The Apache HTTP Server is a “heavy-duty” network server that Subversion can leverage. Via a custom module, httpd makes Subversion repositories available to clients via the WebDAV/DeltaV protocol, which is an extension to HTTP 1.1 (see http://www.webdav.org/ for more information). This protocol takes the ubiquitous HTTP protocol that is the core of the World Wide Web, and adds writing—specifically, versioned writing—capabilities. The result is a standardized, robust system that is conveniently packaged as part of the Apache 2.0 software, supported by numerous operating systems and third-party products, and doesn't require network administrators to open up yet another custom port. [45] While an Apache-Subversion server has more features than svnserve, it's also a bit more difficult to set up. With flexibility often comes more complexity.
Much of the following discussion includes references to Apache configuration directives. While some examples are given of the use of these directives, describing them in full is outside the scope of this chapter. The Apache team maintains excellent documentation, publicly available on their web site at http://httpd.apache.org. For example, a general reference for the configuration directives is located at http://httpd.apache.org/docs-2.0/mod/directives.html.
Also, as you make changes to your Apache setup, it is likely
that somewhere along the way a mistake will be made. If you are
not already familiar with Apache's logging subsystem, you should
become aware of it. In your httpd.conf
file are directives that specify the on-disk locations of the
access and error logs generated by Apache (the
CustomLog and ErrorLog
directives, respectively).
Subversion's mod_dav_svn uses Apache's error
logging interface as well. You can always browse the contents
of those files for information that might reveal the source of a
problem that is not clearly noticeable otherwise.
To network your repository over HTTP, you basically need four components, available in two packages. You'll need Apache httpd 2.0, the mod_dav DAV module that comes with it, Subversion, and the mod_dav_svn filesystem provider module distributed with Subversion. Once you have all of those components, the process of networking your repository is as simple as:
Getting httpd 2.0 up and running with the mod_dav module
Installing the mod_dav_svn backend to mod_dav, which uses Subversion's libraries to access the repository
Configuring your httpd.conf
file to export (or expose) the repository
You can accomplish the first two items either by
compiling httpd and Subversion from
source code or by installing prebuilt binary packages of
them on your system. For the most up-to-date information on
how to compile Subversion for use with the Apache HTTP Server,
as well as how to compile and configure Apache itself for
this purpose, see the INSTALL file in
the top level of the Subversion source code tree.
Once you have all the necessary components installed on
your system, all that remains is the configuration of Apache
via its httpd.conf file. Instruct Apache
to load the mod_dav_svn module using the
LoadModule directive. This directive must
precede any other Subversion-related configuration items. If
your Apache was installed using the default layout, your
mod_dav_svn module should have been
installed in the modules subdirectory of
the Apache install location (often
/usr/local/apache2). The
LoadModule directive has a simple syntax,
mapping a named module to the location of a shared library on
disk:
LoadModule dav_svn_module modules/mod_dav_svn.so
Note that if mod_dav was compiled as a
shared object (instead of statically linked directly to the
httpd binary), you'll need a similar
LoadModule statement for it, too. Be sure
that it comes before the mod_dav_svn line:
LoadModule dav_module modules/mod_dav.so LoadModule dav_svn_module modules/mod_dav_svn.so
At a later location in your configuration file, you now
need to tell Apache where you keep your Subversion repository
(or repositories). The Location directive
has an XML-like notation, starting with an opening tag and
ending with a closing tag, with various other configuration
directives in the middle. The purpose of the
Location directive is to instruct Apache to
do something special when handling requests that are directed
at a given URL or one of its children. In the case of
Subversion, you want Apache to simply hand off support for
URLs that point at versioned resources to the DAV layer. You
can instruct Apache to delegate the handling of all URLs whose
path portions (the part of the URL that follows the server's
name and the optional port number) begin with
/repos/ to a DAV provider whose
repository is located at
/var/svn/repository using the
following httpd.conf syntax:
<Location /repos> DAV svn SVNPath /var/svn/repository </Location>
If you plan to support multiple Subversion repositories
that will reside in the same parent directory on your local
disk, you can use an alternative
directive—SVNParentPath—to
indicate that common parent directory. For example, if you
know you will be creating multiple Subversion repositories in
a directory /var/svn that would be
accessed via URLs such as
http://my.server.com/svn/repos1,
http://my.server.com/svn/repos2, and so on, you
could use the httpd.conf configuration
syntax in the following example:
<Location /svn> DAV svn # any "/svn/foo" URL will map to a repository /var/svn/foo SVNParentPath /var/svn </Location>
Using the previous syntax, Apache will delegate the
handling of all URLs whose path portions begin with
/svn/ to the Subversion DAV provider,
which will then assume that any items in the directory
specified by the SVNParentPath directive
are actually Subversion repositories. This is a particularly
convenient syntax in that, unlike the use of the
SVNPath directive, you don't have to
restart Apache to create and network new repositories.
