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Multiuser Subversion

by Rafael Garcia-Suarez
12/19/2002

Installing a Subversion Server

In a previous article, we saw how to install and use Subversion, in the context of a single user, and use it on a single system. This article will take you one step beyond and show how to install a Subversion server, accessible via the network and to a team of users. We'll see also how Subversion can help to manage a project to which several developers contribute.

Subversion uses Apache as its HTTP transport layer. The command-line client communicates with remote servers with a protocol built on the WebDAV/DeltaV HTTP 1.1 extensions. This means that a Subversion server, while not completely implementing WebDAV, can respond to common HTTP and WebDAV (read-only) clients such as Web browsers and file explorers.

Building a Subversion-powered Apache server

You'll need a recent version of Apache 2 to build a Subversion server. The build process is obviously a bit more complex than for a stand alone client, as you should build an Apache with the specific module mod_dav_svn. That's why I'll detail the necessary steps.

As in my previous article, I'll assume you're building from a source snapshot on an Unix platform and that you will install everything under /usr/local/subversion-rXXXX (where XXXX is the revision number of the snapshot you're using, this directory being linked from /usr/local/subversion). This directory layout makes it easier to upgrade Subversion, an important point when using a software that has not reached a stable version yet. In this case, make sure your system can find shared libraries in /usr/local/subversion/lib. (To build Subversion from the latest sources, refer to the INSTALL file, which explains how to checkout and compile from the development server.) At the time of this writing, the current stable version is 0.16, revision number 3987.

The first step is to set up the subversion directory and build the database. As of subversion 0.15, you will need the Berkeley DB, version 4.0.14. If it's not installed on your system, install it in the same directory as Subversion.

# mkdir /usr/local/subversion-rXXXX
# ln -s /usr/local/subversion-rXXXX /usr/local/subversion

$ gunzip -c db-4.0.14.tar.gz | tar xf -
$ cd db-4.0.14/build_unix
$ ../dist/configure --prefix=/usr/local/subversion-rXXXX
$ make

# make install

Then, install Apache. You should get at least version 2.0.42. The --enable-dav option will build and include mod_dav and --enable-so will enable DSO (Dynamic Shared Object) support. The other options specify that Apache will be installed with Subversion and will use the Berkeley DB you've just built, an important point.

$ gunzip -c httpd-2.0.43.tar.gz | tar xf -
$ cd httpd-2.0.43
$ ./configure --prefix=/usr/local/subversion-rXXXX \
	--enable-dav --enable-so \
	--with-dbm=db4 --with-berkeley-db=/usr/local/subversion-rXXXX
$ make

# make install

Finally, compile and install Subversion. The important option here is --with-apxs, giving the path to the apxs(8) tool, used to build and install Apache extension modules. In this case, the installation process should install mod_dav_svn, the Apache module that handles requests from Subversion clients.

$ cd ..
$ gunzip -c subversion-rXXXX.tar.gz | tar xf -
$ cd subversion-rXXXX
$ ./configure --prefix=/usr/local/subversion-rXXXX \
	--with-berkeley-db=/usr/local/subversion-rXXXX \
	--with-apxs=/usr/local/subversion-rXXXX/bin/apxs
$ make
$ make check         # optional : runs the tests

# make install

That's it.

Configuring the Server

As Apache provides the network layer, a Subversion server gets all the Apache features for free: robustness, HTTP authentication, data compression via mod_deflate, encryption via mod_ssl, etc.

