connection-pooling.txt

Database connections are now pooled, one pool per #establish_connection call.
Pools start out empty and grow as necessary to a maximum size (default is 5,
configure size with key 'pool' in your database configuration). If no
connections are available, a thread will wait up to a 'wait_timeout' time
(default is 5 seconds).

Connections are verified and reset when checked out from the pool (usually
upon first access to ActiveRecord::Base.connection), and returned back to the
pool after each request.

If you would like to use connection pools outside of ActionPack, there is an
ActiveRecord::Base.connection_pool method that gives you access to the pool,
and you can manually checkout/checkin connections, or supply a block to
ActiveRecord::Base.connection_pool.with_connection which takes care of the
checkout/checkin for you.