Layered Library Design
Subversion has a modular design, implemented as a collection of C libraries. Each library has a well-defined purpose and interface, and most modules are said to exist in one of three main layers—the Repository Layer, the Repository Access (RA) Layer, or the Client Layer. We will examine these layers shortly, but first, see our brief inventory of Subversion's libraries in Òàáëèöà 8.1, «A Brief Inventory of the Subversion Libraries». For the sake of consistency, we will refer to the libraries by their extensionless Unix library names (e.g.: libsvn_fs, libsvn_wc, mod_dav_svn).
Òàáëèöà 8.1. A Brief Inventory of the Subversion Libraries
| libsvn_client | Primary interface for client programs |
| libsvn_delta | Tree and byte-stream differencing routines |
| libsvn_diff | Contextual differencing and merging routines |
| libsvn_fs | Filesystem commons and module loader |
| libsvn_fs_base | The Berkeley DB filesystem back-end |
| libsvn_fs_fs | The native filesystem (FSFS) back-end |
| libsvn_ra | Repository Access commons and module loader |
| libsvn_ra_dav | The WebDAV Repository Access module |
| libsvn_ra_local | The local Repository Access module |
| libsvn_ra_svn | The custom protocol Repository Access module |
| libsvn_repos | Repository interface |
| libsvn_subr | Miscellaneous helpful subroutines |
| libsvn_wc | The working copy management library |
| mod_authz_svn | Apache authorization module for Subversion repositories access via WebDAV |
| mod_dav_svn | Apache module for mapping WebDAV operations to Subversion ones |
The fact that the word «miscellaneous» only appears once in Òàáëèöà 8.1, «A Brief Inventory of the Subversion Libraries» is a good sign. The Subversion development team is serious about making sure that functionality lives in the right layer and libraries. Perhaps the greatest advantage of the modular design is its lack of complexity from a developer's point of view. As a developer, you can quickly formulate that kind of «big picture» that allows you to pinpoint the location of certain pieces of functionality with relative ease.
Another benefit of modularity is the ability to replace a given module with a whole new library that implements the same API without affecting the rest of the code base. In some sense, this happens within Subversion already. The libsvn_ra_dav, libsvn_ra_local, and libsvn_ra_svn all implement the same interface. And all three communicate with the Repository Layer—libsvn_ra_dav and libsvn_ra_svn do so across a network, and libsvn_ra_local connects to it directly. The libsvn_fs_base and libsvn_fs_fs libraries are another example of this.
The client itself also highlights modularity in the Subversion design. While Subversion itself comes with only a command-line client program, there are several third party programs which provide various forms of client GUI. These GUIs use the same APIs that the stock command-line client does. Subversion's libsvn_client library is the one-stop shop for most of the functionality necessary for designing a working Subversion client (see «Client Layer»).
