Just got a response back from the SVN user group. According to Ryan Schmidt (thanks Ryan!) “svn switch, like its lesser cousin svn update, is not atomic.”

Oh well. Symlinks are the way to go for me.

Thanks,

Mike

Also if your site is very high concurrency and you have multiple web servers, how about off-lining one while you do the update. The svn update itself doesn’t take long at all and you could be back to full service very quickly.

I hope these ideas have helped.

How safe is this though? Does svn replace the files in an atomic manner so that half written to files are not accidentally served?

The PHP APC docs explain the issue best:

“When you modify a file on a live web server you really should do so in an atomic manner. That is, write to a temporary file and rename (mv) the file into its permanent position when it is ready. Many text editors, cp, tar and other such programs don’t do this. This means that there is a chance that a file is accessed (and cached) while it is still being written to. This apc.file_update_protection setting puts a delay on caching brand new files. The default is 2 seconds which means that if the modification timestamp (mtime) on a file shows that it is less than 2 seconds old when it is accessed, it will not be cached. The unfortunate person who accessed this half-written file will still see weirdness, but at least it won’t persist. If you are certain you always atomically update your files by using something like rsync which does this correctly, you can turn this protection off by setting it to 0. If you have a system that is flooded with io causing some update procedure to take longer than 2 seconds, you may want to increase this a bit. ” - http://ca3.php.net/manual/en/apc.configuration.php

I looked at the svn docs and can’t find any details on the method used to replace files.

Thanks,

Mike