How does the rails connection pooling work?

Question

I'm running a rails 2.3.5 application, which supports me to pool mysql connections to my database. But I remember reading that my mongrel servers are single threaded. What's the point of having a connection pool, to a single threaded application? Is there a way to multi-thread my app?

Also, do connection pools understand, that ruby 1.8 has "green" threads?

Cheers!

Answer 1

Manage Connections

The major benefit of connection pooling for a single-thread server like Mongrel/Passenger/etc is that the connection is established/maintained in a Rack handler outside the main Rails request processing. This allows for a connection to be established once vs. many times as it's used in different ways. The goal is to re-use the established connection and minimize the number of connections. This should prevent having to reconnect within a given request processing cycle and possibly even between requests (if I recall correctly).

Multiple Concurrent Connections

Although most use cases (Mongrel/Passenger) are single threaded and can only use a single connection at a time - there is JRuby and environments/app servers that have full multi-threaded support. Rails has been thread safe since 2.2

TL;DR:

Pool establishes connection automatically. Some people do use multiple concurrent db connections from pool.