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<div class="sect1" title="Layered Library Design">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="svn.developer.layerlib"></a>Layered Library Design</h2>
</div>
</div>
</div>
<p>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
<a class="xref" href="svn.intro.whatis.html#svn.intro.architecture.dia-1" title="Figure 1. Subversion's architecture">Figure 1, “Subversion's architecture”</a> in the Preface).
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
(<code class="filename">libsvn_fs</code>, <code class="filename">libsvn_wc</code>,
<code class="filename">mod_dav_svn</code>, etc.).</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">libsvn_client</span>
</dt>
<dd>
<p>Primary interface for client
programs</p>
</dd>
<dt>
<span class="term">libsvn_delta</span>
</dt>
<dd>
<p>Tree and byte-stream differencing
routines</p>
</dd>
<dt>
<span class="term">libsvn_diff</span>
</dt>
<dd>
<p>Contextual differencing and merging
routines</p>
</dd>
<dt>
<span class="term">libsvn_fs</span>
</dt>
<dd>
<p>Filesystem commons and module
loader</p>
</dd>
<dt>
<span class="term">libsvn_fs_base</span>
</dt>
<dd>
<p>The Berkeley DB filesystem
backend</p>
</dd>
<dt>
<span class="term">libsvn_fs_fs</span>
</dt>
<dd>
<p>The native filesystem (FSFS)
backend</p>
</dd>
<dt>
<span class="term">libsvn_ra</span>
</dt>
<dd>
<p>Repository Access commons and module
loader</p>
</dd>
<dt>
<span class="term">libsvn_ra_local</span>
</dt>
<dd>
<p>The local Repository Access
module</p>
</dd>
<dt>
<span class="term">libsvn_ra_neon</span>
</dt>
<dd>
<p>The WebDAV Repository Access
module</p>
</dd>
<dt>
<span class="term">libsvn_ra_serf</span>
</dt>
<dd>
<p>Another (experimental) WebDAV Repository
Access module</p>
</dd>
<dt>
<span class="term">libsvn_ra_svn</span>
</dt>
<dd>
<p>The custom protocol Repository Access
module</p>
</dd>
<dt>
<span class="term">libsvn_repos</span>
</dt>
<dd>
<p>Repository interface</p>
</dd>
<dt>
<span class="term">libsvn_subr</span>
</dt>
<dd>
<p>Miscellaneous helpful
subroutines</p>
</dd>
<dt>
<span class="term">libsvn_wc</span>
</dt>
<dd>
<p>The working copy management
library</p>
</dd>
<dt>
<span class="term">mod_authz_svn</span>
</dt>
<dd>
<p>Apache authorization module for Subversion
repositories access via WebDAV</p>
</dd>
<dt>
<span class="term">mod_dav_svn</span>
</dt>
<dd>
<p>Apache module for mapping WebDAV operations to
Subversion ones</p>
</dd>
</dl>
</div>
<p>The fact that the word <span class="quote">“<span class="quote">miscellaneous</span>”</span>
appears only 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 <span class="quote">“<span class="quote">big
picture</span>”</span> that allows you to pinpoint the location of
certain pieces of functionality with relative ease.</p>
<p>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
<code class="filename">libsvn_ra_local</code>,
<code class="filename">libsvn_ra_neon</code>,
<code class="filename">libsvn_ra_serf</code>, and
<code class="filename">libsvn_ra_svn</code> libraries each implement the
same interface, all working as plug-ins to
<code class="filename">libsvn_ra</code>. And all four communicate with
the Repository layer—<code class="filename">libsvn_ra_local</code> connects to the
repository directly; the other three do so over a network. The
<code class="filename">libsvn_fs_base</code> and
<code class="filename">libsvn_fs_fs</code> libraries are another pair of
libraries that implement the same functionality in different
ways—both are plug-ins to the common
<code class="filename">libsvn_fs</code> library.</p>
<p>The client itself also highlights the benefits of modularity
in the Subversion design. Subversion's
<code class="filename">libsvn_client</code> library is a one-stop shop
for most of the functionality necessary for designing a working
Subversion client (see <a class="xref" href="svn.developer.layerlib.html#svn.developer.layerlib.client" title="Client Layer">the section called “Client Layer”</a>). So while the
Subversion distribution provides only the <span class="command"><strong>svn</strong></span>
command-line client program, several third-party
programs provide various forms of graphical client UIs.
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.</p>
<div class="sect2" title="Repository Layer">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.developer.layerlib.repos"></a>Repository Layer</h3>
</div>
</div>
</div>
<p>When referring to Subversion's Repository layer, we're
generally talking about two basic concepts—the versioned
filesystem implementation (accessed via
<code class="filename">libsvn_fs</code>, and supported by its
<code class="filename">libsvn_fs_base</code> and
<code class="filename">libsvn_fs_fs</code> plug-ins), and the repository
logic that wraps it (as implemented in
<code class="filename">libsvn_repos</code>). 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
<span class="quote">“<span class="quote">other end of the line.</span>”</span></p>
<p>The Subversion filesystem is not a kernel-level filesystem
that one would install in an operating system (such as the
Linux ext2 or NTFS), but instead is a virtual filesystem.
