Active Record Connection Pool

Connection pool base class for managing Active Record database connections.

Introduction

A connection pool synchronizes thread access to a limited number of database connections. The basic idea is that each thread checks out a database connection from the pool, uses that connection, and checks the connection back in. ConnectionPool is completely thread-safe, and will ensure that a connection cannot be used by two threads at the same time, as long as ConnectionPool’s contract is correctly followed. It will also handle cases in which there are more threads than connections: if all connections have been checked out, and a thread tries to checkout a connection anyway, then ConnectionPool will wait until some other thread has checked in a connection, or the checkout_timeout has expired.

Obtaining (checking out) a connection

Connections can be obtained and used from a connection pool in several ways:

1. Simply use ActiveRecord::Base.connection When you’re done with the connection(s) and wish it to be returned to the pool, you call {ActiveRecord::Base.connection_handler.clear_active_connections!}[rdoc-ref:ConnectionAdapters::ConnectionHandler#clear_active_connections!]. This is the default behavior for Active Record when used in conjunction with Action Pack’s request handling cycle.

2. Manually check out a connection from the pool with {ActiveRecord::Base.connection_pool.checkout}[rdoc-ref:#checkout]. You are responsible for returning this connection to the pool when finished by calling {ActiveRecord::Base.connection_pool.checkin(connection)}[rdoc-ref:#checkin].

3. Use {ActiveRecord::Base.connection_pool.with_connection(&block)}[rdoc-ref:#with_connection], which obtains a connection, yields it as the sole argument to the block, and returns it to the pool after the block completes.

Connections in the pool are actually AbstractAdapter objects (or objects compatible with AbstractAdapter’s interface).

While a thread has a connection checked out from the pool using one of the above three methods, that connection will automatically be the one used by ActiveRecord queries executing on that thread. It is not required to explicitly pass the checked out connection to Rails models or queries, for example.

Options

There are several connection-pooling-related options that you can add to your database connection configuration:

• pool: maximum number of connections the pool may manage (default 5).

• idle_timeout: number of seconds that a connection will be kept unused in the pool before it is automatically disconnected (default 300 seconds). Set this to zero to keep connections forever.

• checkout_timeout: number of seconds to wait for a connection to become available before giving up and raising a timeout error (default 5 seconds).