Subversion implements the concept of a version control repository much as any other modern version control system would. Unlike a working copy, a Subversion repository is an abstract entity, able to be operated upon almost exclusively by Subversion's own libraries and tools. As most of a user's Subversion interactions involve the use of the Subversion client and occur in the context of a working copy, we spend the majority of this book discussing the Subversion working copy and how to manipulate it. For the finer details of the repository, though, check out Chapter 5, Repository Administration.

A Subversion client commits (that is, communicates the changes made to) any number of files and directories as a single atomic transaction. By atomic transaction, we mean simply this: either all of the changes are accepted into the repository, or none of them is. Subversion tries to retain this atomicity in the face of program crashes, system crashes, network problems, and other users' actions.

Each time the repository accepts a commit, this creates a new state of the filesystem tree, called a revision. Each revision is assigned a unique natural number, one greater than the number assigned to the previous revision. The initial revision of a freshly created repository is numbered 0 and consists of nothing but an empty root directory.

Figure 1.6, “Tree changes over time” illustrates a nice way to visualize the repository. Imagine an array of revision numbers, starting at 0, stretching from left to right. Each revision number has a filesystem tree hanging below it, and each tree is a “snapshot” of the way the repository looked after a commit.

Figure 1.6. Tree changes over time

Subversion client programs use URLs to identify versioned files and directories in Subversion repositories. For the most part, these URLs use the standard syntax, allowing for server names and port numbers to be specified as part of the URL.

Subversion repository URLs aren't limited to only the http:// variety. Because Subversion offers several different ways for its clients to communicate with its servers, the URLs used to address the repository differ subtly depending on which repository access mechanism is employed. Table 1.1, “Repository access URLs” describes how different URL schemes map to the available repository access methods. For more details about Subversion's server options, see Chapter 6, Server Configuration.

Subversion's handling of URLs has some noteworthy nuances. For example, URLs containing the file:// access method (used for local repositories) must, in accordance with convention, have either a server name of localhost or no server name at all:

Also, users of the file:// scheme on Windows platforms will need to use an unofficially “standard” syntax for accessing repositories that are on the same machine, but on a different drive than the client's current working drive. Either of the two following URL path syntaxes will work, where X is the drive on which the repository resides:

Note that a URL uses forward slashes even though the native (non-URL) form of a path on Windows uses backslashes. Also note that when using the file:///X|/ form at the command line, you need to quote the URL (wrap it in quotation marks) so that the vertical bar character is not interpreted as a pipe.

The Subversion client will automatically encode URLs as necessary, just like a web browser does. For example, the URL http://host/path with space/project/españa — which contains both spaces and upper-ASCII characters — will be automatically interpreted by Subversion as if you'd provided http://host/path%20with%20space/project/espa%C3%B1a. If the URL contains spaces, be sure to place it within quotation marks at the command line so that your shell treats the whole thing as a single argument to the program.

There is one notable exception to Subversion's handling of URLs which also applies to its handling of local paths in many contexts, too. If the final path component of your URL or local path contains an at sign (@), you need to use a special syntax—described in the section called “Peg and Operative Revisions”—in order to make Subversion properly address that resource.

In Subversion 1.6, a new caret (^) notation was introduced as a shorthand for “the URL of the repository's root directory”. For example, you can use the ^/tags/bigsandwich/ to refer to the URL of the /tags/bigsandwich directory in the root of the repository. Such a URL is called a repository-relative URL. Note that this URL syntax works only when your current working directory is a working copy—the command-line client knows the repository's root URL by looking at the working copy's metadata. Also note that when you wish to refer precisely to the root directory of the repository, you must do so using ^/ (with the trailing slash character), not merely ^. Windows users should not forget that a caret is an escape character on their platform. Therefore, use a double caret ^^ if you run the Subversion client on a Windows machine.

A Subversion working copy is an ordinary directory tree on your local system, containing a collection of files. You can edit these files however you wish, and if they're source code files, you can compile your program from them in the usual way. Your working copy is your own private work area: Subversion will never incorporate other people's changes, nor make your own changes available to others, until you explicitly tell it to do so. You can even have multiple working copies of the same project.

After you've made some changes to the files in your working copy and verified that they work properly, Subversion provides you with commands to “publish” your changes (by writing to the repository), thereby making them available to the other people working with you on your project. If other people publish their own changes, Subversion provides you with commands to merge those changes into your own working copy (by reading from the repository). Notice that the central repository is the broker for everybody's changes in Subversion—changes aren't passed directly from working copy to working copy in the typical workflow.

A working copy also contains some extra files, created and maintained by Subversion, to help it carry out these commands. In particular, each working copy contains a subdirectory named .svn, also known as the working copy's administrative directory. The files in the administrative directory help Subversion recognize which of your versioned files contain unpublished changes, and which files are out of date with respect to others' work.

Fundamental working copy interactions

A typical Subversion repository often holds the files (or source code) for several projects; usually, each project is a subdirectory in the repository's filesystem tree. In this arrangement, a user's working copy will usually correspond to a particular subtree of the repository.

For example, suppose you have a repository that contains two software projects, paint and calc. Each project lives in its own top-level subdirectory, as shown in Figure 1.7, “The repository's filesystem”.

Figure 1.7. The repository's filesystem

To get a working copy, you must check out some subtree of the repository. (The term check out may sound like it has something to do with locking or reserving resources, but it doesn't; it simply creates a working copy of the project for you.) For example, if you check out /calc, you will get a working copy like this:

$ svn checkout http://svn.example.com/repos/calc
A    calc/Makefile
A    calc/integer.c
A    calc/button.c
Checked out revision 56.
$ ls -A calc
Makefile  button.c integer.c .svn/
$

The list of letter As in the left margin indicates that Subversion is adding a number of items to your working copy. You now have a personal copy of the repository's /calc directory, with one additional entry—.svn—which holds the extra information needed by Subversion, as mentioned earlier.

Suppose you make changes to button.c. Since the .svn directory remembers the file's original modification date and contents, Subversion can tell that you've changed the file. However, Subversion does not make your changes public until you explicitly tell it to. The act of publishing your changes is more commonly known as committing (or checking in) changes to the repository.

To publish your changes, you can use Subversion's svn commit command:

$ svn commit button.c -m "Fixed a typo in button.c."
Sending        button.c
Transmitting file data .
Committed revision 57.
$

Now your changes to button.c have been committed to the repository, with a note describing your change (namely, that you fixed a typo). If another user checks out a working copy of /calc, she will see your changes in the latest version of the file.

Suppose you have a collaborator, Sally, who checked out a working copy of /calc at the same time you did. When you commit your change to button.c, Sally's working copy is left unchanged; Subversion modifies working copies only at the user's request.

To bring her project up to date, Sally can ask Subversion to update her working copy, by using the svn update command. This will incorporate your changes into her working copy, as well as any others that have been committed since she checked it out.

$ pwd
/home/sally/calc
$ ls -A
Makefile button.c integer.c .svn/
$ svn update
Updating '.':
U    button.c
Updated to revision 57.
$

The output from the svn update command indicates that Subversion updated the contents of button.c. Note that Sally didn't need to specify which files to update; Subversion uses the information in the .svn directory as well as further information in the repository, to decide which files need to be brought up to date.