Be sure that when you define your new
Location, it doesn't overlap with other
exported locations. For example, if your main
DocumentRoot is exported to
/www, do not export a Subversion
repository in <Location /www/repos>.
If a request comes in for the URI
/www/repos/foo.c, Apache won't know
whether to look for a file repos/foo.c in
the DocumentRoot, or whether to delegate
mod_dav_svn to return
foo.c from the Subversion repository.
The result is often an error from the server of the form
301 Moved Permanently.
At this stage, you should strongly consider the question of permissions. If you've been running Apache for some time now as your regular web server, you probably already have a collection of content—web pages, scripts, and such. These items have already been configured with a set of permissions that allows them to work with Apache, or more appropriately, that allows Apache to work with those files. Apache, when used as a Subversion server, will also need the correct permissions to read and write to your Subversion repository.
You will need to determine a permission system setup that
satisfies Subversion's requirements without messing up any
previously existing web page or script installations. This
might mean changing the permissions on your Subversion
repository to match those in use by other things that Apache
serves for you, or it could mean using the
User and Group
directives in httpd.conf to specify that
Apache should run as the user and group that owns your
Subversion repository. There is no single correct way to set
up your permissions, and each administrator will have
different reasons for doing things a certain way. Just be
aware that permission-related problems are perhaps the most
common oversight when configuring a Subversion repository for
use with Apache.
At this point, if you configured
httpd.conf to contain something such as the
following:
<Location /svn> DAV svn SVNParentPath /var/svn </Location>
your repository is “anonymously”
accessible to the world. Until you configure some
authentication and authorization policies, the Subversion
repositories that you make available via the
Location directive will be generally
accessible to everyone. In other words:
Anyone can use a Subversion client to check out a working copy of a repository URL (or any of its subdirectories).
Anyone can interactively browse the repository's latest revision simply by pointing a web browser to the repository URL.
Anyone can commit to the repository.
Of course, you might have already set up
a pre-commit hook script to prevent
commits (see the section called “Implementing Repository Hooks”).
But as you read on, you'll see that it's also possible to use
Apache's built-in methods to restrict access in specific
ways.
The easiest way to authenticate a client is via the HTTP Basic authentication mechanism, which simply uses a username and password to verify that a user is who she says she is. Apache provides an htpasswd utility for managing the list of acceptable usernames and passwords. Let's grant commit access to Sally and Harry. First, we need to add them to the password file:
$ ### First time: use -c to create the file $ ### Use -m to use MD5 encryption of the password, which is more secure $ htpasswd -cm /etc/svn-auth-file harry New password: ***** Re-type new password: ***** Adding password for user harry $ htpasswd -m /etc/svn-auth-file sally New password: ******* Re-type new password: ******* Adding password for user sally $
Next, you need to add some more
httpd.conf directives inside your
Location block to tell Apache what to do
with your new password file. The
AuthType directive specifies the type of
authentication system to use. In this case, we want to
specify the Basic authentication system.
AuthName is an arbitrary name that you
give for the authentication domain. Most browsers will
display this name in the pop-up dialog box when the browser
is querying the user for her name and password. Finally,
use the AuthUserFile directive to specify
the location of the password file you created using
htpasswd.
After adding these three directives, your
<Location> block should look
something like this:
<Location /svn> DAV svn SVNParentPath /var/svn AuthType Basic AuthName "Subversion repository" AuthUserFile /etc/svn-auth-file </Location>
This <Location> block is not
yet complete, and it will not do anything useful. It's
merely telling Apache that whenever authorization is
required, Apache should harvest a username and password from
the Subversion client. What's missing here, however, are
directives that tell Apache which sorts
of client requests require authorization. Wherever
authorization is required, Apache will demand authentication
as well. The simplest thing to do is protect all requests.
Adding Require valid-user tells Apache
that all requests require an authenticated user:
<Location /svn> DAV svn SVNParentPath /var/svn AuthType Basic AuthName "Subversion repository" AuthUserFile /etc/svn-auth-file Require valid-user </Location>
Be sure to read the next section (the section called “Authorization Options”) for more detail on the
Require directive and other ways to set
authorization policies.
