The Concurrent Versions System (CVS) is an open source version-control system that allows multiple developers to work on code or configuration in a central repository while tracking any changes made. We use it because it's the dominant open source tool, but it's not the only possibility: commercial tools such as Perforce would also work for these purposes.
If you aren't familiar with CVS, you can learn about it from the resources provided at the end of this chapter. CVS is too broad a topic to be covered in this book. Instead, we will concentrate on the CVS techniques that are relevant to our purpose.
Things are much simpler when using CVS for server updates, especially since it allows you to tag each production release. By tagging files, we mean having a group of files under CVS control share a common label. Like RCS and other revision-control systems, CVS gives each file its own version number, which allows us to manipulate different versions of this file. But if we want to operate on a group of many files, chances are that they will have different version numbers. Suppose we want to take snapshots of the whole project so we can refer to these snapshots some time in the future, after the files have been modified and their respective version numbers have been changed. We can do this using tags.
To tag the project whose module name is myproject, execute the following from any directory on any machine:
panic% cvs -rtag PRODUCTION_1_20 myproject
Now when the time comes to update the online version, go to the directory on the live machine that needs to be updated and execute:
panic% cvs update -dP -r PRODUCTION_1_20
The -P option to cvs prunes empty directories and deleted files, the -d option brings in new directories and files (like cvs checkout does), and -r PRODUCTION_1_20 tells CVS to update the current directory recursively to the PRODUCTION_1_20 CVS version of the project.
Suppose that after a while, we have more code updated and we need to make a new release. The currently running version has the tag PRODUCTION_1_20, and the new version has the tag PRODUCTION_1_21. First we tag the files in the current state with a new tag:
panic% cvs -rtag PRODUCTION_1_21 myproject
and update the live machine:
panic% cvs update -dP -r PRODUCTION_1_21
Now if there is a problem, we can go back to the previous working version very easily. If we want to get back to version PRODUCTION_1_20, we can run the command:
panic% cvs update -dP -r PRODUCTION_1_20
As before, the update brings in new files and directories not already present in the local directory (because of the -dP options).
Remember that when you use CVS to update the live server, you should avoid making any minor changes to the code on this server. That's because of potential collisions that might happen during the CVS update. If you modify a single line in a single file and then do cvs update, and someone else modifies the same line at the same time and commits it just before you do, CVS will try to merge the changes. If they are different, it will see a conflict and insert both versions into the file. CVS leaves it to you to resolve the conflict. If this file is Perl code, it won't compile and it will cause temporal troubles until the conflict is resolved. Therefore, the best approach is to think of live server files as being read-only.
Updating the live code directory should be done only if the update is atomic—i.e., if all files are updated in a very short period of time, and when no network problems can occur that might delay the completion of the file update.
The safest approach is to use CVS in conjunction with the safe code update technique presented previously, by working with CVS in a separate directory. When all files are extracted, move them to the directory the live server uses. Better yet, use symbolic links, as described earlier in this chapter: when you update the code, prepare everything in a new directory and, when you're ready, just change the symlink to point to this new directory. This approach will prevent cases where only a partial update happens because of a network or other problem.
The use of CVS needn't apply exclusively to code. It can be of great benefit for configuration management, too. Just as you want your mod_perl programs to be identical between the development and production servers, you probably also want to keep your httpd.conf files in sync. CVS is well suited for this task too, and the same methods apply.
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