class ActiveRecord::ConnectionAdapters::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).
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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 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).
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/active_record/connection_adapters/abstract/connection_pool.rb, line 233 def initialize(spec) super() @spec = spec @checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5 @reaper = Reaper.new(self, (spec.config[:reaping_frequency] && spec.config[:reaping_frequency].to_f)) @reaper.run # default max pool size to 5 @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5 # The cache of reserved connections mapped to threads @reserved_connections = ThreadSafe::Cache.new(:initial_capacity => @size) @connections = [] @automatic_reconnect = true @available = Queue.new self end
Public Instance Methods
Is there an open connection that is being used for the current thread?
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 268 def active_connection? synchronize do @reserved_connections.fetch(current_connection_id) { return false }.in_use? end end
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/active_record/connection_adapters/abstract/connection_pool.rb, line 360 def checkin(conn) synchronize do owner = conn.owner conn._run_checkin_callbacks do conn.expire end release conn, owner @available.add conn 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 and leasing existing connection, or by creating a new connection and leasing it.
If all connections are leased and the pool is at capacity (meaning the number of currently leased connections is greater than or equal to the size limit set), an ActiveRecord::ConnectionTimeoutError exception will be raised.
Returns: an AbstractAdapter object.
Raises:
-
ConnectionTimeoutError: no connection can be obtained from the pool.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 347 def checkout synchronize do conn = acquire_connection conn.lease checkout_and_verify(conn) end end
Clears the cache which maps classes.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 316 def clear_reloadable_connections! synchronize do @reserved_connections.clear @connections.each do |conn| checkin conn conn.disconnect! if conn.requires_reloading? end @connections.delete_if do |conn| conn.requires_reloading? end @available.clear @connections.each do |conn| @available.add conn end end end
Returns true if a connection has already been opened.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 298 def connected? synchronize { @connections.any? } 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/active_record/connection_adapters/abstract/connection_pool.rb, line 259 def connection # this is correctly done double-checked locking # (ThreadSafe::Cache's lookups have volatile semantics) @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/active_record/connection_adapters/abstract/connection_pool.rb, line 303 def disconnect! synchronize do @reserved_connections.clear @connections.each do |conn| checkin conn conn.disconnect! end @connections = [] @available.clear end end
Recover lost connections for the pool. A lost connection can occur if a programmer forgets to checkin a connection at the end of a thread or a thread dies unexpectedly.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 390 def reap stale_connections = synchronize do @connections.select do |conn| conn.in_use? && !conn.owner.alive? end end stale_connections.each do |conn| synchronize do if conn.active? conn.reset! checkin conn else remove conn end end 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/active_record/connection_adapters/abstract/connection_pool.rb, line 279 def release_connection(with_id = current_connection_id) synchronize do conn = @reserved_connections.delete(with_id) checkin conn if conn end end
Remove a connection from the connection pool. The connection will remain open and active but will no longer be managed by this pool.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 376 def remove(conn) synchronize do @connections.delete conn @available.delete conn release conn, conn.owner @available.add checkout_new_connection if @available.any_waiting? 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/active_record/connection_adapters/abstract/connection_pool.rb, line 289 def with_connection connection_id = current_connection_id fresh_connection = true unless active_connection? yield connection ensure release_connection(connection_id) if fresh_connection end
Private Instance Methods
Acquire a connection by one of 1) immediately removing one from the queue of available connections, 2) creating a new connection if the pool is not at capacity, 3) waiting on the queue for a connection to become available.
Raises:
-
ConnectionTimeoutError if a connection could not be acquired
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 418 def acquire_connection if conn = @available.poll conn elsif @connections.size < @size checkout_new_connection else reap @available.poll(@checkout_timeout) end end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 454 def checkout_and_verify(c) c._run_checkout_callbacks do c.verify! end c rescue remove c c.disconnect! raise end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 445 def checkout_new_connection raise ConnectionNotEstablished unless @automatic_reconnect c = new_connection c.pool = self @connections << c c end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 437 def new_connection Base.send(spec.adapter_method, spec.config) end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 429 def release(conn, owner) thread_id = owner.object_id if @reserved_connections[thread_id] == conn @reserved_connections.delete thread_id end end