One word of warning: HTTP Basic Auth passwords pass in very nearly plain text over the network, and thus are extremely insecure.
Another option is to not use Basic authentication, but to use Digest authentication instead. Digest authentication allows the server to verify the client's identity without passing the plain-text password over the network. Assuming that the client and server both know the user's password, they can verify that the password is the same by using it to apply a hashing function to a one-time bit of information. The server sends a small random-ish string to the client; the client uses the user's password to hash the string; the server then looks to see whether the hashed value is what it expected.
Configuring Apache for Digest authentication is also fairly easy, and only a small variation on our prior example. Be sure to consult Apache's documentation for full details.
<Location /svn> DAV svn SVNParentPath /var/svn AuthType Digest AuthName "Subversion repository" AuthDigestDomain /svn/ AuthUserFile /etc/svn-auth-file Require valid-user </Location>
If you're looking for maximum security, public key
cryptography is the best solution. It may be best to use
some sort of SSL encryption, so that clients authenticate
via https:// instead
of http://; at a bare minimum, you can
configure Apache to use a self-signed server certificate.
[46]
Consult Apache's documentation (and OpenSSL documentation)
about how to do that.
Businesses that need to expose their repositories for access outside the company firewall should be conscious of the possibility that unauthorized parties could be “sniffing” their network traffic. SSL makes that kind of unwanted attention less likely to result in sensitive data leaks.
If a Subversion client is compiled to use OpenSSL,
it gains the ability to speak to an Apache server via
https:// URLs. The Neon library used by
the Subversion client is not only able to verify server
certificates, but can also supply client certificates when
challenged. When the client and server have exchanged SSL
certificates and successfully authenticated one another, all
further communication is encrypted via a session key.
It's beyond the scope of this book to describe how to generate client and server certificates and how to configure Apache to use them. Many other books, including Apache's own documentation, describe this task. But what we can cover here is how to manage server and client certificates from an ordinary Subversion client.
When speaking to Apache via https://,
a Subversion client can receive two different types of
information:
A server certificate
A demand for a client certificate
If the client receives a server certificate, it needs to verify that it trusts the certificate: is the server really who it claims to be? The OpenSSL library does this by examining the signer of the server certificate, or certificate authority (CA). If OpenSSL is unable to automatically trust the CA, or if some other problem occurs (such as an expired certificate or hostname mismatch), the Subversion command-line client will ask you whether you want to trust the server certificate anyway:
$ svn list https://host.example.com/repos/project Error validating server certificate for 'https://host.example.com:443': - The certificate is not issued by a trusted authority. Use the fingerprint to validate the certificate manually! Certificate information: - Hostname: host.example.com - Valid: from Jan 30 19:23:56 2004 GMT until Jan 30 19:23:56 2006 GMT - Issuer: CA, example.com, Sometown, California, US - Fingerprint: 7d:e1:a9:34:33:39:ba:6a:e9:a5:c4:22:98:7b:76:5c:92:a0:9c:7b (R)eject, accept (t)emporarily or accept (p)ermanently?
This dialogue should look familiar; it's essentially the
same question you've probably seen coming from your web
browser (which is just another HTTP client like Subversion).
If you choose the (p)ermanent option, the server certificate
will be cached in your private runtime
auth/ area in just the same way your
username and password are cached (see the section called “Client Credentials Caching”). If cached,
Subversion will automatically trust this certificate
in future negotiations.
Your runtime servers file also gives
you the ability to make your Subversion client automatically
trust specific CAs, either globally or on a per-host basis.
Simply set the ssl-authority-files
variable to a semicolon-separated list of PEM-encoded CA
certificates:
[global] ssl-authority-files = /path/to/CAcert1.pem;/path/to/CAcert2.pem
Many OpenSSL installations also have a predefined set
of “default” CAs that are nearly universally
trusted. To make the Subversion client automatically trust
these standard authorities, set the
ssl-trust-default-ca variable to
true.
When talking to Apache, a Subversion client might also receive a challenge for a client certificate. Apache is asking the client to identify itself: is the client really who it says it is? If all goes correctly, the Subversion client sends back a private certificate signed by a CA that Apache trusts. A client certificate is usually stored on disk in encrypted format, protected by a local password. When Subversion receives this challenge, it will ask you for a path to the certificate and the password that protects it:
$ svn list https://host.example.com/repos/project Authentication realm: https://host.example.com:443 Client certificate filename: /path/to/my/cert.p12 Passphrase for '/path/to/my/cert.p12': ******** …
Notice that the client certificate is a “p12” file. To use a client certificate with Subversion, it must be in PKCS#12 format, which is a portable standard. Most web browsers are already able to import and export certificates in that format. Another option is to use the OpenSSL command-line tools to convert existing certificates into PKCS#12.
