catastrophic disk failures or accidental deletions,
|
||
|
|
but they are ill-suited for version control.
|
||
|
|
Conversely, version control tools do not obviate the
|
||
|
|
need for tape archives.
|
||
|
|
.PP
|
||
|
|
A natural technique for keeping several old revisions online is
|
||
|
|
to never delete a file.
|
||
|
|
Editing a file
|
||
|
|
simply creates a new file with the same
|
||
|
|
name, but with a different sequence number.
|
||
|
|
This technique, available as an option in DEC's VMS operating system,
|
||
|
|
turns out to be inadequate for version control.
|
||
|
|
First, it is prohibitively expensive in terms of storage costs,
|
||
|
|
especially since no data compression techniques are employed.
|
||
|
|
Secondly, indiscriminately storing every change produces too many
|
||
|
|
revisions, and programmers have difficulties distinguishing them.
|
||
|
|
The proliferation of revisions forces programmers to spend much time on
|
||
|
|
finding and deleting useless files.
|
||
|
|
Thirdly, most of the support functions like locking, logging,
|
||
|
|
revision selection,
|
||
|
|
and identification described in this paper are not available.
|
||
|
|
.PP
|
||
|
|
An alternative approach is to separate editing from revision control.
|
||
|
|
The user may repeatedly edit a given revision,
|
||
|
|
until freezing it with an explicit command.
|
||
|
|
Once a revision is frozen, it is stored permanently and can no longer be modified.
|
||
|
|
(In RCS, freezing a revisions is done with \fIci\fR.)
|
||
|
|
Editing a frozen revision implicitly creates a new one, which
|
||
|
|
can again be changed repeatedly until it is frozen itself.
|
||
|
|
This approach saves exactly those revisions that the user
|
||
|
|
considers important, and keeps the number of revisions manageable.
|
||
|
|
IBM's CLEAR/CASTER\u7\d,
|
||
|
|
AT&T's SCCS\u3\d,
|
||
|
|
CMU's SDC\u8\d
|
||
|
|
and DEC's CMS\u9\d,
|
||
|
|
are examples of version control systems using this approach.
|
||
|
|
CLEAR/CASTER maintains a data base of programs, specifications,
|
||
|
|
documentation and messages, using deltas.
|
||
|
|
Its goal is to provide control over the development process from a
|
||
|
|
management viewpoint.
|
||
|
|
SCCS stores multiple revisions of source text in an ancestral tree,
|
||
|
|
records a log entry for each revision,
|
||
|
|
provides access control, and has facilities
|
||
|
|
for uniquely identifying each revision.
|
||
|
|
An efficient delta technique
|
||
|
|
reduces the space consumed by each revision group.
|
||
|
|
SDC is much simpler than SCCS because it stores not more than
|
||
|
|
two revisions. However, it maintains a complete log for all old
|
||
|
|
revisions, some of which may be on back-up tape.
|
||
|
|
CMS, like SCCS, manages tree-structured revision groups,
|
||
|
|
but offers no identification mechanism.
|
||
|
|
.PP
|
||
|
|
Tools for dealing with configurations are still in a state of flux.
|
||
|
|
SCCS, SDC and CMS can be combined with MAKE or MAKE-like programs.
|
||
|
|
Since flexible selection rules are missing from all these tools,
|
||
|
|
it is sometimes difficult
|
||
|
|
to specify precisely which revision of each group
|
||
|
|
should be passed to MAKE for building a desired configuration.
|
||
|
|
The Xerox Cedar system\u10\d
|
||
|
|
provides a `System Modeller' that can rebuild
|
||
|
|
a configuration from an arbitrary set of module revisions.
|
||
|
|
The revisions of a module are only distinguished by creation time,
|
||
|
|
and there is no tool for managing groups.
|
||
|
|
Since the selection rules are primitive,
|
||
|
|
the System Modeller appears to be somewhat tedious to use.
|
||
|
|
Apollo's DSEE\u5\d
|
||
|
|
is a sophisticated software engineering environment.
|
||
|
|
It manages revision groups in a way similar to SCCS and CMS. Configurations
|
||
|
|
are built using `configuration threads'.
