@c
@c ================================================================================
@c                               Edition 1.1
@c                      of the Texinfo-manuals for the
@c                      (R)evision (C)ontrol (S)ystem
@c                               Version 5.7
@c
@c                  (c) 1982, 1988, 1989 Walter F. Tichy.
@c           (c) 1990, 1991, 1992, 1993, 1994, 1995 Paul Eggert.
@c        (c) 1996, 1997 Karl Heinz Marbaise (doing converting job)
@c ================================================================================
@c
@c Discription:
@c    A story about RCS and version control
@c
@c Authors:
@c    Walter Tichy,
@c    Paul Eggert,
@c    Karl Heinz Marbaise (doing converting job)
@c
@c e-mail:
@c    Internet: KHMarbaise@p69.ks.fido.de
@c    Fido-net: 2:2452/117.69
@c
@c Bugs, question:
@c    to above e-mail adress.
@c
@c License:
@c    The "Texinfo Edition of the RCS V5.7 manuals" are free
@c    software; you can redistribute it and/or modify it under
@c    the terms of the GNU General Public License as published
@c    by the Free Software Foundation; either version 2, or (at
@c    your option) any later version.
@c
@c    The "Texinfo Edition of the RCS V5.7 manuals" are distributed
@c    in the hope that they will be useful, but WITHOUT ANY WARRANTY;
@c    without even the implied warranty of MERCHANTABILITY or
@c    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
@c    License for more details.
@c
@c    You should have received a copy of the GNU General Public License
@c    along with the "Texinfo Edition of the RCS V5.7 manuals"; see the
@c    file COPYING. If not, write to the:
@c    Free Software Foundation,
@c    59 Temple Place - Suite 330,
@c    Boston, MA 02111-1307, USA.
@c
@c    See \rcstxi.110\COPYING for details.
@c
@c ================================================================================
@c
@c
@c $Id: RCS_DOC.TEX 1.2 1997/03/30 22:53:19 KHM Exp $
@c
@c =============================================================================
@c RCS -- A System for Version Control
@c -----------------------------------------------------------------------------
@node VersionControl,rcsIntroduction,About,Top
@chapter RCS--A System for Version Control

@center Walter F. Tichy
@center Department of Computer Sciences
@center Purdue University
@center West Lafayette, Indiana 47907

An important problem in program development and maintenance is
version control, i.e., the task of keeping a software system
consisting of many versions and configurations well organized.
The Revision Control System (RCS) is a software tool that assists
with that task. RCS manages revisions of text documents, in
particular source programs, documentation, and test data. It
automates the storing, retrieval, logging and identification of
revisions, and it provides selection mechanisms for composing
configurations. This paper introduces basic version control
concepts and discusses the practice of version control using RCS.
For conserving space, RCS stores deltas, i.e., differences
between successive revisions.  Several delta storage methods are
discussed. Usage statistics show that RCS's delta storage method
is space and time efficient. The paper concludes with a detailed
survey of version control tools.

Keywords: configuration management, history management,
version control, revisions, deltas.

An earlier version of this paper was published in
@cite{Software--Practice & Experience, 7 (July 1985), 637-654.}

@ifinfo
@menu
* Introduction::        Introduction to RCS.
* StartRCS::            Getting started with RCS.
* Identification::      Automatic Identification.
* RevisionTree::        The RCS Revision Tree.
* Branches::            When are branches needed?.
* Deltas::              Revisions are represented as deltas.
* Controversial::       Locking: A Controversial Issue.
* Configuration::       Configuration Management.
* Functions::           RCS Selection Functions.
* MAKERCS::             Combining MAKE and RCS.
* Statistics::          Usage Statistics.
* Survey::              Survey of Version Control Tools.
@end menu
@end ifinfo

@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Introduction
@c -----------------------------------------------------------------------------
@node Introduction,StartRCS,,VersionControl
@section Introduction
Version control is the task of keeping software systems
consisting of many versions and configurations well organized.
The Revision Control System (RCS) is a set of UNIX commands that
assist with that task.

RCS' primary function is to manage @b{revision groups}. A
revision group is a set of text documents, called @b{revisions},
that evolved from each other. A new revision is created by
manually editing an existing one. RCS organizes the revisions
into an ancestral tree.  The initial revision is the root of the
tree, and the tree edges indicate from which revision a given one
evolved. Besides managing individual revision groups, RCS
provides flexible selection functions for composing
configurations. RCS may be combined with MAKE (@xref{Feldman}),
resulting in a powerful package for version control.

