8.1 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 Table 8-1. 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).
The fact that the word miscellaneous only appears once in Table 8-1 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 client itself also highlights modularity in the Subversion design. While Subversion currently comes with only a command-line client program, there are already a few other programs being developed by third parties to act as GUIs for Subversion. Again, 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 Section 8.1.3 later in this chapter). 8.1.1 Repository LayerWhen referring to Subversion's Repository Layer, we're generally talking about two libraries the repository library and the filesystem library. These libraries provide the storage and reporting mechanisms for the various revisions of your version-controlled data. This layer is connected to the Client Layer via the Repository Access Layer, and is, from the perspective of the Subversion user, the stuff at the other end of the line. The Subversion Filesystem is accessed via the libsvn_fs API, and is not a kernel-level filesystem that one would install in an operating system (like the Linux ext2 or NTFS), but a virtual filesystem. Rather than storing files and directories as real files and directories (as in, the kind you can navigate through using your favorite shell program), it uses a database system for its back-end storage mechanism. Currently, the database system in use is Berkeley DB.[1] However, there has been considerable interest by the development community in giving future releases of Subversion the ability to use other back-end database systems, perhaps through a mechanism such as Open Database Connectivity (ODBC).
The filesystem API exported by libsvn_fs contains the kinds of functionality you would expect from any other filesystem API: you can create and remove files and directories, copy and move them around, modify file contents, and so on. It also has features that are not quite as common, such as the ability to add, modify, and remove metadata (properties) on each file or directory. Furthermore, the Subversion Filesystem is a versioning filesystem, which means that as you make changes to your directory tree, Subversion remembers what your tree looked like before those changes. And before the previous changes. And the previous ones. And so on, all the way back through versioning time to (and just beyond) the moment you first started adding things to the filesystem. All the modifications you make to your tree are done within the context of a Subversion transaction. The following is a simplified general routine for modifying your filesystem:
Once you have committed your transaction, your filesystem modifications are permanently stored as historical artifacts. Each of these cycles generates a single new revision of your tree, and each revision is forever accessible as an immutable snapshot of the way things were.
Most of the functionality provided by the filesystem interface comes as an action that occurs on a filesystem path. That is, from outside of the filesystem, the primary mechanism for describing and accessing the individual revisions of files and directories comes through the use of path strings like /foo/bar, just as if you were addressing files and directories through your favorite shell program. You add new files and directories by passing their paths-to-be to the right API functions. You query for information about them by the same mechanism. Unlike most filesystems, though, a path alone is not enough information to identify a file or directory in Subversion. Think of a directory tree as a two-dimensional system, where a node's siblings represent a sort of left-and-right motion, and descending into subdirectories a downward motion. Figure 8-1 shows a typical representation of a tree as exactly that. Figure 8-1. Files and directories in two dimensions Of course, the Subversion filesystem has a nifty third dimension that most filesystems do not have time![2] In the filesystem interface, nearly every function that has a path argument also expects a root argument. This svn_fs_root_t argument describes either a revision or a Subversion transaction (which is usually just a revision-to-be), and provides that third-dimensional context needed to understand the difference between /foo/bar in revision 32, and the same path as it exists in revision 98. Figure 8-2 shows revision history as an added dimension to the Subversion filesystem universe.
Figure 8-2. Revisioning time the third dimension! As we mentioned earlier, the libsvn_fs API looks and feels like any other filesystem, except that it has this wonderful versioning capability. It was designed to be usable by any program interested in a versioning filesystem. Not coincidentally, Subversion itself is interested in that functionality. But while the filesystem API should be sufficient for basic file and directory versioning support, Subversion wants more and that is where libsvn_repos comes in. The Subversion repository library (libsvn_repos) is basically a wrapper library around the filesystem functionality. This library is responsible for creating the repository layout, making sure that the underlying filesystem is initialized, and so on. libsvn_repos also implements a set of hooks scripts that are executed by the repository code when certain actions take place. These scripts are useful for notification, authorization, or whatever purposes the repository administrator desires. This type of functionality, and other utility provided by the repository library, is not strictly related to implementing a versioning filesystem, which is why it was placed into its own library. Developers who wish to use the libsvn_repos API find that it is not a complete wrapper around the filesystem interface. That is, only certain major events in the general cycle of filesystem