Connection pool base class for managing Active Record database connections.
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.
Obtaining (checking out) a connection
Connections can be obtained and used from a connection pool in several ways:
Simply use ActiveRecord::Base.connection as with Active Record 2.1 and earlier (pre-connection-pooling). Eventually, when you’re done with the connection(s) and wish it to be returned to the pool, you call ActiveRecord::Base.clear_active_connections!. This will be the default behavior for Active Record when used in conjunction with Action Pack’s request handling cycle.
Manually check out a connection from the pool with ActiveRecord::Base.connection_pool.checkout. You are responsible for returning this connection to the pool when finished by calling ActiveRecord::Base.connection_pool.checkin(connection).
There are several connection-pooling-related options that you can add to your database connection configuration:
pool: number indicating size of connection pool (default 5)
checkout_timeout: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds).
reaping_frequency: frequency in seconds to periodically run the Reaper, which attempts to find and recover connections from dead threads, which can occur if a programmer forgets to close a connection at the end of a thread or a thread dies unexpectedly. Regardless of this setting, the Reaper will be invoked before every blocking wait. (Default nil, which means don’t schedule the Reaper).