|
||
|
|
A configuration thread states which revision of each group
|
||
|
|
named in a configuration should be chosen.
|
||
|
|
A configuration thread may contain dynamic specifiers
|
||
|
|
(e.g., `choose the revisions I am currently working on,
|
||
|
|
and the most recent revisions otherwise'), which are bound
|
||
|
|
automatically at build time.
|
||
|
|
It also provides a notification mechanism for alerting
|
||
|
|
maintainers about the need to rebuild a system after a change.
|
||
|
|
.PP
|
||
|
|
RCS is based on a general model for describing
|
||
|
|
multi-version/multi-configuration systems\u11\d.
|
||
|
|
The model describes systems using AND/OR graphs, where AND nodes represent
|
||
|
|
configurations, and OR nodes represent version groups.
|
||
|
|
The model gives rise to a suit of selection rules for
|
||
|
|
composing configurations, almost all of which are implemented in RCS.
|
||
|
|
The revisions selected by RCS are passed to MAKE for configuration building.
|
||
|
|
Revision group management is modelled after SCCS.
|
||
|
|
RCS retains SCCS's best features,
|
||
|
|
but offers a significantly simpler user interface,
|
||
|
|
flexible selection rules, adequate integration with MAKE
|
||
|
|
and improved identification.
|
||
|
|
A detailed comparison of RCS and SCCS appears in Reference 4.
|
||
|
|
.PP
|
||
|
|
An important component of all revision control systems
|
||
|
|
is a program for computing deltas.
|
||
|
|
SCCS and RCS use the program \fIdiff\fR\u2\d,
|
||
|
|
which first computes the longest common substring of two
|
||
|
|
revisions, and then produces the delta from that substring.
|
||
|
|
The delta is simply an edit script consisting of deletion and
|
||
|
|
insertion commands that generate one revision from the other.
|
||
|
|
.PP
|
||
|
|
A delta based on a longest common substring is not necessarily minimal,
|
||
|
|
because it does not take advantage of crossing block moves.
|
||
|
|
Crossing block moves arise if two or more blocks of lines
|
||
|
|
(e.g., procedures)
|
||
|
|
appear in a different order in two revisions.
|
||
|
|
An edit script derived from a longest common substring
|
||
|
|
first deletes the shorter of the two blocks, and then reinserts it.
|
||
|
|
Heckel\u12\d
|
||
|
|
proposed an algorithm for detecting block moves, but
|
||
|
|
since the algorithm is based on heuristics,
|
||
|
|
there are conditions
|
||
|
|
under which the generated delta is far from minimal.
|
||
|
|
DSEE uses this algorithm combined with blank compression,
|
||
|
|
apparently with satisfactory overall results.
|
||
|
|
A new algorithm that is guaranteed to produce a minimal delta based on
|
||
|
|
block moves appears in Reference 13.
|
||
|
|
A future release of RCS will use this algorithm.
|
||
|
|
.PP
|
||
|
|
\fIAcknowledgements\fR:
|
||
|
|
Many people have helped make RCS a success by contributed criticisms, suggestions,
|
||
|
|
corrections, and even whole new commands (including manual pages).
|
||
|
|
The list of people is too long to be
|
||
|
|
reproduced here, but my sincere thanks for their help and
|
||
|
|
goodwill goes to all of them.
|
||
|
|
.sp
|
||
|
|
.nr VS 12p
|
||
|
|
.vs 12p
|
||
|
|
.SH
|
||
|
|
Appendix: Synopsis of RCS Operations
|
||
|
|
.LP
|
||
|
|
.IP "\fIci\fP \fB\- check in revisions\fP"
|
||
|
|
.sp 0
|
||
|
|
\fICi\fR stores the contents of a working file into the
|
||
|
|
corresponding RCS file as a new revision.
|
||
|
|
If the RCS file doesn't exist, \fIci\fR creates it.
|
||
|
|
\fICi\fR removes the working file, unless one of the options
|
||
|
|
\fI\-u\fR or \fI\-l\fR is present.
|
||
|
|
For each check-in, \fIci\fR asks for a commentary
|
||
|
|
describing the changes relative to the previous revision.
