The Passenger installation process really is as easy as it’s been described, particularly if you’re as comfortable with Apache configuration as we are, but by moving responsibility for serving your apps from Mongrel to Apache, file and process ownership becomes an issue in a way it wasn’t when Mongrel and Apache talked to each other and otherwise minded their own stores. Ben Hughes hinted at this in his walk-through of the Passenger installation, and we were not immune.

Our problem turned out to be session files. The first application we moved to Passenger (and it’s worth noting that apps running on Passenger and apps running Mongrel cluster can coexist under the same Apache) was Redmine, which uses file-based session store by default. If ownership of those session files creates a problem, and it did for us, switching to ActiveRecord session storage resolves the problem.

For those wondering about the point of Passenger, it’s memory use. A single Mongrel instance and an app running under Passenger have comparable RAM requirements, but because Passenger is an Apache module, it can spawn new instances as requests arrive; the Mongrel cluster solution requires multiple mongrel instances to be running all the time. This means Mongrel apps essentially require the RAM for peak capacity all the time. Passenger gives that RAM back at non-peak times, which allows more apps to share the same system, allowing us to get more mileage from our VPSes. (It’s worth noting that the Phusion guys claim a 33% memory savings per instance if Ruby Enterprise Edition is also used, so that will probably be our next step when all the apps are Passenger-ized.)

Update, 12/10: Another gotcha we found was file permissions. Again because of the permissions issues, Rails wasn’t writing to the shared/logs/production.log file. We didn’t notice this immediately, because the applications ran, they just didn’t log anything. The solution here was to change the ownership and permissions on the files. Specifically, we changed group ownership to “nobody” (the group Apache runs under) and the mode of the log file to 664.