RCS also offers facilities for merging updates with customer
modifications, for distributed software development, and for
automatic identification. Identification is the @file{stamping}
of revisions and configurations with unique markers. These
markers are akin to serial numbers, telling software maintainers
unambiguously which configuration is before them.

RCS is designed for both production and experimental
environments. In production environments, access controls detect
update conflicts and prevent overlapping changes. In experimental
environments, where strong controls are counterproductive, it is
possible to loosen the controls.

Although RCS was originally intended for programs, it is useful
for any text that is revised frequently and whose previous
revisions must be preserved.  RCS has been applied successfully
to store the source text for drawings, VLSI layouts,
documentation, specifications, test data, form letters and
articles.

This paper discusses the practice of version control using RCS.
It also introduces basic version control concepts, useful for
clarifying current practice and designing similar systems.
Revision groups of individual components are treated in the next
three sections, and the extensions to configurations follow.
Because of its size, a survey of version control tools appears at
the end of the paper.

@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Getting started with RCS
@c -----------------------------------------------------------------------------
@node StartRCS,Identification,Introduction,VersionControl
@section Getting started with RCS
Suppose a text file @file{f.c} is to be placed under control of
RCS. Invoking the check-in command


@example
ci  f.c
@end example


creates a new revision group with the contents of @file{f.c} as
the initial revision (numbered 1.1) and stores the group into the
file @file{f.c,v}. Unless told otherwise, the command deletes
@file{f.c}. It also asks for a description of the group. The
description should state the common purpose of all revisions in
the group, and becomes part of the group's documentation. All
later check-in commands will ask for a log entry, which should
summarize the changes made. (The first revision is assigned a
default log message, which just records the fact that it is the
initial revision.)

Files ending in @file{,v} are called @b{RCS files}
(@b{v} stands for @b{Rersions}); the others are called working
files. To get back the working file @file{f.c} in the previous
example, execute the check-out command:


@example
co  f.c
@end example


This command extracts the latest revision from the revision group
@file{f.c,v} and writes it into @file{f.c}. The file @file{f.c}
can now be edited and, when finished, checked back in with
@code{ci}:


@example
ci  f.c
@end example


@code{Ci} assigns number 1.2 to the new revision.
If @code{ci} complains with the message


@example
ci error: no lock set by <login>
@end example


then the system administrator has decided to configure RCS for a
production environment by enabling the @file{strict locking
feature}. If this feature is enabled, all RCS files are
initialized such that check-in operations require a lock on the
previous revision (the one from which the current one evolved).
Locking prevents overlapping modifications if several people work
on the same file. If locking is required, the revision should
have been locked during the check-out by using the option
@code{-l}:


@example
co -l  f.c
@end example


Of course it is too late now for the check-out with locking,
because @file{f.c} has already been changed; checking out the
file again would overwrite the modifications. (To prevent
accidental overwrites, @code{co} senses the presence of a working
file and asks whether the user really intended to overwrite it.
The overwriting check-out is sometimes useful for backing up to
the previous revision.) To be able to proceed with the check-in
in the present case, first execute


@example
rcs  -l  f.c
@end example


command retroactively locks the latest revision, unless someone
else locked it in the meantime.  In this case, the two
programmers involved have to negotiate whose modifications should
take precedence.


If an RCS file is private, i.e., if only the
owner of the file is expected to deposit revisions into it, the
strict locking feature is unnecessary and may be disabled. If
strict locking is disabled, the owner of the RCS file need not
have a lock for check-in. For safety reasons, all others still
do.  Turning strict locking off and on is done with the commands:


@example
rcs  -U  f.c   and rcs  -L  f.c
@end example


These commands enable or disable the strict locking feature for each
RCS file individually. The system administrator only decides
whether strict locking is enabled initially.


To reduce the clutter in a working directory, all RCS files can
be moved to a subdirectory with the name @code{RCS}. RCS commands
look first into that directory for RCS files. All the commands
presented above work with the @code{RCS}
subdirectory@footnote{Pairs of RCS and working files can actually
be specified in 3 ways: a) both are given, b) only the working
file is given, c) only the RCS file is given. If a pair is given,
both files may have arbitrary path prefixes; RCS commands pair
them up intelligently}. without change.


