Problem: We had three internal rails applications running in one physical serve. During some of the deployments, one of the applications was not booting up. Though occurrence of such an event was very low I looked into the issue as it stimulated me.

We had a master passenger.conf and each application had their own application.conf, which looked something like this:

Master

PassengerRoot /usr/local/rvm/gems/ruby-2.6.0/gems/passenger-6.0.2
PassengerDefaultRuby /usr/local/rvm/gems/ruby-2.6.0/wrappers/ruby
PassengerMaxPoolSize 18
PassengerPoolIdleTime 300

Application specific configs

PassengerAppEnv qa
PassengerRuby /usr/local/rvm/gems/gemset/wrappers/ruby
PassengerMinInstances 9
PassengerPreStart http://localhost:81/

If we carefully look at the PassengerMaxPoolSize and PassengerMinInstances for three applications, they didn't add up. Forget request load balancing, passenger didn't even have space for booting the application because Max was set to 18 and each application's min was 9. This meant, at any given time there could be only two applications running.

Fortunately, that wasn't the case. The deployment sequence decided the number of instances of each application that will occupy the 18 available slots. If all the three were deployed simultaneously then each occupied x,y,z number of instances based on the application's booting time.

Some of the examples of numbers were:

Solution: We had to change PassengerMinInstances to a number that was suitable for the application. It was also made sure that passenger get a couple of free instances for Request load balancing.