Developer Information

In addition to the standard HTTP/WebDAV protocol, Subversion also provides an RA implementation that uses a custom protocol. The libsvn_ra_svn module implements its own network socket connectivity, and communicates with a stand-alone server—the svnserve program—on the machine that hosts the repository. Clients access the repository using the svn:// schema.

This RA implementation lacks most of the advantages of Apache mentioned in the previous section; however, it may be appealing to some sysadmins nonetheless. It is dramatically easier to configure and run; setting up an svnserve process is nearly instantaneous. It is also much smaller (in terms of lines of code) than Apache, making it much easier to audit, for security reasons or otherwise. Furthermore, some sysadmins may already have an SSH security infrastructure in place, and want Subversion to use it. Clients using ra_svn can easily tunnel the protocol over SSH.

RA-Local (Direct Repository Access)

Not all communications with a Subversion repository require a powerhouse server process and a network layer. For users who simply wish to access the repositories on their local disk, they may do so using file: URLs and the functionality provided by libsvn_ra_local. This RA module binds directly with the repository and filesystem libraries, so no network communication is required at all.

Subversion requires that the server name included as part of the file: URL be either localhost or empty, and that there be no port specification. In other words, your URLs should look like either file://localhost/path/to/repos or file:///path/to/repos.

Also, be aware that Subversion's file: URLs cannot be used in a regular web browser the way typical file: URLs can. When you attempt to view a file: URL in a regular web browser, it reads and displays the contents of the file at that location by examining the filesystem directly. However, Subversion's resources exist in a virtual filesystem (see the section called “Repository Layer”), and your browser will not understand how to read that filesystem.

Your RA Library Here

For those who wish to access a Subversion repository using still another protocol, that is precisely why the Repository Access Layer is modularized! Developers can simply write a new library that implements the RA interface on one side and communicates with the repository on the other. Your new library can use existing network protocols, or you can invent your own. You could use inter-process communication (IPC) calls, or—let's get crazy, shall we?—you could even implement an email-based protocol. Subversion supplies the APIs; you supply the creativity.

Client Layer

On the client side, the Subversion working copy is where all the action takes place. The bulk of functionality implemented by the client-side libraries exists for the sole purpose of managing working copies—directories full of files and other subdirectories which serve as a sort of local, editable “reflection” of one or more repository locations—and propagating changes to and from the Repository Access layer.

Subversion's working copy library, libsvn_wc, is directly responsible for managing the data in the working copies. To accomplish this, the library stores administrative information about each working copy directory within a special subdirectory. This subdirectory, named .svn, is present in each working copy directory and contains various other files and directories which record state and provide a private workspace for administrative action. For those familiar with CVS, this .svn subdirectory is similar in purpose to the CVS administrative directories found in CVS working copies. For more information about the .svn administrative area, see the section called “Inside the Working Copy Administration Area”in this chapter.

The Subversion client library, libsvn_client, has the broadest responsibility; its job is to mingle the functionality of the working copy library with that of the Repository Access Layer, and then to provide the highest-level API to any application that wishes to perform general revision control actions. For example, the function svn_client_checkout takes a URL as an argument. It passes this URL to the RA layer and opens an authenticated session with a particular repository. It then asks the repository for a certain tree, and sends this tree into the

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