|
||
|
|
.sp 1
|
||
|
|
\fICi\fR assigns the revision number given by the \fI\-r\fR option;
|
||
|
|
if that option is missing, it derives the number from the
|
||
|
|
lock held by the user; if there is no lock and locking is not strict,
|
||
|
|
\fIci\fR increments the number of the latest revision on the trunk.
|
||
|
|
A side branch can only be started by explicitly specifying its
|
||
|
|
number with the \fI\-r\fR option during check-in.
|
||
|
|
.sp 1
|
||
|
|
\fICi\fR also determines
|
||
|
|
whether the revision to be checked in is different from the
|
||
|
|
previous one, and asks whether to proceed if not.
|
||
|
|
This facility simplifies check-in operations for large systems,
|
||
|
|
because one need not remember which files were changed.
|
||
|
|
.sp 1
|
||
|
|
The option \fI\-k\fR searches the checked in file for identification
|
||
|
|
markers containing
|
||
|
|
the attributes
|
||
|
|
revision number, check-in date, author and state, and assigns these
|
||
|
|
to the new revision rather than computing them. This option is
|
||
|
|
useful for software distribution: Recipients of distributed software
|
||
|
|
using RCS should check in updates with the \fI\-k\fR option.
|
||
|
|
This convention guarantees that revision numbers, check-in dates,
|
||
|
|
etc., are the same at all sites.
|
||
|
|
.IP "\fIco\fP \fB\- check out revisions\fP"
|
||
|
|
.sp 0
|
||
|
|
\fICo\fR retrieves revisions according to revision number,
|
||
|
|
date, author and state attributes. It either places the revision
|
||
|
|
into the working file, or prints it on the standard output.
|
||
|
|
\fICo\fR always expands the identification markers.
|
||
|
|
.IP "\fIident\fP \fB\- extract identification markers\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIIdent\fR extracts the identification markers expanded by \fIco\fR
|
||
|
|
from any file and prints them.
|
||
|
|
.IP "\fIrcs\fP \fB\- change RCS file attributes\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRcs\fR is an administrative operation that changes access lists,
|
||
|
|
locks, unlocks, breaks locks, toggles the strict-locking feature,
|
||
|
|
sets state attributes and symbolic revision numbers, changes the
|
||
|
|
description, and deletes revisions. A revision can
|
||
|
|
only be deleted if it is not the fork of a side branch.
|
||
|
|
.br
|
||
|
|
.ne 10
|
||
|
|
.IP "\fIrcsclean\fP \fB\- clean working directory\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRcsclean\fR removes working files that were checked out but never changed.*
|
||
|
|
.FS *
|
||
|
|
The \fIrcsclean\fP and \fIrcsfreeze\fP commands
|
||
|
|
are optional and are not always installed.
|
||
|
|
.FE
|
||
|
|
.IP "\fIrcsdiff\fP \fB\- compare revisions\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRcsdiff\fR compares two revisions and prints their
|
||
|
|
difference, using the UNIX tool \fIdiff\fR.
|
||
|
|
One of the revisions compared may be checked out.
|
||
|
|
This command is useful for finding out about changes.
|
||
|
|
.IP "\fIrcsfreeze\fP \fB\- freeze a configuration\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRcsfreeze\fR assigns the same symbolic revision number
|
||
|
|
to a given revision in all RCS files.
|
||
|
|
This command is useful for accurately recording a configuration.*
|
||
|
|
.IP "\fIrcsmerge\fP \fB\- merge revisions\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRcsmerge\fR merges two revisions, \fIrev1\fR and \fIrev2\fR,
|
||
|
|
with respect to a common ancestor.
|
||
|
|
A 3-way file comparison determines the segments of lines that
|
||
|
|
are (a) the same in all three revisions, or (b) the same in 2 revisions,
|
||
|
|
or (c) different in all three. For all segments of type (b) where
|
||
|
|
\fIrev1\fR is the differing revision,
|
||
|
|
the segment in \fIrev1\fR replaces the corresponding segment of \fIrev2\fR.
|
||
|
|
Type (c) indicates an overlapping change, is flagged as an error, and requires user
|
||
|
|
intervention to select the correct alternative.
|
||
|
|
.IP "\fIrlog\fP \fB\- read log messages\fP"
|
||
|
|
.sp 0
|
||
|
|
\fIRlog\fR prints the log messages and other information in an RCS file.