It may be undesirable that @code{ci} deletes the working file.
For instance, sometimes one would like
to save the current revision, but continue editing. Invoking


@example
ci  -l  f.c
@end example


checks in @file{f.c} as usual, but performs an additional
check-out with locking afterwards.  Thus, the working file does
not disappear after the check-in. Similarly, the option @code{-u}
does a check-in followed by a check-out without locking.  This
option is useful if the file is needed for compilation after the
check-in. Both options update the identification markers in the
working file (see below).


Besides the operations @code{ci} and @code{co}, RCS provides the
following commands:


@table @code
@item ident
extract identification markers

@item rcs
change RCS file attributes

@item rcsclean
remove unchanged working files (optional)

@item rcsdiff
compare revisions

@item rcsfreeze
record a configuration (optional)

@item rcsmerge
merge revisions

@item rlog
read log messages and other information in RCS files

@end table


A synopsis of these commands appears in the Appendix.


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Getting started with RCS
@c         Automatic Indentification
@c -----------------------------------------------------------------------------
@node Identification,RevisionTree,StartRCS,VersionControl
@subsection Automatic Identification

RCS can stamp source and object code with special identification
strings, similar to product and serial numbers. To obtain such
identification, place the marker


@example
@value{RCSID}
@end example


into the text of a revision, for instance inside a comment.
The check-out operation will replace this marker with a string of the form


@example
@value{RCSD}Id:  filename  revisionnumber  date  time  author state  locker @value{RCSD}
@end example


This string need never be touched, because @code{co} keeps it
up to date automatically. To propagate the marker into object code,
simply put it into a literal character string.
In C, this is done as follows:


@example
static char rcsid[] = "@value{RCSID}";
@end example


The command @code{ident} extracts such markers from any file,
in particular from object code.
@code{Ident} helps to find out which revisions of which modules
were used in a given program. It returns a complete and unambiguous
component list, from which a copy of the program can be reconstructed.
This facility is invaluable for program maintenance.

There are several additional identification markers, one for each component
of @value{RCSID}. The marker


@example
@value{RCSLOG}
@end example


has a similar function.  It accumulates
the log messages that are requested during check-in.
Thus, one can maintain the complete history of a revision directly inside it,
by enclosing it in a comment.
Figure 1 is an edited version of a log contained in revision 4.1 of
the file @file{ci.c}.  The log appears at the beginning of the file,
and makes it easy to determine what the recent modifications were.


@example
/*
 * @value{RCSD}Log: ci.c,v @value{RCSD}
 * Revision 4.1  1983/05/10 17:03:06  wft
 * Added option -d and -w, and updated assignment of date, etc.
 * to new delta. Added handling of default branches.
 *
 * Revision 3.9  1983/02/15 15:25:44  wft
 * Added call to fastcopy() to copy remainder of RCS file.
 *
 * Revision 3.8  1983/01/14 15:34:05  wft
 * Added ignoring of interrupts while new RCS file is renamed;
 * avoids deletion of RCS files by interrupts.
 *
 * Revision 3.7  1982/12/10 16:09:20  wft
 * Corrected checking of return code from diff.
 * An RCS file now inherits its mode during the first ci from the
 * working file, except that write permission is removed.
 */
@end example
@center Figure 1.  Log entries produced by the marker @value{RCSLOG}


Since revisions are stored in the form of differences, each log
message is physically stored once, independent of the number of
revisions present. Thus, the @value{RCSLOG} marker incurs
negligible space overhead.

@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     The RCS Revision Tree
@c -----------------------------------------------------------------------------
@node RevisionTree,Branches,Identification,VersionControl
@section The RCS Revision Tree

RCS arranges revisions in an ancestral tree. The @code{ci}
command builds this tree; the auxiliary command @code{rcs} prunes
it. The tree has a root revision, normally numbered 1.1, and
successive revisions are numbered 1.2, 1.3, etc.  The first field
of a revision number is called the @code{release number} and the
second one the @code{level number}.  Unless given explicitly, the
@code{ci} command assigns a new revision number by incrementing
the level number of the previous revision. The release number
must be incremented explicitly, using the @code{-r} option of
@code{ci}. Assuming there are revisions 1.1, 1.2, and 1.3 in the
RCS file @file{f.c,v}, the command


@example
ci  -r2.1  f.c       or       ci  -r2  f.c
@end example


assigns the number 2.1 to the new revision. Later check-ins
without the @code{-r} option will assign the numbers 2.2, 2.3,
and so on. The release number should be incremented only at major
transition points in the development, for instance when a new
release of a software product has been completed.