To enable some URLs to be handled by mod_dav_svn, you only have to add in the httpd.conf file:

<Location /svn>
	DAV svn
	SVNPath /home/rafael/svn
</Location>

This block tells Apache to serve paths that begin with /svn via mod_dav_svn, and that they map to the Subversion repository kept at the physical location /home/rafael/svn. To check out a working copy of the trunk of the frobnizer project from this repository, the appropriate command becomes:

$ svn checkout http://my.host/svn/frobnizer/trunk work-dir

By default, Apache listens on port 80, and this means that it must be started by the root user. You can change this if you want. Note, however, that the Apache processes must be able to write to the Subversion repository. One of the clean ways to do this is to create an account svn, on your system, to own the Subversion repository. Configure Apache to run as this user via the User configuration directive. Once you've done this, you're ready to start the server:

$ /usr/local/subversion/bin/apachectl start

To restrict read or write access to your repository, you can use Apache's access control features. You can limit the hosts that are allowed to access the repository and require a username and a password for some operations (via HTTP authentication). For example, the following lines, inserted in the <Location> block, limit write access to a specific group of users:

<LimitExcept GET PROPFIND OPTIONS REPORT>
	Require group commiters
</LimitExcept>

This specifies that all requests but those using the listed HTTP methods should be issued by a authenticated user, a member of the commiters group. (See http://httpd.apache.org/docs-2.0/mod/core.html for a detailed description of access control and of the related configuration directives.) When the commit has been done via HTTP, the Subversion-generated change logs will record the HTTP username.

Note that the Subversion client caches your username and password in your working copy by default, so you won't have to enter it each time you access the repository. If you're worried about security, you can disable this behavior via the user configuration file $HOME/.subversion/config or in the global configuration file /etc/subversion/config on the client machine.

Related Reading

Apache: The Definitive Guide
By Ben Laurie, Peter Laurie

If you want to deploy Subversion in your organization, you may find it easier to compile a statically linked svn client. This way, you only have to copy this binary file to the users machines, without having to install the required libraries. (You'll need, however, to compile this client separately using the --enable-all-static configure option. It's not possible to build a statically linked svn and mod_dav_svn at the same time. The Apache module needs to be built as a shared library.)

Collaborative Use of Subversion

When several users have commit access to the Subversion repository, they need, at some point, to integrate in their working copies the changes committed by others. The svn update command performs this task, updating its targets (by default, the current directory and its contents, recursively) to the latest revision.

What happens when you update a file that you've edited locally, but on which another user has committed his own edits? If you've read the first article, you already know that Subversion doesn't require that you lock a file when you want to modify it. This behavior is different from some other source control software that prevent you from editing a file that has been reserved by another user. Subversion attempts to merge the changes you've made with the ones made by others.

While performing an update, svn will print a list of updated files, marking each line with a letter to indicate the action taken on the corresponding file. It was (A)dded, (D)eleted, (U)pdated, and, when your local copy has changes, they have been mer(G)ed, or the file remains in a (C)onflicted state. For example, the following output:

U  foo.c
G  bar.c
C  hot.c

indicates that foo.c was updated, and that it didn't have any local change. Your local edits to bar.c were successfully merged. Subversion didn't know how to merge on hot.c, because the server's changes overlapped with yours.

In this last case, Subversion will create three temporary files containing your local version, the repository version you used as a basis for your edits, and the latest version from the repository. Your original file will be modified as well: mergeable changes will be merged, and other changes will be indicated with conflict markers similar to those inserted by the diff3(1) command (specifically, diff3 -E).

You will then have to resolve those conflicts manually and delete the temporary files. (To protect against unwanted commits, Subversion will not let you commit a file when it detects this temporary file lying around.) For convenience, the svn resolve command deletes those temporary files for you.

Note also that Subversion will not let you commit files for which a more recent revision exists. You'll need to update them first.

Metadata

An interesting feature of Subversion is that it allows an arbitrary amount of metadata to be attached to any versioned resource (files and directories). Metadata, like files, are also versioned. This metadata is a set of key/value pairs, known as properties.

You can use properties to annotate files (let's say, review-status to this file is being reviewed by Joe), or directories. For example, on a tag directory frobnizer-1.09b, you can add a note beta release, for internal use only.

