Correct setting of database connection pool database.yml for single-threaded Rails applications

Question

I was wondering about the following setting in the Rails database.yml:

By default the number of database connections for the connection pool of ActiveRecord is set to 5:

development:
  ...
  pool: 5

But by default, Rails 3 is single threaded. Why would you need 5 connections by default?

As far as I understand, a single threaded Rails app can't trigger multiple database operations at once, why would do you need to keep more connection open?

I would assume that 2 connections would make sense, so you always have one active connection even if the other one times out, but holding five connections seems a little odd to me.

Am I missing something?

UPDATE If anyone else is curious, I just found a commit that explains it: https://github.com/rails/rails/commit/b700153507b7d539a57a6e3bcf03c84776795051

In fact these default settings don't make any sense, it was fixed but then temporarily reverted (one year ago) because of the test suite.

Answer 1

Quite late to the party here, but I ran out of database connections today in production.

Like a lot of people, I use Sidekiq to perform asynchronous jobs like sending emails for example. It is important to note that Sidekiq runs as a multithread process.

So, I don't just have a single-threaded Rails application, therefore this answer does not directly apply to the question asked but I thought it was worth saying something here as I think multithreaded Rails apps are relatively normal nowadays.

This means you need to adjust your pool size in such a way as to create enough connections to handle all the jobs that can be enqueued and take longer than 5 seconds (the default timeout period to wait for a database connection before throwing an error).

Answer 2

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

• Connection pooling is handled inside of ActiveRecord, so all application servers should behave basically the same.

• The database connection pool starts out empty and creates connections over time according to demand. The maximum size of this pool defaults to 5 and is configured in database.yml.

• Requests and users share connections from this pool. A request checks out a connection the first time it needs to access the database and then checks the connection back in at the end of the request.

• If you use Rails.threadsafe! mode, then multiple threads might be accessing multiple connections at the same time, so depending on the request load you might have multiple threads contending for a few connections.

You can change accordingly, If you are using single threaded app. Default is 5 as per most user's need, as now a day its normal to have a multithreaded app.