How does Phusion Passenger reuse threads and processes?

Question

I am setting up an Apache2 webserver running multiple Ruby on Rails web applications with Phusion Passenger. I know that Passenger spawns Ruby processes for handling requests. I have the following questions:

• If more than one request has to be handled at the same time, will Passenger spawn multiple processes or multiple (Ruby) threads? How do I configure it so it always spawns single-threaded processes?
• If I have two Rails applications, imagine that a request for app A goes to process 1, then later request for app B arrives. Is it possible that process 1 will handle this request as well? When and how is this possible? In other words, is one process allowed to handle requests for multiple Rails applications?
• I have the same Rails application exported in multiple URLs and multiple virtual hosts (such as http:// and https://). Will the same process be able to serve different virtual hosts? (The answer to this seems to be yes, I've set a global variable in answering a request to virtual host A, and I was able to retrieve the value in virtual host B.)

Answer 1

Generally speaking, Passenger spawns new processes by forking an ApplicationSpawner, which has the framework and application code pre-loaded into memory, or a FrameworkSpawner, which just has the framework code.

Passenger, as far as I know, doesn't deal in threads. Instead, as the load increases on an application, it will fork that Application's ApplicationSpawner and initialize another instance. When load decreases, one or more application instances are killed off.

If Passenger is configured in a certain way (I believe by choosing the "smart" spawn method), it will create a FrameworkSpawner, which loads the rails code, but no application code, which can then be forked to load and application using that version of Rails.

So to answer your questions:

• It will serve them sequentially, then spawn additional processes if it decides the load is high enough.

• No. One process can only belong to a single Rails Application.

• I'm kind of sketchy on this one, but your experiment makes sense. Passenger should be smart enough to figure out that even though it's running from different places in the server config, you're talking about the same application. It's probably based on the application's filesystem path.

EDIT: I went and read up on this a bit. Turns out I was mostly right, but the technical details were a bit off. See the Passenger documentation

Answer 2

Yup, Burke is right. In case of the third question, Phusion Passenger recognizes applications by their application root path. So even if you have two virtual hosts, if they both point to the same DocumentRoot then Phusion Passenger will think that they're the same app.