Again, the runtime servers file
allows you to automate this challenge on a per-host basis.
Either or both pieces of information can be described in
runtime variables:
[groups] examplehost = host.example.com [examplehost] ssl-client-cert-file = /path/to/my/cert.p12 ssl-client-cert-password = somepassword
Once you've set the
ssl-client-cert-file and
ssl-client-cert-password variables, the
Subversion client can automatically respond to a client
certificate challenge without prompting you.
[47]
At this point, you've configured authentication, but not authorization. Apache is able to challenge clients and confirm identities, but it has not been told how to allow or restrict access to the clients bearing those identities. This section describes two strategies for controlling access to your repositories.
The simplest form of access control is to authorize certain users for either read-only access to a repository or read/write access to a repository.
You can restrict access on all repository operations by
adding the Require valid-user directive
to your <Location> block. Using
our previous example, this would mean that only clients that
claimed to be either harry or
sally and that provided the correct
password for their respective username would be allowed to
do anything with the Subversion repository:
<Location /svn> DAV svn SVNParentPath /var/svn # how to authenticate a user AuthType Basic AuthName "Subversion repository" AuthUserFile /path/to/users/file # only authenticated users may access the repository Require valid-user </Location>
Sometimes you don't need to run such a tight ship. For
example, Subversion's own source code repository at
http://svn.collab.net/repos/svn allows anyone
in the world to perform read-only repository tasks (such as
checking out working copies and browsing the repository with
a web browser), but restricts all write operations to
authenticated users. To do this type of selective
restriction, you can use the Limit and
LimitExcept configuration directives.
Like the Location directive, these blocks
have starting and ending tags, and you would nest them
inside your <Location>
block.
The parameters present on the Limit
and LimitExcept directives are HTTP
request types that are affected by that block. For example,
if you wanted to disallow all access to your repository
except the currently supported read-only operations, you
would use the LimitExcept directive,
passing the GET,
PROPFIND, OPTIONS, and
REPORT request type parameters. Then the
previously mentioned Require valid-user
directive would be placed inside the
<LimitExcept> block instead of just
inside the <Location> block.
<Location /svn>
DAV svn
SVNParentPath /var/svn
# how to authenticate a user
AuthType Basic
AuthName "Subversion repository"
AuthUserFile /path/to/users/file
# For any operations other than these, require an authenticated user.
<LimitExcept GET PROPFIND OPTIONS REPORT>
Require valid-user
</LimitExcept>
</Location>
These are only a few simple examples. For more in-depth
information about Apache access control and the
Require directive, take a look at the
Security section of the Apache
documentation's tutorials collection at http://httpd.apache.org/docs-2.0/misc/tutorials.html.
It's possible to set up finer-grained permissions using a second Apache httpd module, mod_authz_svn. This module grabs the various opaque URLs passing from client to server, asks mod_dav_svn to decode them, and then possibly vetoes requests based on access policies defined in a configuration file.
If you've built Subversion from source code,
mod_authz_svn is automatically built
and installed alongside mod_dav_svn.
Many binary distributions install it automatically as well.
To verify that it's installed correctly, make sure it comes
right after mod_dav_svn's
LoadModule directive in
httpd.conf:
LoadModule dav_module modules/mod_dav.so LoadModule dav_svn_module modules/mod_dav_svn.so LoadModule authz_svn_module modules/mod_authz_svn.so
To activate this module, you need to configure your
Location block to use the
AuthzSVNAccessFile directive, which
specifies a file containing the permissions policy for paths
within your repositories. (In a moment, we'll discuss the
format of that file.)
Apache is flexible, so you have the option to configure your block in one of three general patterns. To begin, choose one of these basic configuration patterns. (The following examples are very simple; look at Apache's own documentation for much more detail on Apache authentication and authorization options.)
The simplest block is to allow open access to everyone. In this scenario, Apache never sends authentication challenges, so all users are treated as “anonymous.” (See Example 6.1, “A sample configuration for anonymous access”.)
Example 6.1. A sample configuration for anonymous access
<Location /repos>
DAV svn
SVNParentPath /var/svn
# our access control policy
AuthzSVNAccessFile /path/to/access/file
</Location>
On the opposite end of the paranoia scale, you can
configure your block to demand authentication from everyone.
