class Pools::ConnectionPool
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.
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).
-
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).
Options¶ ↑
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).
Attributes
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 lib/pools/connection_pool.rb, line 71 def initialize(pooled, options) super() @pooled = pooled @options = options # The cache of reserved connections mapped to threads @reserved_connections = {} # The mutex used to synchronize pool access @queue = self.new_cond # default 5 second timeout unless on ruby 1.9 @timeout = options[:wait_timeout] || 5 # default max pool size to 5 @size = (options[:pool] && options[:pool].to_i) || 5 @connections = [] @checked_out = [] 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 lib/pools/connection_pool.rb, line 213 def checkin(conn) synchronize do @checked_out.delete conn @queue.signal end 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.
Raises:
-
ConnectionTimeoutError: no connection can be obtained from the pool within the timeout period.
# File lib/pools/connection_pool.rb, line 182 def checkout # Checkout an available connection synchronize do loop do conn = if @checked_out.size < @connections.size checkout_existing_connection elsif @connections.size < @size checkout_new_connection end return conn if conn @queue.wait(@timeout) if(@checked_out.size < @connections.size) next else clear_stale_cached_connections! if @size == @checked_out.size raise ConnectionTimeoutError, "could not obtain a pooled connection#{" within #{@timeout} seconds" if @timeout}. The max pool size is currently #{@size}; consider increasing it." end end end end end
Return any checked-out connections back to the pool by threads that are no longer alive.
# File lib/pools/connection_pool.rb, line 154 def clear_stale_cached_connections! synchronize do keys = @reserved_connections.keys - Thread.list.find_all { |t| t.alive? }.map { |thread| thread.object_id } keys.each do |key| checkin @reserved_connections[key] @reserved_connections.delete(key) end end end
Returns true if a connection has already been opened.
# File lib/pools/connection_pool.rb, line 125 def connected? synchronize { !@connections.empty? } 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 lib/pools/connection_pool.rb, line 97 def connection @reserved_connections[current_connection_id] || synchronize do @reserved_connections[current_connection_id] ||= checkout end end
Disconnects all connections in the pool, and clears the pool.
# File lib/pools/connection_pool.rb, line 130 def disconnect! synchronize do @reserved_connections = {} @connections.each do |conn| checkin conn @pooled.__disconnect(conn) end @connections = [] end 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 lib/pools/connection_pool.rb, line 106 def release_connection(with_id = current_connection_id) synchronize do conn = @reserved_connections.delete(with_id) checkin conn if conn end end
If a connection already exists yield it to the block. If no connection exists checkout a connection, yield it to the block, and checkin the connection when finished.
# File lib/pools/connection_pool.rb, line 116 def with_connection connection_id = current_connection_id fresh_connection = synchronize { !@reserved_connections[connection_id] } yield connection ensure release_connection(connection_id) if fresh_connection end
Private Instance Methods
# File lib/pools/connection_pool.rb, line 237 def checkout_connection(c) @checked_out << c c end
# File lib/pools/connection_pool.rb, line 232 def checkout_existing_connection c = (@connections - @checked_out).first checkout_connection(c) end
# File lib/pools/connection_pool.rb, line 225 def checkout_new_connection c = @pooled.__connection @pooled.__prepare(c) @connections << c checkout_connection(c) end