[troels]

I wonder what problem this is really solving. I mean, delete+create in a script will happen pretty fast after each other, so the moment of inconsistency is really very short. If this is an actual problem for you, chances are that your setup is rather large and you have multiple nodes behind a load balancer. In that case, you have bigger issues, such as making sure the individual nodes are updated at the same time. Usually this would be solved by taking them out of rotation while updating, in which case the atomic symlink switch becomes moot.

[regularfry]

The problem is that the issues created by out-of-sync code that happens to get loaded in the wrong order can be an absolute nightmare to debug. You'll have transient, unreproducible, potentially data-destroying bugs which vary with each release. Some releases you might get lucky, some not. If you don't think about atomicity of the deployment, you can chase your tail for days trying to figure out what went wrong.

That being said, this strikes me as more of a pain under the traditional PHP model, where reloading code from disc per request is normal, than for something like Rails which loads everything into memory once at launch.

[martinaglv]

With APC (and stat disabled) PHP behaves in exactly the same way. Bytecode is kept in memory between requests and you can then safely push a new version of your directory tree. All that is required is to flush the cache to have the new version go live.

[DougWebb]

At my old job our webapp handled 6+ million requests per day, and during the 8hr peak period it handled just over 100 requests per second. When you're dealing with request rates like that, even really brief periods of inconsistency will cause problems for someone.

We handled deployments a different way though. Each release went into a different directory and was made available under a different url, and users got redirected to the newest release when they logged in. Once in a session they stayed with the same version until they logged out. This also allowed us to do limited deployments; we could choose which version each customer group was sent to.