All clients must supply credentials to identify themselves.
Your block unconditionally requires authentication via the
Require valid-user directive, and it
defines a means to authenticate. (See
Example 6.2, “A sample configuration for authenticated access”.)
Example 6.2. A sample configuration for authenticated access
<Location /repos>
DAV svn
SVNParentPath /var/svn
# our access control policy
AuthzSVNAccessFile /path/to/access/file
# only authenticated users may access the repository
Require valid-user
# how to authenticate a user
AuthType Basic
AuthName "Subversion repository"
AuthUserFile /path/to/users/file
</Location>
A third very popular pattern is to allow a combination
of authenticated and anonymous access. For example, many
administrators want to allow anonymous users to read certain
repository directories, but want only authenticated users to
read (or write) more sensitive areas. In this setup, all
users start out accessing the repository anonymously. If
your access control policy demands a real username at any
point, Apache will demand authentication from the client.
To do this, use both the Satisfy Any
and Require valid-user directives
together. (See
Example 6.3, “A sample configuration for mixed
authenticated/anonymous access”.)
Example 6.3. A sample configuration for mixed authenticated/anonymous access
<Location /repos>
DAV svn
SVNParentPath /var/svn
# our access control policy
AuthzSVNAccessFile /path/to/access/file
# try anonymous access first, resort to real
# authentication if necessary.
Satisfy Any
Require valid-user
# how to authenticate a user
AuthType Basic
AuthName "Subversion repository"
AuthUserFile /path/to/users/file
</Location>
Once you've settled on one of these three
basic httpd.conf templates, you need to
create your file containing access rules for particular
paths within the repository. We describe this later in
this chapter, in
the section called “Path-Based Authorization”.
The mod_dav_svn module goes through a
lot of work to make sure that data you've marked
“unreadable” doesn't get accidentally leaked.
This means it needs to closely monitor all of the paths
and file-contents returned by commands such as svn
checkout and svn update.
If these commands encounter a path that isn't
readable according to some authorization policy, the
path is typically omitted altogether. In the case of
history or rename tracing—for example, running a command such
as svn cat -r OLD foo.c on a file that
was renamed long ago—the rename tracking will simply
halt if one of the object's former names is determined to be
read-restricted.
All of this path checking can sometimes be quite
expensive, especially in the case of svn
log. When retrieving a list of revisions, the
server looks at every changed path in each revision and
checks it for readability. If an unreadable path is
discovered, it's omitted from the list of the revision's
changed paths (normally seen with
the --verbose (-v) option),
and the whole log message is suppressed. Needless to say,
this can be time-consuming on revisions that affect a large
number of files. This is the cost of security: even if you
haven't configured a module such as
mod_authz_svn at all, the
mod_dav_svn module is still asking Apache
httpd to run authorization checks on
every path. The mod_dav_svn module has
no idea what authorization modules have been installed, so
all it can do is ask Apache to invoke whatever might be
present.
On the other hand, there's also an escape hatch of
sorts, which allows you to trade security features for
speed. If you're not enforcing any sort of per-directory
authorization (i.e., not using
mod_authz_svn or similar module),
you can disable all of this path checking. In your
httpd.conf file, use the
SVNPathAuthz directive as shown in
Example 6.4, “Disabling path checks altogether”.
Example 6.4. Disabling path checks altogether
<Location /repos>
DAV svn
SVNParentPath /var/svn
SVNPathAuthz off
</Location>
The SVNPathAuthz directive
is “on” by default. When
set to “off,” all path-based authorization
checking is disabled;
mod_dav_svn stops invoking authorization
checks on every path it discovers.
We've covered most of the authentication and authorization options for Apache and mod_dav_svn. But there are a few other nice features that Apache provides.
One of the most useful benefits of an Apache/WebDAV
configuration for your Subversion repository is that the
youngest revisions of your versioned files and directories
are immediately available for viewing via a regular web
browser. Since Subversion uses URLs to identify versioned
resources, those URLs used for HTTP-based repository access
can be typed directly into a web browser. Your browser will
issue an HTTP GET request for that URL;
based on whether that URL represents a versioned directory
or file, mod_dav_svn will respond with a
directory listing or with file contents.
Since the URLs do not contain any information about which version of the resource you wish to see, mod_dav_svn will always answer with the youngest version. This functionality has the wonderful side effect that you can pass around Subversion URLs to your peers as references to documents, and those URLs will always point at the latest manifestation of that document. Of course, you can even use the URLs as hyperlinks from other web sites, too.