Properties whose names begin with svn: are reserved by Subversion. Some of them are specifically recognized and handled. For example, svn:executable marks a file to be checked-out with the executable flag set on filesystems that support it. Other interesting properties include:

Branches, Switching and Merging

I presented an overview of tags and branches in the first article. Branches prove to be useful mainly on multiuser projects. With branches, a developer or a team of developers, can work on an experimental feature without interfering with the main development. Branches can also be used to keep track of changes in a maintenance version of a product. They can be used to handle beta releases and the returns of beta-testers. Subversion makes it easy and cheap to create branches, so why shouldn't you use them?

A branch is typically created outside a working copy to spare disk space with the form of the copy command that operates directly on URLs. The following creates a maintenance branch from the current project trunk (by convention, the root directory of your project files):

$ svn copy http://my.host/svn/frobnizer/trunk \
	http://my.host/svn/frobnizer/maint-1.0

To start working on a branch, you can of course check out a whole new working copy corresponding to it, but it's easier to switch your working copy (or parts of it) to the branch. Assuming that your working copy already contains the trunk, you can make it point at the maint-1.0 branch with the following command, issued at the root of your working copy:

$ svn switch http://my.host/svn/frobnizer/maint-1.0 .

svn switch works a bit like update, except that it doesn't conceptually move your working copy through time (revisions), but through space (branches). Consistently, the output of switch is similar to that of update.

Finally, to integrate changes from another branch in your working copy, you can use the powerful svn merge command. merge can be compared to diff, but instead of outputting changes as a unified diff to standard output, it applies them to your working copy (as you would apply a patch).

For example, the following commands integrate a batch of changes from the trunk into your maintenance working copy:

$ svn merge -r149:155 http://my.host/svn/frobnizer/trunk
U  foo.c
U  foo.h
A  bar.c

$ svn status
M      foo.c
M      foo.h
A  +   bar.c

$ svn commit -m 'Integrate revisions 150 to 155 from the trunk'

The output of svn merge indicates which files have been affected by the changes, just like svn update. The svn status command reports the status of files in your working copy. In this exmaple, foo.c and foo.h have local modifications, and bar.c is scheduled for addition in the next commit. The + sign in the status line indicates that Subversion knows that this file has been branched from elsewhere, and will retain this information when committing it.

Note that you may have to resolve manually potential conflicts between the merge and the commit.

An interesting use of merge is to fetch changes from the current branch. This way you can roll back a change. The following command applies the changes of revision 200 to your working copy, in reverse:

$ svn merge -r200:199 .

Meanwhile, in the Real World

Once you're getting used to Subversion, you'll find that it can help you in ways you didn't expect.

A first example would be dealing with runtime configuration files during upgrades. Working on a software project where some global configuration variables (IP addresses, environment variables or trace levels) are read in a file, I have to modify this file almost every time the application is installed on another machine. A default configuration file is provided with the sources, and it's kept under version control because the set of configuration variables changes over versions. What to do when I want to upgrade a snapshot installed on a test machine, but without losing the local changes to the config file?

The solution is simple. Instead of installing the application on the test machine from a source tarball, I copied my statically linked svn client there and performed a check-out. Then, I adjusted the configuration files to my needs, without commiting the changes. Upgrading the software to another development snapshot is as easy as an svn update followed by a make all . The update process integrates the common changes to the configuration file (e.g., new variables) with the local changes (e.g., user/password to access a local database). This can be applied to any evolving configuration file you need to deploy: an /etc/profile, a .vimrc, or an httpd.conf.

Branches can also be used in creative ways. I work on an intranet application that ships with several sets of style sheets. For example, some are designed for low resolution screens and provide different font sizes and background images.

I set up two directories, /html/css/800 and /html/css/1024, to hold those CSS. The 1024 directory, for style sheets adapted to a higher screen resolution, is a branch of the other. When a CSS is augmented or modified, it's easy to incorporate its changes into the other branch while leaving the specific font or image settings untouched.

I'm sure Subversion will be used in ways its designers didn't imagine. That's what makes the success of a tool.

Hints and Links

Thanks to Karl Fogel for having kindly reviewed this article.

Rafael Garcia-Suarez is a French software engineer and Unix system administrator.


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