|
||
|
|
.bp
|
||
|
|
.LP
|
||
|
|
.nr VS 12p
|
||
|
|
.vs 12p
|
||
|
|
.]<
|
||
|
|
.ds [F 1
|
||
|
|
.]-
|
||
|
|
.ds [K FELD02
|
||
|
|
.ds [K MakeArticle
|
||
|
|
.ds [A Feldman, Stuart I.
|
||
|
|
.ds [D March 1979
|
||
|
|
.ds [T Make\*-A Program for Maintaining Computer Programs
|
||
|
|
.ds [J Software\*-Practice & Experience
|
||
|
|
.ds [V 9
|
||
|
|
.ds [N 3
|
||
|
|
.ds [P 255-265
|
||
|
|
.nr [P 1
|
||
|
|
.nr [T 0
|
||
|
|
.nr [A 1
|
||
|
|
.nr [O 0
|
||
|
|
.][ 1 journal-article
|
||
|
|
.ds [F 2
|
||
|
|
.]-
|
||
|
|
.ds [K HUNT01
|
||
|
|
.ds [T An Algorithm for Differential File Comparison
|
||
|
|
.ds [A Hunt, James W.
|
||
|
|
.as [A " and McIlroy, M. D.
|
||
|
|
.ds [I Computing Science Technical Report, Bell Laboratories
|
||
|
|
.ds [R 41
|
||
|
|
.ds [D June 1976
|
||
|
|
.nr [T 0
|
||
|
|
.nr [A 1
|
||
|
|
.nr [O 0
|
||
|
|
.][ 4 tech-report
|
||
|
|
.ds [F 3
|
||
|
|
.]-
|
||
|
|
.ds [K SCCS
|
||
|
|
.ds [A Rochkind, Marc J.
|
||
|
|
.ds [D Dec. 1975
|
||
|
|
.ds [T The Source Code Control System
|
||
|
|
.ds [J IEEE Transactions on Software Engineering
|
||
|
|
.ds [V SE-1
|
||
|
|
.ds [N 4
|
||
|
|
.ds [P 364-370
|
||
|
|
.nr [P 1
|
||
|
|
.nr [T 0
|
||
|
|
.nr [A 1
|
||
|
|
.nr [O 0
|
||
|
|
.][ 1 journal-article
|
||
|
|
.ds [F 4
|
||
|
|
.]-
|
||
|
|
.ds [K TICH08
|
||
|
|
.ds [T Design, Implementation, and Evaluation of a Revision Control System
|
||
|
|
.ds [A Tichy, Walter F.
|
||
|
|
.ds [B Proceedings of the 6th International Conference on Software Engineering
|
||
|
|
.ds [I ACM, IEEE, IPS, NBS
|
||
|
|
.ds [D September 1982
|
||
|
|
.ds [P 58-67
|
||
|
|
.nr [P 1
|
||
|
|
.nr [T 0
|
||
|
|
.nr [A 1
|
||
|
|
.nr [O 0
|
||
|
|
.][ 3 article-in-book
|
||
|
|
.ds [F 5
|
||
|
|
.]-
|
||
|
|
.ds [K LEBL01
|
||
|
|
.ds [A Leblang, David B.
|
||
|
|
.as [A " and Chase, Robert P.
|
||
|
|
.ds [T Computer-Aided Software Engineering in a Distributed Workstation Environment
|
||
|
|
.ds [O Proceedings of the ACM SIGSOFT/SIGPLAN Software Engineering Symposium
|
||
|
|
.as [O " on Practical Software Development Environments.
|
||
|
|
.ds [J SIGPLAN Notices
|
||
|
|
.ds [V 19
|
||
|
|
.ds [N 5
|
||
|
|
.ds [D May 1984
|
||
|
|
.ds [P 104-112
|
||
|
|
.nr [P 1
|
||
|
|
.nr [T 0
|
||
|
|
.nr [A 1
|
||
|
|
.nr [O 0
|
||
|
|
.][ 1 journal-article
|
||
|
|
.ds [F 1
|
||
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.ds [F 3
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.ds [F 6
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.]-
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.ds [K SCCSEval
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.ds [A Glasser, Alan L.