When browsing a Subversion repository, the web browser
gets a clue about how to render a file's contents by
looking at the Content-Type: header
returned in Apache's response to the
HTTP GET request. The value of this
header is some sort of MIME type. By default, Apache will
tell the web browsers that all repository files are of
the “default” MIME type,
typically text/plain. This can be
frustrating, however, if a user wishes repository files to
render as something more meaningful—for example,
it might be nice to have a foo.html file
in the repository actually render as HTML when
browsing.
To make this happen, you need only to make sure that
your files have the
proper svn:mime-type set. We discuss this
in more detail in
the section called “File Content Type”,
and you can even configure your client to automatically
attach proper svn:mime-type properties
to files entering the repository for the first time; see
the section called “Automatic Property Setting”.
So in our example, if one were to set
the svn:mime-type property
to text/html on
file foo.html, Apache would
properly tell your web browser to render the file as HTML.
One could also attach proper image/*
MIME-type properties to image files and ultimately get an
entire web site to be viewable directly from a repository!
There's generally no problem with this, as long as the web
site doesn't contain any dynamically generated
content.
You generally will get more use out of URLs to
versioned files—after all, that's where the
interesting content tends to lie. But you might have
occasion to browse a Subversion directory listing, where
you'll quickly note that the generated HTML used to
display that listing is very basic, and certainly not
intended to be aesthetically pleasing (or even
interesting). To enable customization of these directory
displays, Subversion provides an XML index feature. A
single SVNIndexXSLT directive in your
repository's Location block of
httpd.conf will
instruct mod_dav_svn to generate XML
output when displaying a directory listing, and to
reference the XSLT stylesheet of your choice:
<Location /svn> DAV svn SVNParentPath /var/svn SVNIndexXSLT "/svnindex.xsl" … </Location>
Using the SVNIndexXSLT directive and
a creative XSLT stylesheet, you can make your directory
listings match the color schemes and imagery used in other
parts of your web site. Or, if you'd prefer, you can use
the sample stylesheets provided in the Subversion source
distribution's tools/xslt/ directory.
Keep in mind that the path provided to the
SVNIndexXSLT directory is actually a URL
path—browsers need to be able to read your
stylesheets to make use of them!
If you're serving a collection of repositories from a
single URL via the SVNParentPath
directive, then it's also possible to have Apache display
all available repositories to a web browser. Just
activate the SVNListParentPath
directive:
<Location /svn> DAV svn SVNParentPath /var/svn SVNListParentPath on … </Location>
If a user now points her web browser to the
URL http://host.example.com/svn/, she'll
see a list of all Subversion repositories sitting
in /var/svn. Obviously, this can
be a security problem, so this feature is turned off by
default.
Because Apache is an HTTP server at heart, it contains
fantastically flexible logging features. It's beyond the
scope of this book to discuss all of the ways logging can be
configured, but we should point out that even the most
generic httpd.conf file will cause
Apache to produce two logs:
error_log
and access_log. These logs may appear
in different places, but are typically created in the
logging area of your Apache installation. (On Unix, they
often live
in /usr/local/apache2/logs/.)
The error_log describes any internal
errors that Apache runs into as it works.
The access_log file records every
incoming HTTP request received by Apache. This makes it
easy to see, for example, which IP addresses Subversion
clients are coming from, how often particular clients use
the server, which users are authenticating properly, and
which requests succeed or fail.
Unfortunately, because HTTP is a stateless protocol,
even the simplest Subversion client operation generates
multiple network requests. It's very difficult to look at
the access_log and deduce what the
client was doing—most operations look like a series
of cryptic PROPPATCH, GET,
PUT, and REPORT
requests. To make things worse, many client operations send
nearly identical series of requests, so it's even harder to
tell them apart.
mod_dav_svn, however, can come to your aid. By activating an “operational logging” feature, you can ask mod_dav_svn to create a separate log file describing what sort of high-level operations your clients are performing.
To do this, you need to make use of
Apache's CustomLog directive (which is
explained in more detail in Apache's own documentation).
Be sure to invoke this
directive outside your
Subversion Location block:
<Location /svn>
DAV svn
…
</Location>
CustomLog logs/svn_logfile "%t %u %{SVN-ACTION}e" env=SVN-ACTION
In this example, we're asking Apache to create a special
logfile, svn_logfile, in the standard
Apache logs directory.
The %t and %u
variables are replaced by the time and username of the
request, respectively. The really important parts are the
two instances of SVN-ACTION.