Rather than storing <span class="quote">“<span class="quote">files</span>”</span> and
<span class="quote">“<span class="quote">directories</span>”</span> as real files and directories (the
kind you can navigate through using your favorite shell
program), it uses one of two available abstract storage
backends—either a Berkeley DB database environment or a
flat-file representation. (To learn more about the two
repository backends, see <a class="xref" href="svn.reposadmin.planning.html#svn.reposadmin.basics.backends" title="Choosing a Data Store">the section called “Choosing a Data Store”</a>.) There has even
been considerable interest by the development community in
giving future releases of Subversion the ability to use other
backend database systems, perhaps through a mechanism such as
Open Database Connectivity (ODBC). In fact, Google did
something similar to this before launching the Google Code
Project Hosting service: they announced in mid-2006 that
members of its open source team had written a new proprietary
Subversion filesystem plug-in that used Google's ultra-scalable
Bigtable database for its storage.</p>
<p>The filesystem API exported by
<code class="filename">libsvn_fs</code> 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
(<span class="quote">“<span class="quote">properties</span>”</span>) 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.</p>
<p>All the modifications you make to your tree are done
within the context of a Subversion commit transaction. The
following is a simplified general routine for modifying your
filesystem:</p>
<div class="orderedlist">
<ol class="orderedlist" type="1">
<li class="listitem">
<p>Begin a Subversion commit transaction.</p>
</li>
<li class="listitem">
<p>Make your changes (adds, deletes, property
modifications, etc.).</p>
</li>
<li class="listitem">
<p>Commit your transaction.</p>
</li>
</ol>
</div>
<p>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 <span class="quote">“<span class="quote">the way things were.</span>”</span></p>
<div class="sidebar" title="The Transaction Distraction">
<div class="titlepage">
<div>
<div>
<p class="title">
<strong>The Transaction Distraction</strong>
</p>
</div>
</div>
</div>
<p>The notion of a Subversion transaction can become easily
confused with the transaction support provided by the
underlying database itself, especially given the former's
close proximity to the Berkeley DB database code in
<code class="filename">libsvn_fs_base</code>. Both types of
transaction exist to provide atomicity and isolation. In
other words, transactions give you the ability to perform a
set of actions in an all-or-nothing fashion—either all
the actions in the set complete with success, or they all
get treated as though <span class="emphasis"><em>none</em></span> of them ever
happened—and in a way that does not interfere with
other processes acting on the data.</p>
<p>Database transactions generally encompass small
operations related specifically to the modification of data
in the database itself (such as changing the contents of a
table row). Subversion transactions are larger in scope,
encompassing higher-level operations such as making
modifications to a set of files and directories that are
intended to be stored as the next revision of the filesystem
tree. If that isn't confusing enough, consider the fact
that Subversion uses a database transaction during the
creation of a Subversion transaction (so that if the
creation of a Subversion transaction fails, the database will
look as though we had never attempted that creation in the first
place)!</p>
<p>Fortunately for users of the filesystem API, the
transaction support provided by the database system itself
is hidden almost entirely from view (as should be expected
from a properly modularized library scheme). It is only
when you start digging into the implementation of the
filesystem itself that such things become visible (or
interesting).</p>
</div>
<p>Most of the functionality the filesystem
interface provides deals with actions that occur on individual
filesystem paths. That is, from outside the filesystem, the
primary mechanism for describing and accessing the individual
revisions of files and directories comes through the use of
path strings such as <code class="filename">/foo/bar</code>, just as though
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.</p>
<p>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 navigating into the node's
subdirectories represents a downward motion. <a class="xref" href="svn.developer.layerlib.html#svn.developer.layerlib.repos.dia-1" title="Figure 8.1. Files and directories in two dimensions">Figure 8.1, “Files and directories in two dimensions”</a> shows a typical
representation of a tree as exactly that.</p>
<div class="figure">
<a id="svn.developer.layerlib.repos.dia-1"></a>
<p class="title">
<strong>Figure 8.1. Files and directories in two dimensions</strong>
</p>
<div class="figure-contents">
<div>
<img src="images/ch08dia1.png" alt="Files and directories in two dimensions" />
</div>
</div>
</div>
<br class="figure-break" />
<p>The difference here is that the Subversion filesystem has
a nifty third dimension that most filesystems do not
have—Time!<sup>[<a id="idp17281040" href="#ftn.idp17281040" class="footnote">66</a>]</sup> In the filesystem
