The Subversion Project: Buiding a Better CVS
Subversion has a modular design; it's implemented as a collection of C libraries. Each layer has a well defined purpose and interface. In general, code flow begins at the top of the diagram and flows downward—each layer provides an interface to the layer above it (see Figure 1). Let's take a short tour of these layers, starting at the bottom.
The Subversion Filesystem is not a kernel-level filesystem that one would install in an operating system (like the Linux ext2 fs). Instead, it refers to the design of Subversion's repository. The repository is built on top of a database, currently Berkeley DB, and thus is a collection of .db files. However, a library accesses these files and exports a C API that simulates a filesystem, specifically a versioned filesystem.
This means that writing a program to access the repository is like writing against other filesystem APIs: you can open files and directories for reading and writing as usual. The main difference is that this particular filesystem never loses data when written to; old versions of files and directories always are saved as historical artifacts.
Whereas CVS's back end (RCS) stores revision numbers on a per-file basis, Subversion numbers entire trees. Each atomic commit to the repository creates a completely new filesystem tree and is individually labeled with a single, global revision number. Files and directories that have changed are rewritten (and older versions are backed up and stored as differences against the latest version), while unchanged entries are pointed to via a shared-storage mechanism. This is how the repository is able to version tree structures, not just file contents.
Finally, it should be mentioned that using a database like Berkeley DB immediately provides other nice features that Subversion needs: data integrity, atomic writes, recoverability and hot backups. See www.sleepycat.com for more information.
Subversion has the mark of Apache all over it. At its very core, the client uses the Apache Portable Runtime (APR) library. In fact, this means that a Subversion client should compile and run anywhere Apache httpd does. Right now, this list includes all flavors of UNIX, Win32, BeOS, OS/2, Mac OS X and possibly NetWare.
However, Subversion depends on more than just APR; the Subversion server is Apache httpd itself. Why was Apache chosen? Ultimately, the decision was about not re-inventing the wheel. Apache is a time-tested, open-source server process ready for serious use, yet is still extensible. It can sustain a high-network load. It runs on many platforms and can operate through firewalls. It's able to use a number of different authentication protocols. It can do network pipelining and caching. By using Apache as a server, Subversion gets all these features for free. Why start from scratch?
Subversion uses WebDAV as its network protocol. DAV (distributed authoring and versioning) is a whole discussion in itself (www.webdav.org), but in short, it's an extension to HTTP that allows reads/writes and versioning of files over the Web. The Subversion Project is hoping to ride a slowly rising tide of support for this protocol; all of the latest file browsers for Win32, Mac OS and GNOME speak this protocol already. Interoperability will (hopefully) become more and more of a bonus over time.
For users who simply wish to access Subversion repositories on local disk, the client can do this too; no network is required. The Repository Access (RA) layer is an abstract API implemented by both the DAV and local-access RA libraries. This is a specific benefit of writing a “librarized” revision control system; it's a big win over CVS, which has two very different, difficult-to-maintain code paths for local vs. network repository-access. Feel like writing a new network protocol for Subversion? Just write a new library that implements the RA API.
On the client side, the Subversion working copy library maintains administrative information within special /SVN subdirectories, similar in purpose to the /CVS administrative directories found in CVS working copies.
A glance inside the typical /SVN directory turns up a bit more than usual, however. The entries file contains XML that describes the current state of the working copy directory (and that basically serves the purposes of CVS's Entries, Root and Repository files combined). But, other items present (and not found in /CVS) include storage locations for the versioned properties (the metadata mentioned in the “Subversion Features” section above) and private caches of pristine versions of each file. This latter feature provides the ability to report local modifications and do reversions without network access. Authentication data also is stored within /SVN, rather than in a single .cvspass-like file.
The Subversion client library has the broadest responsibility. Its job is to mingle the functionality of the working-copy library with that of the repository-access library, and then to provide a highest-level API to any application that wishes to perform general revision control actions.
For example, the C routine svn_client_checkout() takes a URL as an argument. It passes this URL to the repository-access library and opens an authenticated session with a particular repository. It then asks the repository for a certain tree and sends this tree into the working-copy library, which then writes a full working copy to disk (/SVN directories and all).
The client library is designed to be used by any application. While the Subversion source code includes a standard command-line client, it should be easy to write any number of GUI clients on top of the client library. Hopefully, these GUIs should someday prove to be much better than the current crop of CVS GUI applications, which are no more than fragile wrappers around the CVS command-line client.
In addition, proper SWIG bindings (www.swig.org) should make the Subversion API available for any number of languages: Java, Perl, Python, Guile and so on. In order to Subvert CVS, it helps to be ubiquitous.
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