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Version Control with Subversion
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Version Control with Subversion - Locking

Locking

Subversion's “copy-modify-merge” model is optimal when users are collaborating on projects that consist of line-based text files, such as program source code. However, as discussed in When Locking is Necessary, sometimes one has to use the “lock-modify-unlock” model instead of Subversion's standard concurrent model. When a file consists of binary data, it's often difficult or impossible to merge two sets of changes made in parallel by different users. For this reason, Subversion 1.2 and later offers a feature known as locking, often known as “reserved checkouts” in other version control systems.

Subversion's locking feature has two main goals:

  • Serializing access to a resource . Allow a user to grab an exclusive right to change to a file in the repository. If Harry reserves the right to change foo.jpg, then Sally should not be able to commit a change to it.

  • Aiding communication . Prevent users from wasting time on unmergeable changes. If Harry has reserved the right to change foo.jpg, then it should be easy for Sally to notice this fact and avoid working on the file.

Subversion's locking feature is currently limited to files only—it's not yet possible to reserve access to a whole directory tree.

Creating locks

In the Subversion repository, a lock is a piece of metadata which grants exclusive access to one user to change a file. This user is said to be the lock owner. Each lock also has a unique identifier, typically a long string of characters, known as the lock token. The repository manages locks in a separate table, and enforces locks during a commit operation. If any commit transaction attempts to modify or delete the file (or delete a parent of the file), the repository will demand two pieces of information:

  1. User authentication . The client performing the commit must be authenticated as the lock owner.

  2. Software authorization . The user's working copy must send the lock token with the commit, proving that it knows exactly which lock it's using.

An example is in order, to demonstrate. Let's say that Harry has decided to change a JPEG image. To prevent other people from committing changes to the file, he locks the file in the repository using the svn lock command:

$ svn lock banana.jpg --message "Editing file for tomorrow's release."
'banana.jpg' locked by user 'harry'.

$ svn status
     K banana.jpg

$ svn info banana.jpg
Path: banana.jpg
Name: banana.jpg
URL: https://svn.example.com/repos/project/banana.jpg
Repository UUID: edb2f264-5ef2-0310-a47a-87b0ce17a8ec
Revision: 2198
Node Kind: file
Schedule: normal
Last Changed Author: frank
Last Changed Rev: 1950
Last Changed Date: 2005-03-15 12:43:04 -0600 (Tue, 15 Mar 2005)
Text Last Updated: 2005-06-08 19:23:07 -0500 (Wed, 08 Jun 2005)
Properties Last Updated: 2005-06-08 19:23:07 -0500 (Wed, 08 Jun 2005)
Checksum: 3b110d3b10638f5d1f4fe0f436a5a2a5
Lock Token: opaquelocktoken:0c0f600b-88f9-0310-9e48-355b44d4a58e
Lock Owner: harry
Lock Created: 2005-06-14 17:20:31 -0500 (Tue, 14 Jun 2005)
Lock Comment (1 line):
Editing file for tomorrow's release.

There are a number of new things demonstrated in the previous example. First, notice that Harry passed the --message option to svn lock . Similar to svn commit , the svn lock command can take comments (either via --message (-m) or --file (-F)) to describe the reason for locking the file. Unlike svn commit , however, svn lock will not demand a message by launching your preferred text editor. Lock comments are optional, but still recommended to aid communication.

Second, the lock attempt succeeded. This means that the file wasn't already locked, and that Harry had the latest version of the file. If Harry's working copy of the file had been out-of-date, the repository would have rejected the request, forcing harry to svn update and reattempt the locking command.

Also notice that after creating the lock in the repository, the working copy has cached information about the lock—most importantly, the lock token. The presence of the lock token is critical. It gives the working copy authorization to make use of the lock later on. The svn status command shows a K next to the file (short for locKed), indicating that the lock token is present.

Now that Harry has locked banana.jpg, Sally is unable to change or delete that file:

$ whoami
sally

$ svn delete banana.jpg
D         banana.jpg

$ svn commit -m "Delete useless file."
Deleting       banana.jpg
svn: Commit failed (details follow):
svn: DELETE of
'/repos/project/!svn/wrk/64bad3a9-96f9-0310-818a-df4224ddc35d/banana.jpg':
423 Locked (https://svn.example.com)

But Harry, after touching up the banana's shade of yellow, is able to commit his changes to the file. That's because he authenticates as the lock owner, and also because his working copy holds the correct lock token:

$ whoami
harry

$ svn status
M    K banana.jpg

$ svn commit -m "Make banana more yellow"
Sending        banana.jpg
Transmitting file data .
Committed revision 2201.

$ svn status
$

Notice that after the commit is finished, svn status shows that the lock token is no longer present in working copy. This is the standard behavior of svn commit : it walks the working copy (or list of targets, if you provide such a list), and sends all lock tokens it encounters to the server as part of the commit transaction. After the commit completes successfully, all of the repository locks that were mentioned are released— even on files that weren't committed. The rationale here is to discourage users from being sloppy about locking, or from holding locks for too long. For example, suppose Harry were to haphazardly lock thirty files in a directory named images, because he's unsure of which files he needs to change. He ends up making changes to only four files. When he runs svn commit images , the process would still release all thirty locks.

This behavior of automatically releasing locks can be overridden with the --no-unlock option to svn commit . This is best used for those times when you want to commit changes, but still plan to make more changes and thus need to retain existing locks. This behavior is also semi-permanently tweakable, by setting no-unlock = yes in your run-time config file (see the section called “Runtime Configuration Area”).

Of course, locking a file doesn't oblige one to commit a change to it. The lock can be released at any time with a simple svn unlock command:

$ svn unlock banana.c
'banana.c' unlocked.

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Version Control with Subversion
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