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| Newer page: | version 18 | Last edited on Monday, June 22, 2009 8:25:30 pm | by LawrenceDoliveiro | Revert |
| Older page: | version 11 | Last edited on Saturday, June 20, 2009 3:15:45 am | by AristotlePagaltzis | Revert |
@@ -8,29 +8,35 @@
* GnuArch
* [RCS]
* SubVersion
-See EricRaymond's essay ''[Understanding Version Control|http://www.catb.org/~esr/writings/version-control/version-control.html]'' for an introduction to the main concepts and a comparison of the major systems. The main stages of development can be summarized as follows:
+See EricRaymond's essay ''[Understanding Version Control|http://www.catb.org/~esr/writings/version-control/version-control.html]'' for an introduction to the main concepts and a comparison of the major systems.
+
+!!! Evolutionary stages of Version Control
+
+
The main stages of development can be summarized as follows:
# The design of the earliest systems revolved around versioning a single working copy, directly edited by all users. To prevent attempts at simultaneous modification of a single file, editing was not allowed without checking files out, which only one user at a time could do for any given file.
- Having to give each user access to the same machine and FileSystem in order to work on code was natural at the time these systems were design
, in the mainframe era, but today would obviously be a problem. Also, the requirement to check files out causes
a lot
of friction,
even in normal operations
, since everyone has to wait on one another – not to mention that someone might forget to check a file back in before leaving on vacation.
+ Having to give each user access to the same machine and FileSystem in order to work on code was natural at the time these systems were designed
, in the mainframe era, but today would obviously be a problem. Also, the requirement to check files out was
a cause
of friction even at the time
, since everyone has to wait on one another – not to mention that someone might forget to check a file back in before leaving on vacation.
+
+# The next evolutionary step was to decouple the repository from the working copy, so that there may then be many working copies. The exemplar in this class of systems, known as centralised [VCS]s, is [CVS]. It lifts the obvious restrictions of earlier systems with a design in which the repository is mediated by a server. Multiple users can collaborate by each checking out a private working copy of the project
.
-# The next evolutionary step was to decouple the repository from the working copy, of which there may then be many. The exemplar in this class of systems, known as centralised [VCS]s, is [CVS]. It lifts the obvious restrictions of earlier systems with a design in which the repository is mediated by a server. Multiple users can collaborate by
checking out their own private working copy of the versioned tree each, but this
no longer implies a
locking. Checking in changes is simply blocked if someone else has already checked in other changes in the meantime. The latecomer has to manually incorporate
the upstream changes before being
allowed to check in their own changes.
+ Note that “
checking out”
no longer implies locking. Checking in changes is simply blocked if someone else has already checked in other changes in the meantime. The latecomer simply
has to update their working copy with
the upstream changes, resolving any conflicts manually,
before they are
allowed to check in their own changes.
This works reasonably well, so [CVS] was the de facto standard for a decade. However, its single-repository nature, subsequently adopted by most following major systems, perpetuates problems harking back to the earlier model and adds new ones.
- Checking
in changes under such a system requires a network connection, as do most operations related to the project history;
networked operations are inevitably
slow. (Systems
like SubVersion tries
to selectively speed up some of these operations)
.
+ Firstly, checking
in changes under such a system requires a network connection, as do most operations related to the project history. Besides the fact that this makes offline work nearly impossible, it also imposes a major performance penalty, since
networked operations are inescapably
slow. Some systems,
like SubVersion, try
to selectively speed up some of these operations by keeping more data in the working copy, but the benefit of this is uneven across operations. Further, high traffic repositories may require rather beefy servers and connections to sustain
.
- Anything checked is
is always public; this means one has to be very careful about the state of commits and
makes it impossible to touch up history (eg. to fix common mistakes like forgetting to include a new file in a commit). This also makes branches
a big deal;
no matter how experimental, the commits have to be published
. Also, branch names are forced into a global namespace, and every experiment ends up getting published visibly
.
+ Anything checked in
is always public; this means one has to be very careful about the state of commits. It also
makes it impossible to touch up history (eg. to fix common mistakes like forgetting to include a new file in a commit). Branches become
a big deal: all commits are publicly visible,
no matter how experimental. Also, branch names are forced into a global namespace so a lot of thought has to be given to choosing them
.
- Branching is problematic for more reasons too. Most of these systems do not support merging branches
very well: after you do it once, the changes from the merged-in branch are mixed in without any tracking, so if you
later attempts to merge the same branch will result in lots of conflicts. This makes it very difficult to keep branches in synch; but
the more
branches fall out of synch
, the more effort it takes to merge them. All this adds up to a large barrier, psychological and otherwise, against branching.
+ Branching is problematic for more reasons too. Most of these systems do not support branch
merging very well: after you do it once, the changes from the merged-in branch are mixed in without any tracking, so if later attempts to merge the same branch will result in lots of artificial
conflicts. This makes it very difficult to keep branches in synch. But
the longer
branches go without merging
, the more effort it takes to merge them. All this adds up to a large barrier, psychological and otherwise, against branching.
- Finally, the single-repository nature means that anyone who wants the safety of revision control needs to have write access to the same repository. And since branching is badly supported, everyone with access to the repository is generally going to be working on the same trunk. This means write access has to be given out selectively to competent, resulting in political headaches within projects, while outsiders are forced to create their patches in an unversioned ghetto.
+ Finally, the single-repository nature means that anyone who wants the safety of revision control needs to have write access to the same repository. And since branching is badly supported, everyone with access to the repository is generally going to be working on the same trunk. This means write access has to be given out selectively,
to competent people only
, resulting in political headaches within projects, while outsiders are forced to create their patches in an unversioned ghetto.
# The solution to all this was to not only give each collaborator a separate working copy, but a separate repository also. This class of system is known as DistributedVersionControlSystem~s. The technical basis that allows this is algorithmic merging: 3-way merging allows combining non-overlapping changes automatically, and merge point tracking allows repeatedly merging branches without unnecessary conflicts.
- Since each collaborator has their own repository and can make commits, the effect is that everyone has their own private branch, with full versioning for local changes, and these branches can easily
published and merged. Actually, each collaborator often has several local branches – since merging is easy and branches never ''need'' be published, it is easy
to create short-lived branches for experiments or tests, to use them as a general workflow aspect (eg. start a new branch for every separate bug fix), or for any other purpose, whether intended for public consumption or not.
+ Since each collaborator has their own repository and can make commits, the effect is that everyone has their own private branch, with full versioning for local changes, and these branches can be
published at the discretion of their author
and can be
merged by others easily
. Actually, each collaborator often has several local branches – since merging is easy and branches never ''need'' be published, it is painless
to create short-lived branches for experiments or tests, to use them as a general workflow aspect (eg. start a new branch for every separate bug fix), or for any other purpose, whether intended for public consumption or not.
Everyone has full offline access to the project history, and all repository operations (except pushing or pulling changes, obviously) take place at full local disk speed.
All this immensely accelerates collaborative development and removes the political headaches surrounding commit access.
