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).
- A
- C
- D
- N
- P
- R
- W
- MonitorMixin START:includes
| [RW] | automatic_reconnect | |
| [R] | spec | |
| [R] | connections |
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 71 71: def initialize(spec) 72: super() 73: 74: @spec = spec 75: 76: # The cache of reserved connections mapped to threads 77: @reserved_connections = {} 78: 79: @queue = new_cond 80: @timeout = spec.config[:wait_timeout] || 5 81: 82: # default max pool size to 5 83: @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5 84: 85: @connections = [] 86: @automatic_reconnect = true 87: end
Is there an open connection that is being used for the current thread?
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.
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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 233 233: def checkout 234: synchronize do 235: waited_time = 0 236: 237: loop do 238: conn = @connections.find { |c| c.lease } 239: 240: unless conn 241: if @connections.size < @size 242: conn = checkout_new_connection 243: conn.lease 244: end 245: end 246: 247: if conn 248: checkout_and_verify conn 249: return conn 250: end 251: 252: if waited_time >= @timeout 253: 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." 254: end 255: 256: # Sometimes our wait can end because a connection is available, 257: # but another thread can snatch it up first. If timeout hasn't 258: # passed but no connection is avail, looks like that happened -- 259: # loop and wait again, for the time remaining on our timeout. 260: before_wait = Time.now 261: @queue.wait( [@timeout - waited_time, 0].max ) 262: waited_time += (Time.now - before_wait) 263: 264: # Will go away in Rails 4, when we don't clean up 265: # after leaked connections automatically anymore. Right now, clean 266: # up after we've returned from a 'wait' if it looks like it's 267: # needed, then loop and try again. 268: if(active_connections.size >= @connections.size) 269: clear_stale_cached_connections! 270: end 271: end 272: end 273: end
Clears the cache which maps classes.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 146 146: def clear_reloadable_connections! 147: synchronize do 148: @reserved_connections = {} 149: @connections.each do |conn| 150: checkin conn 151: conn.disconnect! if conn.requires_reloading? 152: end 153: @connections.delete_if do |conn| 154: conn.requires_reloading? 155: end 156: end 157: 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 200 200: def clear_stale_cached_connections! 201: keys = @reserved_connections.keys - Thread.list.find_all { |t| 202: t.alive? 203: }.map { |thread| thread.object_id } 204: keys.each do |key| 205: conn = @reserved_connections[key] 206: ActiveSupport::Deprecation.warn("Database connections will not be closed automatically, please close your\ndatabase connection at the end of the thread by calling `close` on your\nconnection. For example: ActiveRecord::Base.connection.close\n") if conn.in_use? 207: checkin conn 208: @reserved_connections.delete(key) 209: end 210: end
Returns true if a connection has already been opened.
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.
Disconnects all connections in the pool, and clears the pool.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 134 134: def disconnect! 135: synchronize do 136: @reserved_connections = {} 137: @connections.each do |conn| 138: checkin conn 139: conn.disconnect! 140: end 141: @connections = [] 142: end 143: 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.
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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 120 120: def with_connection 121: connection_id = current_connection_id 122: fresh_connection = true unless active_connection? 123: yield connection 124: ensure 125: release_connection(connection_id) if fresh_connection 126: end