Each of Subversion's core libraries can be said to exist in one of three main layers—the Repository Layer, the Repository Access (RA) Layer, or the Client Layer (see Figure 1, “Subversion's Architecture”). We will examine these layers shortly, but first, let's briefly summarize Subversion's various libraries. For the sake of consistency, we will refer to the libraries by their extensionless Unix library names (libsvn_fs, libsvn_wc, mod_dav_svn, etc.).
Primary interface for client programs
Tree and byte-stream differencing routines
Contextual differencing and merging routines
Filesystem commons and module loader
The Berkeley DB filesystem back-end
The native filesystem (FSFS) back-end
Repository Access commons and module loader
The WebDAV Repository Access module
The local Repository Access module
Another (experimental) WebDAV Repository Access module
The custom protocol Repository Access module
Miscellaneous helpful subroutines
The working copy management library
Apache authorization module for Subversion repositories access via WebDAV
Apache module for mapping WebDAV operations to Subversion ones
The fact that the word “miscellaneous” only appears once in the previous list 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, libsvn_ra_serf, and libsvn_ra_svn libraries each implement the same interface, all working as plugins to libsvn_ra. And all four communicate with the Repository Layer—libsvn_ra_local connects to the repository directly; the other three do so over a network. The libsvn_fs_base and libsvn_fs_fs libraries are another pair of libraries that implement the same functionality in different ways—both are plugins to the common libsvn_fs library.
The client itself also highlights the benefits of modularity in the Subversion design. Subversion's libsvn_client library is a one-stop shop for most of the functionality necessary for designing a working Subversion client (see the section called “Client Layer”). So while the Subversion distribution provides only the svn command-line client program, there are several third-party programs which provide various forms of graphical client UI. These GUIs use the same APIs that the stock command-line client does. This type of modularity has played a large role in the proliferation of available Subversion clients and IDE integrations and, by extension, to the tremendous adoption rate of Subversion itself.