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     The RCS Revision Tree
@c         When are branches needed?
@c -----------------------------------------------------------------------------
@node Branches,Deltas,RevisionTree,VersionControl
@subsection When are branches needed?
A young revision tree is slender: It consists of only one branch,
called the trunk. As the tree ages, side branches may form.
Branches are needed in the following 4 situations.

@enumerate
@item Temporary fixes

Suppose a tree has 5 revisions grouped in 2 releases, as
illustrated in Figure 2. Revision 1.3, the last one of release 1,
is in operation at customer sites, while release 2 is in active
development.
@*
@example
@group
@w{+-----+     +-----+     +-----+     +-----+     +-----+}
@w{! 1.1 !---->! 1.2 !---->! 1.3 !---->! 2.1 !---->! 2.2 !--->>}
@w{+-----+     +-----+     +-----+     +-----+     +-----+}
@w{}
@end group
@end example
@center Figure 2.  A slender revision tree.
@*
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:
@*
@*
@example
@w{co -r1.3 f.c     -- check out revision 1.3}
@w{edit f.c         -- change it}
@w{ci  -r1.3.1  f.c -- check it in on branch 1.3.1}
@end example
@*
@*
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 @code{rcsmerge} automates this process (see the
Appendix).
@*
@example
@group
@w{+-----+     +-----+     +-----+     +-----+     +-----+}
@w{! 1.1 !---->! 1.2 !---->! 1.3 !---->! 2.1 !---->! 2.2 !--->>}
@w{+-----+     +-----+     +--+--+     +-----+     +-----+}
@w{                           !}
@w{                           !        +---------+}
@w{                           +------->! 1.3.1.1 !---->>}
@w{                                    +---------+}
@end group
@end example
@center Figure 3.  A revision tree with one side branch
@*
@item Distributed development and customer modifications

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.
@*
@example
@group
@w{    +-----+                 +-----+}
@w{--->! 1.3 !---------------->! 2.4 !---->>}
@w{    +--+--+                 +---+-+}
@w{       !                        !}
@w{       !     +---------+        !       +---------+}
@w{       +---->! 1.3.1.1 !        +------>! 2.4.1.1 !}
@w{             +---------+                +---------+}
@end group
@end example
@center Figure 4.  A customer's revision tree with local modifications.
@*
This approach is actually practiced in the CSNET project, where
several universities and a company cooperate in developing a
national computer network.


@item Parallel development


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.

@item Conflicting updates


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.


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.

@end enumerate


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     The RCS Revision Tree
@c         Revisions are represented as deltas
@c -----------------------------------------------------------------------------
@node Deltas,Controversial,Branches,VersionControl
@subsection Revisions are represented as deltas
For conserving space, RCS stores revisions in the form of deltas,
i.e., as differences between revisions. The user interface
completely hides this fact.


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
@code{diff} 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.


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: @code{ci} simply
adds the new revision intact, replaces the previous revision with
a reverse delta, and keeps the rest of the old deltas. Thus,
@code{ci} requires the computation of only one new delta.


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
@code{forward} 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(with five exclamation marks) represent
reverse and forward deltas,
respectively.
@*
@example
@group
@w{      !           !           !           !           !}
@w{+-----!     +-----!     +-----!     +-----!     +-----!}
@w{! 1.1 !---->! 1.2 !---->! 1.3 !---->! 2.1 !---->! 2.2 !--->>}
@w{+-----+     +-----!     +--+--!     +-----!     +-----!}
@w{      !           !        !  !           !           !}
@w{                           !}
@w{                           !            !               !}
@w{                           !  +---------!     +---------!}
@w{                           +->! 1.3.1.1 !---->! 1.3.1.2 !}
@w{                              +---------!     +---------!}
@w{                                        !               !}
@end group
@end example
@center Figure 3.  A revision tree with one side branch
@*