activity are wrapped by the repository interface. Some of these include the creation and commit of Subversion transactions, and the modification of revision properties. These particular events are wrapped by the repository layer because they have hooks associated with them. In the future, other events may be wrapped by the repository API. All of the remaining filesystem interaction continues to occur directly with libsvn_fs API, though. For example, Example 8-1 is a code segment that illustrates the use of both the repository and filesystem interfaces to create a new revision of the filesystem in which a directory is added. Note that in this example (and all others throughout this book), the SVN_ERR macro simply checks for a non-successful error return from the function it wraps, and returns that error if it exists. Example 8-1. Using the repository layer/* Create a new directory at the path NEW_DIRECTORY in the Subversion repository located at REPOS_PATH. Perform all memory allocation in POOL. This function will create a new revision for the addition of NEW_DIRECTORY. */ static svn_error_t * make_new_directory (const char *repos_path, const char *new_directory, apr_pool_t *pool) { svn_error_t *err; svn_repos_t *repos; svn_fs_t *fs; svn_revnum_t youngest_rev; svn_fs_txn_t *txn; svn_fs_root_t *txn_root; const char *conflict_str; /* Open the repository located at REPOS_PATH. */ SVN_ERR (svn_repos_open (&repos, repos_path, pool)); /* Get a pointer to the filesystem object that is stored in REPOS. */ fs = svn_repos_fs (repos); /* Ask the filesystem to tell us the youngest revision that currently exists. */ SVN_ERR (svn_fs_youngest_rev (&youngest_rev, fs, pool)); /* Begin a new transaction that is based on YOUNGEST_REV. We are less likely to have our later commit rejected as conflicting if we always try to make our changes against a copy of the latest snapshot of the filesystem tree. */ SVN_ERR (svn_fs_begin_txn (&txn, fs, youngest_rev, pool)); /* Now that we have started a new Subversion transaction, get a root object that represents that transaction. */ SVN_ERR (svn_fs_txn_root (&txn_root, txn, pool)); /* Create our new directory under the transaction root, at the path NEW_DIRECTORY. */ SVN_ERR (svn_fs_make_dir (txn_root, new_directory, pool)); /* Commit the transaction, creating a new revision of the filesystem which includes our added directory path. */ err = svn_repos_fs_commit_txn (&conflict_str, repos, &youngest_rev, txn, pool); if (! err) { /* No error? Excellent! Print a brief report of our success. */ printf ("Directory '%s' was successfully added as new revision " "'%" SVN_REVNUM_T_FMT "'.\n", new_directory, youngest_rev); } else if (err->apr_err = = SVN_ERR_FS_CONFLICT) { /* Uh-oh. Our commit failed as the result of a conflict (someone else seems to have made changes to the same area of the filesystem that we tried to modify). Print an error message. */ printf ("A conflict occurred at path '%s' while attempting " "to add directory '%s' to the repository at '%s'.\n", conflict_str, new_directory, repos_path); } else { /* Some other error has occurred. Print an error message. */ printf ("An error occurred while attempting to add directory '%s' " "to the repository at '%s'.\n", new_directory, repos_path); } /* Return the result of the attempted commit to our caller. */ return err; }In the previous code segment, calls were made to both the repository and filesystem interfaces. We could just as easily have committed the transaction using svn_fs_commit_txn. But the filesystem API knows nothing about the repository library's hook mechanism. If you want your Subversion repository to automatically perform some set of non-Subversion tasks every time you commit a transaction (such as sending an email that describes all the changes made in that transaction to your developer mailing list), you need to use the libsvn_repos-wrapped version of that function svn_repos_fs_commit_txn. This function actually first run the pre-commit hook script if one exists, then commit the transaction, and finally runs a post-commit hook script. The hooks provide a special kind of reporting mechanism that does not really belong in the core filesystem library itself. (For more information regarding Subversion's repository hooks, see Section 5.2.1). The hook mechanism requirement is but one of the reasons for the abstraction of a separate repository library from the rest of the filesystem code. The libsvn_repos API provides several other important utilities to Subversion. These include the abilities to:
As Subversion continues to evolve, the repository library grows with the filesystem library to offer increased functionality and configurable option support. 8.1.2 Repository Access LayerIf the Subversion Repository Layer is at the other end of the line, the Repository Access Layer is the line itself. Charged with marshalling data between the client libraries and the repository, this layer includes the libsvn_ra module loader library, the RA modules themselves (which currently includes libsvn_ra_dav, libsvn_ra_local, and libsvn_ra_svn), and any additional libraries needed by one or more of those RA modules, such as the mod_dav_svn Apache module with which libsvn_ra_dav communicates or libsvn_ra_svn's server, svnserve. Since Subversion uses URLs to identify its repository resources, the protocol portion of the URL schema (usually file:, http:, https:, or svn:) is used to determine which RA module will handle the communications. Each module registers a list of the protocols it knows how to speak so that the RA loader can, at runtime, determine which module to use for the task at hand. You can determine which RA modules are available to the Subversion command-line client, and what protocols they claim to support, by running svn version: $ svn --version svn, version 1.0.1 (r9023) compiled Mar 17 2004, 09:31:13 Copyright (C) 2000-2004 CollabNet. Subversion is