When Apache sees that variable, it substitutes the value of
the SVN-ACTION environment variable,
which is automatically set by mod_dav_svn
whenever it detects a high-level client action.
So, instead of having to interpret a
traditional access_log like
this:
[26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc/!svn/vcc/default HTTP/1.1" 207 398 [26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc/!svn/bln/59 HTTP/1.1" 207 449 [26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc HTTP/1.1" 207 647 [26/Jan/2007:22:25:29 -0600] "REPORT /svn/calc/!svn/vcc/default HTTP/1.1" 200 607 [26/Jan/2007:22:25:31 -0600] "OPTIONS /svn/calc HTTP/1.1" 200 188 [26/Jan/2007:22:25:31 -0600] "MKACTIVITY /svn/calc/!svn/act/e6035ef7-5df0-4ac0-b811-4be7c823f998 HTTP/1.1" 201 227 …
you can peruse a much more
intelligible svn_logfile like
this:
[26/Jan/2007:22:24:20 -0600] - get-dir /tags r1729 props [26/Jan/2007:22:24:27 -0600] - update /trunk r1729 depth=infinity send-copyfrom-args [26/Jan/2007:22:25:29 -0600] - status /trunk/foo r1729 depth=infinity [26/Jan/2007:22:25:31 -0600] sally commit r1730
For an exhaustive list of all actions logged, see the section called “High-level logging”.
One of the nice advantages of using Apache as a Subversion server is that it can be set up for simple replication. For example, suppose that your team is distributed across four offices around the globe. The Subversion repository can exist only in one of those offices, which means the other three offices will not enjoy accessing it—they're likely to experience significantly slower traffic and response times when updating and committing code. A powerful solution is to set up a system consisting of one master Apache server and several slave Apache servers. If you place a slave server in each office, users can check out a working copy from whichever slave is closest to them. All read requests go to their local slave. Write requests get automatically routed to the single master server. When the commit completes, the master then automatically “pushes” the new revision to each slave server using the svnsync replication tool.
This configuration creates a huge perceptual speed increase for your users, because Subversion client traffic is typically 80–90% read requests. And if those requests are coming from a local server, it's a huge win.
In this section, we'll walk you through a standard setup of this single-master/multiple-slave system. However, keep in mind that your servers must be running at least Apache 2.2.0 (with mod_proxy loaded) and Subversion 1.5 (mod_dav_svn).
First, configure your master server's
httpd.conf file in the usual way.
Make the repository available at a certain URI location,
and configure authentication and authorization however
you'd like. After that's done, configure each of your
“slave” servers in the exact same way, but
add the special SVNMasterURI directive
to the block:
<Location /svn> DAV svn SVNPath /var/svn/repos SVNMasterURI http://master.example.com/svn … </Location>
This new directive tells a slave server to redirect all write requests to the master. (This is done automatically via Apache's mod_proxy module.) Ordinary read requests, however, are still serviced by the slaves. Be sure that your master and slave servers all have matching authentication and authorization configurations; if they fall out of sync, it can lead to big headaches.
Next, we need to deal with the problem of infinite recursion. With the current configuration, imagine what will happen when a Subversion client performs a commit to the master server. After the commit completes, the server uses svnsync to replicate the new revision to each slave. But because svnsync appears to be just another Subversion client performing a commit, the slave will immediately attempt to proxy the incoming write request back to the master! Hilarity ensues.
The solution to this problem is to have the master
push revisions to a different
<Location> on the slaves. This
location is configured to not proxy
write requests at all, but to accept normal commits from
(and only from) the master's IP address:
<Location /svn-proxy-sync> DAV svn SVNPath /var/svn/repos Order deny,allow Deny from all # Only let the server's IP address access this Location: Allow from 10.20.30.40 … </Location>
Now that you've configured
your Location blocks on master and
slaves, you need to configure the master to replicate to
the slaves. This is done the usual way—
using svnsync. If you're not familiar
with this tool, see
the section called “Repository Replication” for
details.
