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I have a Ruby on Rails application deployed with Phusion passenger + Apache web server. Does each request runs in its own thread spawned by Phusion Passenger?
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Phusion Passenger® is an open source web application server. It handles HTTP requests, manages processes and resources, and enables administration, monitoring and problem diagnosis. Passenger is very easy to use, makes deploying in production much easier and is scalable.
Ruby makes it easy to write multi-threaded programs with the Thread class. Ruby threads are a lightweight and efficient way to achieve concurrency in your code.
The Ruby Interpreter is single threaded, which is to say that several of its methods are not thread safe. In the Rails world, this single-thread has mostly been pushed to the server.
In particular, Ruby concurrency is when two tasks can start, run, and complete in overlapping time periods. It doesn't necessarily mean, though, that they'll ever both be running at the same instant (e.g., multiple threads on a single-core machine).
Passenger (along with most other application servers) runs no more than one request per thread. Typically there is also only one thread per process. From the Phusion Passenger docs:
Phusion Passenger supports two concurrency models:
process: single-threaded, multi-processed I/O concurrency. Each application process only has a single thread and can only handle 1 request at a time. This is the concurrency model that Ruby applications traditionally used. It has excellent compatibility (can work with applications that are not designed to be thread-safe) but is unsuitable workloads in which the application has to wait for a lot of external I/O (e.g. HTTP API calls), and uses more memory because each process has a large memory overhead.
thread: multi-threaded, multi-processed I/O concurrency. Each application process has multiple threads (customizable via PassengerThreadCount). This model provides much better I/O concurrency and uses less memory because threads share memory with each other within the same process. However, using this model may cause compatibility problems if the application is not designed to be thread-safe.
(Emphasis my own)
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