open source software, see http://subversion.tigris.org/ This product includes software developed by CollabNet (http://www.Collab.Net/). The following repository access (RA) modules are available: * ra_dav : Module for accessing a repository via WebDAV (DeltaV) protocol. - handles 'http' schema - handles 'https' schema * ra_local : Module for accessing a repository on local disk. - handles 'file' schema * ra_svn : Module for accessing a repository using the svn network protocol. - handles 'svn' schema 8.1.2.1 RA-DAV (repository access using HTTP/DAV)The libsvn_ra_dav library is designed for use by clients that are being run on different machines than the servers with which they communicating, specifically machines reached using URLs that contain the http: or https: protocol portions. To understand how this module works, we should first mention a couple of other key components in this particular configuration of the Repository Access Layer the powerful Apache HTTP Server, and the Neon HTTP/WebDAV client library. Subversion's primary network server is the Apache HTTP Server. Apache is a time-tested, extensible open-source server process that is ready for serious use. It can sustain a high network load and runs on many platforms. The Apache server supports a number of different standard authentication protocols, and can be extended through the use of modules to support many others. It also supports optimizations like network pipelining and caching. By using Apache as a server, Subversion gets all of these features for free. And, since most firewalls already allow HTTP traffic to pass through, sysadmins typically don't even have to change their firewall configurations to allow Subversion to work. Subversion uses HTTP and WebDAV (with DeltaV) to communicate with an Apache server. You can read more about this in the WebDAV section of this chapter, but in short, WebDAV and DeltaV are extensions to the standard HTTP 1.1 protocol that enable sharing and versioning of files over the web. Apache 2.0 comes with mod_dav, an Apache module that understands the DAV extensions to HTTP. Subversion itself supplies mod_dav_svn, though, which is another Apache module that works in conjunction with (really, as a back-end to) mod_dav to provide Subversion's specific implementations of WebDAV and DeltaV. When communicating with a repository over HTTP, the RA loader library chooses libsvn_ra_dav as the proper access module. The Subversion client makes calls into the generic RA interface, and libsvn_ra_dav maps those calls (which embody rather large-scale Subversion actions) to a set of HTTP/WebDAV requests. Using the Neon library, libsvn_ra_dav transmits those requests to the Apache server. Apache receives these requests (exactly as it does generic HTTP requests that your web browser might make), notices that the requests are directed at a URL that is configured as a DAV location (using the <Location> directive in httpd.conf), and hands the request off to its own mod_dav module. When properly configured, mod_dav knows to use Subversion's mod_dav_svn for any filesystem-related needs, as opposed to the generic mod_dav_fs that comes with Apache. So ultimately, the client is communicating with mod_dav_svn, which binds directly to the Subversion Repository Layer. That was a simplified description of the actual exchanges taking place, though. For example, the Subversion repository might be protected by Apache's authorization directives. This could result in initial attempts to communicate with the repository being rejected by Apache on authorization grounds. At this point, libsvn_ra_dav gets back the notice from Apache that insufficient identification was supplied, and calls back into the Client Layer to get some updated authentication data. If the data is supplied correctly, and the user has the permissions that Apache seeks, libsvn_ra_dav's next automatic attempt at performing the original operation will be granted, and all will be well. If sufficient authentication information cannot be supplied, the request ultimately fails, and the client reports the failure to the user. By using Neon and Apache, Subversion gets free functionality in several other complex areas, too. For example, if Neon finds the OpenSSL libraries, it allows the Subversion client to attempt to use SSL-encrypted communications with the Apache server (whose own mod_ssl can speak the language). Also, both Neon itself and Apache's mod_deflate can understand the deflate algorithm (the same used by the PKZIP and gzip programs), so requests can be sent in smaller, compressed chunks across the wire. Other complex features that Subversion hopes to support in the future include the ability to automatically handle server-specified redirects (for example, when a repository has been moved to a new canonical URL) and taking advantage of HTTP pipelining. 8.1.2.2 RA-SVN (custom protocol repository access)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. 8.1.2.3 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 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 Section 8.1.1 earlier in this chapter), and your browser will not understand how to read that filesystem. 8.1.2.4 Your RA library hereFor 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 interprocess 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. 8.1.3 Client LayerOn 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 Section 8.3 later 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 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 very easy to write any number of GUI clients on top of the client library. New GUIs (or any new client, really) for Subversion need not be clunky wrappers around the included command-line client they have full access via the libsvn_client API to same functionality, data, and callback mechanisms that the command-line client uses.
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