# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 190 def primary_keys with_connection do |c| c.schema_cache.primary_keys end end
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).
-
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) -
+checkout _timeout+: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds). ('wait_timeout' supported for backwards compatibility, but conflicts with key used for different purpose by mysql2 adapter).
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 74 def initialize(spec) super() @spec = spec # The cache of reserved connections mapped to threads @reserved_connections = {} @queue = new_cond # 'wait_timeout', the backward-compatible key, conflicts with spec key # used by mysql2 for something entirely different, checkout_timeout # preferred to avoid conflict and allow independent values. @timeout = spec.config[:checkout_timeout] || spec.config[:wait_timeout] || 5 # default max pool size to 5 @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5 @connections = [] @automatic_reconnect = true 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 107 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 285 def checkin(conn) synchronize do conn.run_callbacks :checkin do conn.expire @queue.signal end release 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 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/active_record/connection_adapters/abstract/connection_pool.rb, line 238 def checkout synchronize do waited_time = 0 loop do conn = @connections.find { |c| c.lease } unless conn if @connections.size < @size conn = checkout_new_connection conn.lease end end if conn checkout_and_verify conn return conn end if waited_time >= @timeout raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout} (waited #{waited_time} seconds). The max pool size is currently #{@size}; consider increasing it." end # Sometimes our wait can end because a connection is available, # but another thread can snatch it up first. If timeout hasn't # passed but no connection is avail, looks like that happened -- # loop and wait again, for the time remaining on our timeout. before_wait = Time.now @queue.wait( [@timeout - waited_time, 0].max ) waited_time += (Time.now - before_wait) # Will go away in Rails 4, when we don't clean up # after leaked connections automatically anymore. Right now, clean # up after we've returned from a 'wait' if it looks like it's # needed, then loop and try again. if(active_connections.size >= @connections.size) clear_stale_cached_connections! end end end end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 197 def clear_cache! with_connection do |c| c.schema_cache.clear! end end
Clears the cache which maps classes.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 152 def clear_reloadable_connections! synchronize do @reserved_connections = {} @connections.each do |conn| checkin conn conn.disconnect! if conn.requires_reloading? end @connections.delete_if do |conn| conn.requires_reloading? end end end
Return any checked-out connections back to the pool by threads that are no longer alive.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 206 def clear_stale_cached_connections! keys = @reserved_connections.keys - Thread.list.find_all { |t| t.alive? }.map { |thread| thread.object_id } keys.each do |key| conn = @reserved_connections[key] ActiveSupport::Deprecation.warn("Database connections will not be closed automatically, please close your database connection at the end of the thread by calling `close` on your connection. For example: ActiveRecord::Base.connection.close ") if conn.in_use? checkin conn @reserved_connections.delete(key) end end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 176 def columns with_connection do |c| c.schema_cache.columns end end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 183 def columns_hash with_connection do |c| c.schema_cache.columns_hash end end
Returns true if a connection has already been opened.
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 135 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 100 def connection 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 140 def disconnect! synchronize do @reserved_connections = {} @connections.each do |conn| checkin conn conn.disconnect! 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/active_record/connection_adapters/abstract/connection_pool.rb, line 118 def release_connection(with_id = current_connection_id) conn = synchronize { @reserved_connections.delete(with_id) } checkin conn if conn 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 126 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
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 338 def active_connections @connections.find_all { |c| c.in_use? } end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 331 def checkout_and_verify(c) c.run_callbacks :checkout do c.verify! end c end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 322 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 314 def new_connection ActiveRecord::Base.send(spec.adapter_method, spec.config) end
# File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 298 def release(conn) synchronize do thread_id = nil if @reserved_connections[current_connection_id] == conn thread_id = current_connection_id else thread_id = @reserved_connections.keys.find { |k| @reserved_connections[k] == conn } end @reserved_connections.delete thread_id if thread_id end end