interface, nearly every function that has a
<em class="parameter"><code>path</code></em> argument also expects a
<em class="parameter"><code>root</code></em> argument. This
<code class="literal">svn_fs_root_t</code> argument describes
either a revision or a Subversion transaction (which is simply
a revision in the making) and provides that third dimension
of context needed to understand the difference between
<code class="filename">/foo/bar</code> in revision 32, and the same
path as it exists in revision 98. <a class="xref" href="svn.developer.layerlib.html#svn.developer.layerlib.repos.dia-2" title="Figure 8.2. Versioning time—the third dimension!">Figure 8.2, “Versioning time—the third dimension!”</a> shows revision
history as an added dimension to the Subversion filesystem
universe.</p>
<div class="figure">
<a id="svn.developer.layerlib.repos.dia-2"></a>
<p class="title">
<strong>Figure 8.2. Versioning time—the third dimension!</strong>
</p>
<div class="figure-contents">
<div>
<img src="images/ch08dia2.png" alt="Versioning time—the third dimension!" />
</div>
</div>
</div>
<br class="figure-break" />
<p>As we mentioned earlier, the
<code class="filename">libsvn_fs</code> 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 <code class="filename">libsvn_repos</code>
comes in.</p>
<p>The Subversion repository library
(<code class="filename">libsvn_repos</code>) sits (logically speaking)
atop the <code class="filename">libsvn_fs</code> API, providing
additional functionality beyond that of the underlying
versioned filesystem logic. It does not completely wrap each
and every filesystem function—only certain major steps
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. A repository hook system is not strictly related to
implementing a versioning filesystem, so it lives in the
repository wrapper library.</p>
<p>The hooks mechanism is but one of the reasons for the
abstraction of a separate repository library from the rest of
the filesystem code. The <code class="filename">libsvn_repos</code>
API provides several other important utilities to Subversion.
These include the abilities to:</p>
<div class="itemizedlist">
<ul class="itemizedlist" type="disc">
<li class="listitem">
<p>Create, open, destroy, and perform recovery steps on a
Subversion repository and the filesystem included in that
repository.</p>
</li>
<li class="listitem">
<p>Describe the differences between two filesystem
trees.</p>
</li>
<li class="listitem">
<p>Query for the commit log messages associated with all
(or some) of the revisions in which a set of files was
modified in the filesystem.</p>
</li>
<li class="listitem">
<p>Generate a human-readable <span class="quote">“<span class="quote">dump</span>”</span> of the
filesystem—a complete representation of the revisions in
the filesystem.</p>
</li>
<li class="listitem">
<p>Parse that dump format, loading the dumped revisions
into a different Subversion repository.</p>
</li>
</ul>
</div>
<p>As Subversion continues to evolve, the repository library
will grow with the filesystem library to offer increased
functionality and configurable option support.</p>
</div>
<div class="sect2" title="Repository Access Layer">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.developer.layerlib.ra"></a>Repository Access Layer</h3>
</div>
</div>
</div>
<p>If the Subversion Repository layer is at <span class="quote">“<span class="quote">the other
end of the line,</span>”</span> the Repository Access (RA) layer is
the line itself. Charged with marshaling data between the
client libraries and the repository, this layer includes the
<code class="filename">libsvn_ra</code> module loader library, the RA
modules themselves (which currently includes
<code class="filename">libsvn_ra_neon</code>,
<code class="filename">libsvn_ra_local</code>,
<code class="filename">libsvn_ra_serf</code>, and
<code class="filename">libsvn_ra_svn</code>), and any additional
libraries needed by one or more of those RA modules (such as
the <code class="filename">mod_dav_svn</code> Apache module or
<code class="filename">libsvn_ra_svn</code>'s server,
<span class="command"><strong>svnserve</strong></span>).</p>
<p>Since Subversion uses URLs to identify its repository
resources, the protocol portion of the URL scheme (usually
<code class="literal">file://</code>, <code class="literal">http://</code>,
<code class="literal">https://</code>, <code class="literal">svn://</code>, or
<code class="literal">svn+ssh://</code>) is used to determine which RA
module will handle the communications. Each module registers
a list of the protocols it knows how to <span class="quote">“<span class="quote">speak</span>”</span>
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
<strong class="userinput"><code>svn --version</code></strong>:</p>
<div class="informalexample">
<pre class="screen">
$ svn --version
svn, version 1.7.0
compiled Nov 15 2011, 10:10:24
Copyright (C) 2011 The Apache Software Foundation.
This software consists of contributions made by many people; see the NOTICE
file for more information.
Subversion is open source software, see http://subversion.apache.org/
The following repository access (RA) modules are available:
* ra_neon : Module for accessing a repository via WebDAV protocol using Neon.