@c
@c This part have to be "translated" into TeXinfo or plain ASCII
@c
@c .ne 8
@c .PS 4i
@c .ps -2
@c define BD X [line invis $1 right .5;
@c line up .3 then left .5 down .3 then right .5 down .3 then up .3] X
@c
@c define FD X [line invis $1 right .5;
@c line left .5 down .3 then up .6 then right .5 down .3;] X
@c
@c right
@c D11:    BD(" 1.1")
@c         arrow right from D11.e
@c D12:    BD(" 1.2")
@c         arrow right from D12.e
@c D13:    BD(" 1.3")
@c         arrow right from D13.e
@c D21:    BD(" 2.1")
@c         arrow right from D21.e
@c D22:    box "2.2"
@c         line invis down from D21.s
@c F1:     FD("1.3.1.1 ")
@c         arrow from D13.se to F1.w
@c         arrow from F1.e right
@c         right
@c F2:     FD("1.3.1.2 ")
@c .ps +2
@c .PE
@c .ce 1
@c Figure 5.  A revision tree with reverse and forward deltas.
@c .sp 0
@c

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.


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 @code{ed} 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 @code{ed}.  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.


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 @code{piece table}. A piece table is a one-dimensional
array, specifying how a given revision is @file{pieced-together}
from lines in the RCS file. Suppose piece table @code{PTr}
represents revision @code{r}. Then @code{PTr[i]} contains the
starting position of line @code{i} of revision @code{r}.
Application of the next delta transforms piece table @code{PTr}
into @code{PTr+1}.  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.

@c
@c This part have to be "translated" into TeXinfo or plain ASCII
@c
@c .ne 13
@c .sp 6
@c .ce 1
@c \fIFigure 6 is not available.\fP
@c .sp 5
@c .ce 1
@c Figure 6.  An RCS file and its piece tables
@c .sp 0
@c


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.)


There is an important alternative for representing deltas that
affects performance.  @code{SCCS}, a precursor of RCS, uses
@code{interleaved} 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
@code{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.


Additional speed-up for both delta methods can be obtained by
caching the most recently generated revision, as has been
implemented in @code{DSEE} With caching, access time to
frequently used revisions can approach normal file access time,
at the cost of some additional space.

@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Locking: A Controlversial Issue
@c -----------------------------------------------------------------------------
@node Controversial,Configuration,Deltas,VersionControl
@section Locking: A Controversial Issue

The locking mechanism for RCS was difficult to design. The
problem and its solution are first presented in their @file{pure}
form, followed by a discussion of the complications caused by
@file{real-world} considerations.


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{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@footnote{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.}.


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
@code{-l} option on @code{co}.  On subsequent check-in, @code{ci}
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.


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:

@itemize @minus
@item find out who is holding the lock and ask that person to release it;

@item check out the locked revision, modify it, check it
in on a branch, and merge the changes later;

@item 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.

@end itemize


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
@file{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.


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 @code{co}.  This approach is consistent with the UNIX
philosophy of openness, which contributes to a productive
software development environment.


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Configuration Management
@c -----------------------------------------------------------------------------
@node Configuration,Functions,Controversial,VersionControl
@section Configuration Management

The preceding sections described how @code{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. @code{RCS}, in conjunction with
@code{MAKE}, provides a number of facilities to effect a smooth
selection.

@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Configuration Management
@c         RCS Selection Functions
@c -----------------------------------------------------------------------------
@node Functions,MAKERCS,Configuration,VersionControl
@subsection RCS Selection Functions

@itemize @minus
@item Default selection

During development, the usual selection criterion is to choose
the latest revision of all components.  The @code{co} command
makes this selection by default.  For example, the command


@example
co  *,v
@end example


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.

@item Release based selection

Specifying a release or branch number selects the latest revision
in that release or branch. For instance,


@example
co  -r2  *,v
@end example


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.

@item State and author based selection

If the highest level number within a given release number is not
the desired one, the state attribute can help.  For example,


@example
co  -r2  -sReleased  *,v
@end example


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 @code{ci} or @code{rcs} commands.
Another alternative is to select a revision by its author, using
the @code{-w} option.

@item Date based selection

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


@example
co  -d'March 4, 1:00 pm LT'  *,v
@end example


checks out all the components of that release, independent of the
numbering. The @code{-d} 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.


