The secret to perfectly calculate Rails database connection pool size

Ruby on Rails maintains a pool of database connections for Active Record. When a database connection is needed for querying the database, usually one per thread (though that’s changing to per-transaction), a connection is checked out of the pool, used, and then returned to the pool. The size of the pool is configured in the config/database.yml. The default, as of Rails 7.2, is pool: <%%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>.

The database connection pool size is frequently misconfigured. A lot. How to calculate the database connection pool size is one of the most common questions I get on GoodJob (Hi! I’m the author of GoodJob 👋). I have spent an embarrassingly large amount of time trying to come up with a precise pool size calculator and give advice to take into account Puma threads, and GoodJob async jobs, and load_async queries and everything that might be asking for a database connection at the same time. It’s nearly impossible to get the number exactly right.

If the connection pool is misconfigured to be too small , it can slow down web requests and jobs while waiting for a connection to become available, or raise ActiveRecord::ConnectionTimeoutError if there isn’t a connection available within a reasonable amount of time (5 seconds by default). That’s bad! We never want that to happen. Here’s what you should do:

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