- handles 'http' scheme
- handles 'https' scheme
* ra_svn : Module for accessing a repository using the svn network protocol.
- with Cyrus SASL authentication
- handles 'svn' scheme
* ra_local : Module for accessing a repository on local disk.
- handles 'file' scheme
* ra_serf : Module for accessing a repository via WebDAV protocol using serf.
- handles 'http' scheme
- handles 'https' scheme
$
</pre>
</div>
<p>The public API exported by the RA layer contains
functionality necessary for sending and receiving versioned
data to and from the repository. And each of the available RA
plug-ins is able to perform that task using a specific
protocol—<code class="filename">libsvn_ra_neon</code>
and <code class="filename">libsvn_ra_serf</code> speak HTTP/WebDAV
(optionally using SSL encryption) with an Apache HTTP Server
that is running the <code class="filename">mod_dav_svn</code>
Subversion server module; <code class="filename">libsvn_ra_svn</code>
speaks a custom network protocol with the
<span class="command"><strong>svnserve</strong></span> program; and so on.</p>
<p>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 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.</p>
</div>
<div class="sect2" title="Client Layer">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.developer.layerlib.client"></a>Client Layer</h3>
</div>
</div>
</div>
<p>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 that serve as a sort of local,
editable <span class="quote">“<span class="quote">reflection</span>”</span> of one or more repository
locations—and propagating changes to and from the
Repository Access layer.</p>
<p>Subversion's working copy library,
<code class="filename">libsvn_wc</code>, is directly responsible for
managing the data in the working copies. To accomplish this,
the library stores administrative information about the
working copy within a special subdirectory. This
subdirectory, named <code class="filename">.svn</code>, is present in
each working copy and contains various other files
and directories that record state and provide a private
workspace for administrative action. For those familiar with
CVS, this <code class="filename">.svn</code> subdirectory is similar in
purpose to the <code class="filename">CVS</code> administrative
directories found in CVS working copies.</p>
<p>The Subversion client library,
<code class="filename">libsvn_client</code>, 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
<code class="function">svn_client_checkout()</code> 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 (<code class="filename">.svn</code> directories
and all).</p>
<p>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
<code class="filename">libsvn_client</code> API to the same functionality,
data, and callback mechanisms that the command-line client
uses. In fact, the Subversion source code tree contains a
small C program (which you can find at
<code class="filename">tools/examples/minimal_client.c</code>) that
exemplifies how to wield the Subversion API to create a simple
client program.</p>
<div class="sidebar" title="Binding Directly—A Word About Correctness">
<div class="titlepage">
<div>
<div>
<p class="title">
<strong>Binding Directly—A Word About Correctness</strong>
</p>
</div>
</div>
</div>
<p>Why should your GUI program bind directly with a
<code class="filename">libsvn_client</code> instead of acting as a
wrapper around a command-line program? Besides simply being
more efficient, it can be more correct as well. A
command-line program (such as the one supplied with
Subversion) that binds to the client library needs to
effectively translate feedback and requested data bits from
C types to some form of human-readable output. This type of
translation can be lossy. That is, the program may not
display all of the information harvested from the API or may
combine bits of information for compact
representation.</p>
<p>If you wrap such a command-line program with yet another
program, the second program has access only to
already interpreted (and as we mentioned, likely incomplete)
information, which it must <span class="emphasis"><em>again</em></span>
translate into <span class="emphasis"><em>its</em></span> representation
format. With each layer of wrapping, the integrity of the
original data is potentially tainted more and more, much
like the result of making a copy of a copy (of a copy…)
of a favorite audio or video cassette.</p>
<p>But the most compelling argument for binding directly to
the APIs instead of wrapping other programs is that the
Subversion project makes compatibility promises regarding
its APIs. Across minor versions of those APIs (such as
between 1.3 and 1.4), no function's prototype will change.
In other words, you aren't forced to update your program's
source code simply because you've upgraded to a new version
of Subversion. Certain functions might be deprecated, but
they still work, and this gives you a buffer of time to
eventually embrace the newer APIs. These kinds of
compatibility promises do not exist for Subversion
command-line program output, which is subject to change from
release to release.</p>
</div>
</div>
<div class="footnotes">
<br />
<hr width="100" align="left" />
<div class="footnote">
<p><sup>[<a id="ftn.idp17281040" href="#idp17281040" class="para">66</a>] </sup>We understand that this may
come as a shock to sci-fi fans who have long been under the
impression that Time was actually
the <span class="emphasis"><em>fourth</em></span> dimension, and we apologize
for any emotional trauma induced by our assertion of a
different theory.</p>
</div>
</div>
</div>
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<p>You are reading <em>Version Control with Subversion</em> (for Subversion 1.7), by Ben Collins-Sussman, Brian W. Fitzpatrick, and C. Michael Pilato.<br />
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