Connection pool base class for managing ActiveRecord 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:

  1. Simply use ActiveRecord::Base.connection as with ActiveRecord 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 ActiveRecord when used in conjunction with ActionPack‘s request handling cycle.
  2. 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).
  3. Use ActiveRecord::Base.connection_pool.with_connection(&block), 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).


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

  • pool: number indicating size of connection pool (default 5)
  • wait_timeout: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds).
[R] spec
Public Class methods

Creates a new ConnectionPool object. spec is a ConnectionSpecification object which describes database connection information (e.g. adapter, host name, username, password, etc), as well as the maximum size for this ConnectionPool.

The default ConnectionPool maximum size is 5.

    # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 66
66:       def initialize(spec)
67:         @spec = spec
69:         # The cache of reserved connections mapped to threads
70:         @reserved_connections = {}
72:         # The mutex used to synchronize pool access
73:         @connection_mutex =
74:         @queue = @connection_mutex.new_cond
76:         # default 5 second timeout unless on ruby 1.9
77:         @timeout =
78:           if RUBY_VERSION < '1.9'
79:             spec.config[:wait_timeout] || 5
80:           end
82:         # default max pool size to 5
83:         @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5
85:         @connections = []
86:         @checked_out = []
87:       end
Public Instance methods

Check-in a database connection back into the pool, indicating that you no longer need this connection.

conn: an AbstractAdapter object, which was obtained by earlier by calling checkout on this pool.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 210
210:       def checkin(conn)
211:         @connection_mutex.synchronize do
212:           conn.run_callbacks :checkin
213:           @checked_out.delete conn
214:           @queue.signal
215:         end
216:       end

Check-out a database connection from the pool, indicating that you want to use it. You should call checkin when you no longer need this.

This is done by either returning an existing connection, or by creating a new connection. If the maximum number of connections for this pool has already been reached, but the pool is empty (i.e. they‘re all being used), then this method will wait until a thread has checked in a connection. The wait time is bounded however: if no connection can be checked out within the timeout specified for this pool, then a ConnectionTimeoutError exception will be raised.

Returns: an AbstractAdapter object.


     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 181
181:       def checkout
182:         # Checkout an available connection
183:         @connection_mutex.synchronize do
184:           loop do
185:             conn = if @checked_out.size < @connections.size
186:                      checkout_existing_connection
187:                    elsif @connections.size < @size
188:                      checkout_new_connection
189:                    end
190:             return conn if conn
191:             # No connections available; wait for one
192:             if @queue.wait(@timeout)
193:               next
194:             else
195:               # try looting dead threads
196:               clear_stale_cached_connections!
197:               if @size == @checked_out.size
198:                 raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout}.  The max pool size is currently #{@size}; consider increasing it."
199:               end
200:             end
201:           end
202:         end
203:       end

Clears the cache which maps classes

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 137
137:       def clear_reloadable_connections!
138:         @reserved_connections.each do |name, conn|
139:           checkin conn
140:         end
141:         @reserved_connections = {}
142:         @connections.each do |conn|
143:           conn.disconnect! if conn.requires_reloading?
144:         end
145:         @connections = []
146:       end

Return any checked-out connections back to the pool by threads that are no longer alive.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 159
159:       def clear_stale_cached_connections!
160:         remove_stale_cached_threads!(@reserved_connections) do |name, conn|
161:           checkin conn
162:         end
163:       end

Returns true if a connection has already been opened.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 120
120:       def connected?
121:         !@connections.empty?
122:       end

Retrieve the connection associated with the current thread, or call checkout to obtain one if necessary.

connection can be called any number of times; the connection is held in a hash keyed by the thread id.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 94
 94:       def connection
 95:         if conn = @reserved_connections[current_connection_id]
 96:           conn
 97:         else
 98:           @reserved_connections[current_connection_id] = checkout
 99:         end
100:       end

Disconnects all connections in the pool, and clears the pool.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 125
125:       def disconnect!
126:         @reserved_connections.each do |name,conn|
127:           checkin conn
128:         end
129:         @reserved_connections = {}
130:         @connections.each do |conn|
131:           conn.disconnect!
132:         end
133:         @connections = []
134:       end

Signal that the thread is finished with the current connection. release_connection releases the connection-thread association and returns the connection to the pool.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 105
105:       def release_connection
106:         conn = @reserved_connections.delete(current_connection_id)
107:         checkin conn if conn
108:       end
with_connection() {|conn| ...}

Reserve a connection, and yield it to a block. Ensure the connection is checked back in when finished.

     # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 112
112:       def with_connection
113:         conn = checkout
114:         yield conn
115:       ensure
116:         checkin conn
117:       end