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.ds [D Nov. 1978
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.ds [T The Evolution of a Source Code Control System
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.ds [J Software Engineering Notes
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.ds [V 3
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.ds [N 5
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.ds [P 122-125
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.nr [P 1
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.ds [O Proceedings of the Software Quality and Assurance Workshop.
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.nr [T 0
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.nr [A 1
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.nr [O 1
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.][ 1 journal-article
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.ds [F 5
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.ds [F 7
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.]-
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.ds [K IBMClearCaster
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.ds [A Brown, H.B.
|
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.ds [D 1970
|
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.ds [T The Clear/Caster System
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.ds [J Nato Conference on Software Engineering, Rome
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.nr [T 0
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.nr [A 1
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.nr [O 0
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.][ 1 journal-article
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.ds [F 3
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.ds [F 8
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.]-
|
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.ds [K HabermannSDC
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.ds [A Habermann, A. Nico
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.ds [D Jan. 1979
|
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.ds [T A Software Development Control System
|
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.ds [I Technical Report, Carnegie-Mellon University, Department of Computer Science
|
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.nr [T 0
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.nr [A 0
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.nr [O 0
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.][ 2 book
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.ds [F 9
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.]-
|
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.ds [K CMS
|
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.ds [A DEC
|
||
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.ds [T Code Management System
|
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.ds [I Digital Equipment Corporation
|
||
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.ds [O Document No.\ EA-23134-82
|
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.ds [D 1982
|
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.nr [T 0
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.nr [A 0
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.nr [O 0
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.][ 2 book
|
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.ds [F 10
|
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.]-
|
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.ds [K LAMP01
|
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.ds [A Lampson, Butler W.
|
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.as [A " and Schmidt, Eric E.
|
||
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.ds [T Practical Use of a Polymorphic Applicative Language
|
||
|
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.ds [B Proceedings of the 10th Symposium on Principles of Programming Languages
|
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.ds [I ACM
|
||
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.ds [P 237-255
|
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.nr [P 1
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.ds [D January 1983
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.nr [T 0
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.nr [A 1
|
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.nr [O 0
|
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.][ 3 article-in-book
|
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.ds [F 5
|
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.ds [F 11
|
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.]-
|
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.ds [K TICH07
|
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.ds [T A Data Model for Programming Support Environments and its Application
|
||
|
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.ds [A Tichy, Walter F.
|
||
|
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.ds [B Automated Tools for Information System Design and Development
|
||
|
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.ds [E Hans-Jochen Schneider and Anthony I. Wasserman
|
||
|
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.ds [C Amsterdam
|
||
|
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.ds [I North-Holland Publishing Company
|
||
|
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.ds [D 1982
|
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.nr [T 0
|
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.nr [A 1
|
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.nr [O 0
|
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.][ 3 article-in-book
|
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.ds [F 4
|
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.ds [F 2
|
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.ds [F 12
|
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.]-
|
||
|
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.ds [K HECK01
|
||
|
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.ds [T A Technique for Isolating Differences Between Files
|
||
|
|
.ds [A Heckel, Paul
|
||
|
|
.ds [J Communications of the ACM
|
||
|
|
.ds [D April 1978
|
||
|
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.ds [V 21
|
||
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.ds [N 4
|
||
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.ds [P 264-268
|
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.nr [P 1
|
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.nr [T 0
|
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.nr [A 0
|
||
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.nr [O 0
|
||
|
|
.][ 1 journal-article
|
||
|
|
.ds [F 13
|
||
|
|
.]-
|
||
|
|
.ds [K TICH11
|
||
|
|
.ds [T The String-to-String Correction Problem with Block Moves
|
||
|
|
.ds [A Tichy, Walter F.
|
||
|
|
.ds [D Nov. 1984
|
||
|
|
.ds [J ACM Transactions on Computer Systems
|
||
|
|
.ds [V 2
|
||
|
|
.ds [N 4
|
||
|
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.ds [P 309-321
|
||
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.nr [P 1
|
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.nr [T 0
|
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.nr [A 1
|
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.nr [O 0
|
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.][ 1 journal-article
|
||
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|
.]>
|