First, make sure that each slave repository has a
pre-revprop-change hook script which
allows remote revision property changes. (This is
standard procedure for being on the receiving end of
svnsync.) Then log into the master
server and configure each of the slave repository URIs to
receive data from the master repository on the local
disk:
$ svnsync init http://slave1.example.com/svn-proxy-sync file:///var/svn/repos Copied properties for revision 0. $ svnsync init http://slave2.example.com/svn-proxy-sync file:///var/svn/repos Copied properties for revision 0. $ svnsync init http://slave3.example.com/svn-proxy-sync file:///var/svn/repos Copied properties for revision 0. # Perform the initial replication $ svnsync sync http://slave1.example.com/svn-proxy-sync Transmitting file data .... Committed revision 1. Copied properties for revision 1. Transmitting file data ....... Committed revision 2. Copied properties for revision 2. … $ svnsync sync http://slave2.example.com/svn-proxy-sync Transmitting file data .... Committed revision 1. Copied properties for revision 1. Transmitting file data ....... Committed revision 2. Copied properties for revision 2. … $ svnsync sync http://slave3.example.com/svn-proxy-sync Transmitting file data .... Committed revision 1. Copied properties for revision 1. Transmitting file data ....... Committed revision 2. Copied properties for revision 2. …
After this is done, we configure the master server's
post-commit hook script to invoke
svnsync on each slave server:
#!/bin/sh # Post-commit script to replicate newly committed revision to slaves svnsync sync http://slave1.example.com/svn-proxy-sync > /dev/null 2>&1 & svnsync sync http://slave2.example.com/svn-proxy-sync > /dev/null 2>&1 & svnsync sync http://slave3.example.com/svn-proxy-sync > /dev/null 2>&1 &
The extra bits on the end of each line aren't
necessary, but they're a sneaky way to allow the sync
commands to run in the background so that the Subversion
client isn't left waiting forever for the commit to
finish. In addition to this
post-commit hook, you'll need a
post-revprop-change hook as well so
that when a user, say, modifies a log message, the slave
servers get that change also:
#!/bin/sh
# Post-revprop-change script to replicate revprop-changes to slaves
REV=${2}
svnsync copy-revprops http://slave1.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1 &
svnsync copy-revprops http://slave2.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1 &
svnsync copy-revprops http://slave3.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1 &
The only thing we've left out here is what to do about
locks. Because locks are strictly enforced by the master
server (the only place where commits happen), we don't
technically need to do anything. Many teams don't use
Subversion's locking features at all, so it may be a
nonissue for you. However, if lock changes aren't
replicated from master to slaves, it means that clients
won't be able to query the status of locks
(e.g., svn status -u will show no
information about repository locks). If this bothers you,
you can write post-lock and
post-unlock hook scripts that run
svn lock and svn
unlock on each slave machine, presumably through
a remote shell method such as SSH. That's left as an
exercise for the reader!
Your master/slave replication system should now be ready to use. A couple of words of warning are in order, however. Remember that this replication isn't entirely robust in the face of computer or network crashes. For example, if one of the automated svnsync commands fails to complete for some reason, the slaves will begin to fall behind. For example, your remote users will see that they've committed revision 100, but then when they run svn update, their local server will tell them that revision 100 doesn't yet exist! Of course, the problem will be automatically fixed the next time another commit happens and the subsequent svnsync is successful—the sync will replicate all waiting revisions. But still, you may want to set up some sort of out-of-band monitoring to notice synchronization failures and force svnsync to run when things go wrong.
Several of the features already provided by Apache in
its role as a robust web server can be leveraged for
increased functionality or security in Subversion as well.
The Subversion client is able to use SSL (the Secure Sockets
Layer, discussed earlier). If your Subversion client is
built to support SSL, it can access your Apache server
using https:// and enjoy a high-quality
encrypted network session.
Equally useful are other features of the Apache and Subversion relationship, such as the ability to specify a custom port (instead of the default HTTP port 80) or a virtual domain name by which the Subversion repository should be accessed, or the ability to access the repository through an HTTP proxy.
Finally, because mod_dav_svn is speaking a subset of the WebDAV/DeltaV protocol, it's possible to access the repository via third-party DAV clients. Most modern operating systems (Win32, OS X, and Linux) have the built-in ability to mount a DAV server as a standard network “shared folder.” This is a complicated topic, but also wondrous when implemented. For details, read Appendix C, WebDAV and Autoversioning.
Note that there are a number of other small tweaks one can
make to mod_dav_svn that are too obscure
to mention in this chapter. For a complete list of
all httpd.conf directives
that mod_dav_svn responds to, see
the section called “Directives”.
[45] They really hate doing that.
[46] While self-signed server certificates are still vulnerable to a “man-in-the-middle” attack, such an attack is much more difficult for a casual observer to pull off, compared to sniffing unprotected passwords.
[47] More security-conscious folk might not want to store
the client certificate password in the runtime
servers file.
[48] Back then, it was called ViewCVS.