@item Name based selection

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
@code{co} 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 @code{V3} is
bound to release number 2 in file @file{s,v}, and to revision
number 15.9 in @file{t,v}. Then the single command


@example
co  -rV3  s,v  t,v
@end example


retrieves the latest revision of release 2 from @file{s,v}, and
revision 15.9 from @file{t,v}. In a large system with many
modules, checking out all revisions with one command greatly
simplifies configuration management.

Judicious use of symbolic revision numbers helps with organizing
large configurations.


A special command, @code{rcsfreeze}, assigns a symbolic revision
number to a selected revision in every RCS file. @code{rcsfreeze}
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, @code{V3.5} in the above example would select
revision 2.5 in @file{s,v} and branch 15.9.5 in @file{t,v}.


The options @code{-r}, @code{-s}, @code{-w} and @code{-d} 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.

@end itemize


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Configuration Management
@c         Combining MAKE and RCS
@c -----------------------------------------------------------------------------
@node MAKERCS,Statistics,Functions,VersionControl
@subsection Combining MAKE and RCS

MAKE (@ref{Feldman}) 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.


MAKE has been extended with an auto-checkout
feature@footnote{This auto-checkout extension is available only
in some versions of MAKE, e.g. GNU MAKE.} for RCS.* 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
@code{RCS} 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.
@code{Rcsclean} can be used for that purpose.)


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.


Each programmer chooses a working directory and places into it a
symbolic link, named @code{RCS}, to the directory containing the
relevant RCS files. The symbolic link makes sure that @code{co}
and @code{ci} 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 @code{RCS}. 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.


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.


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
@code{rcsfreeze}, 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.


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Configuration Management
@c         Usage Statistics
@c -----------------------------------------------------------------------------
@node Statistics,Survey,MAKERCS,VersionControl
@section Usage Statistics

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.


For the first experiment, the @code{ci} and @code{co} 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.


@example
@group
@w{Oper.  !   Total  !Total deltas!mean deltas! Operations  !Branch }
@w{       !operations!   applied  !  applied  !with >1 delta!operations}
@w{-------+----------+------------+-----------+-------------+----------}
@w{co     !     7867 !    9320    !    1.18   !   509 (6%)  ! 203 (3%)}
@w{ci     !     3468 !    2207    !    0.64   !    85 (2%)  !  75 (2%)}
@w{ci & co!    11335 !   11527    !    1.02   !   594 (5%)  ! 278 (2%)}
@w{ }
@w{ }
@center Table I.  Statistics for @code{co} and @code{ci} operations
@end group
@end example
@*
@*

The first two lines show statistics for check-out and check-in;
the third line shows the combination. Recall that @code{ci}
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.


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 @code{ci},
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 @code{ci} 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.


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 @code{co}.
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.)

@smallexample
@group
@w{          !Total RCS!  Total  !Mean     !Means size!Mean size!Overhead}
@w{          ! files   !revisions!revisions!RCS files !revisions!}
@w{----------+---------+---------+---------+----------+---------+--------}
@w{All Files !    8033 !   11133 !   1.39  !   6156   !  5585   !  1.10}
@w{Files with!    1477 !    4578 !   3.10  !   8074   !  6041   !  1.34}
@w{>= 2 delta!         !         !         !          !         !}
@w{ }
@w{ }
@center Table II. Statistics for RCS files
@end group
@end smallexample
@*
@*
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.


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(@ref{Rochkind}) 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 (@ref{Glasser}) observed an
average of 7 revisions per group in a single, large project, but
provided no overhead figure. In his paper on DSEE , Leblang
(@ref{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.


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.


@c -----------------------------------------------------------------------------
@c RCS -- A System for Version Control
@c     Survey of Version Control Tools
@c -----------------------------------------------------------------------------
@node Survey,,Statistics,VersionControl
@section Survey of Version Control Tools
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.


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.


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.


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 @code{ci}.) 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 (@ref{Brown}),
AT&T's SCCS (@ref{Rochkind}), CMU's SDC (@ref{Habermann}), and
DEC's CMS (@ref{DEC}), 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.


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 (@ref{Lampson})
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 (@ref{Leblang}) 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.


RCS is based on a general model for describing
multi-version/multi-configuration systems (@ref{Tichy1}). 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.


An important component of all revision control systems is a
program for computing deltas. SCCS and RCS use the program
@code{diff} (@ref{Rochkind}), 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.


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 (@ref{Heckel}) 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.
@example
@group
@code{Acknowledgements}:@*
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.
@end group
@end example

