Active Record Connection Pool

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, or the checkout_timeout has expired.

Obtaining (checking out) a connection

Connections can be obtained and used from a connection pool in several ways:

  1. Simply use ActiveRecord::Base.connection. When you’re done with the connection(s) and wish it to be returned to the pool, you call ActiveRecord::Base.connection_handler.clear_active_connections!. This is the default behavior for Active Record when used in conjunction with Action Pack’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).

While a thread has a connection checked out from the pool using one of the above three methods, that connection will automatically be the one used by ActiveRecord queries executing on that thread. It is not required to explicitly pass the checked out connection to Rails models or queries, for example.

Options

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

  • pool: maximum number of connections the pool may manage (default 5).

  • idle_timeout: number of seconds that a connection will be kept unused in the pool before it is automatically disconnected (default 300 seconds). Set this to zero to keep connections forever.

  • checkout_timeout: number of seconds to wait for a connection to become available before giving up and raising a timeout error (default 5 seconds).

Namespace

Class

Methods

Included Modules

Attributes

[R] async_executor
[RW] automatic_reconnect
[RW] checkout_timeout
[R] db_config
[R] pool_config
[R] reaper
[R] role
[R] shard
[R] size

Class Public methods

new(pool_config)

Creates a new ConnectionPool object. pool_config is a PoolConfig 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.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 120
      def initialize(pool_config)
        super()

        @pool_config = pool_config
        @db_config = pool_config.db_config
        @role = pool_config.role
        @shard = pool_config.shard

        @checkout_timeout = db_config.checkout_timeout
        @idle_timeout = db_config.idle_timeout
        @size = db_config.pool

        # This variable tracks the cache of threads mapped to reserved connections, with the
        # sole purpose of speeding up the +connection+ method. It is not the authoritative
        # registry of which thread owns which connection. Connection ownership is tracked by
        # the +connection.owner+ attr on each +connection+ instance.
        # The invariant works like this: if there is mapping of <tt>thread => conn</tt>,
        # then that +thread+ does indeed own that +conn+. However, an absence of such
        # mapping does not mean that the +thread+ doesn't own the said connection. In
        # that case +conn.owner+ attr should be consulted.
        # Access and modification of <tt>@thread_cached_conns</tt> does not require
        # synchronization.
        @thread_cached_conns = Concurrent::Map.new(initial_capacity: @size)

        @connections         = []
        @automatic_reconnect = true

        # Connection pool allows for concurrent (outside the main +synchronize+ section)
        # establishment of new connections. This variable tracks the number of threads
        # currently in the process of independently establishing connections to the DB.
        @now_connecting = 0

        @threads_blocking_new_connections = 0

        @available = ConnectionLeasingQueue.new self

        @lock_thread = false

        @async_executor = build_async_executor

        @reaper = Reaper.new(self, db_config.reaping_frequency)
        @reaper.run
      end
🔎 See on GitHub

Instance Public methods

active_connection?()

Returns true if there is an open connection being used for the current thread.

This method only works for connections that have been obtained through connection or with_connection methods. Connections obtained through checkout will not be detected by active_connection?

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 196
      def active_connection?
        @thread_cached_conns[connection_cache_key(current_thread)]
      end
🔎 See on GitHub

checkin(conn)

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.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 363
      def checkin(conn)
        conn.lock.synchronize do
          synchronize do
            remove_connection_from_thread_cache conn

            conn._run_checkin_callbacks do
              conn.expire
            end

            conn.lock_thread = nil
            @available.add conn
          end
        end
      end
🔎 See on GitHub

checkout(checkout_timeout = @checkout_timeout)

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:

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 352
      def checkout(checkout_timeout = @checkout_timeout)
        connection = checkout_and_verify(acquire_connection(checkout_timeout))
        connection.lock_thread = @lock_thread
        connection
      end
🔎 See on GitHub

clear_reloadable_connections(raise_on_acquisition_timeout = true)

Clears the cache which maps classes and re-connects connections that require reloading.

Raises:

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 310
      def clear_reloadable_connections(raise_on_acquisition_timeout = true)
        with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
          synchronize do
            @connections.each do |conn|
              if conn.in_use?
                conn.steal!
                checkin conn
              end
              conn.disconnect! if conn.requires_reloading?
            end
            @connections.delete_if(&:requires_reloading?)
            @available.clear
          end
        end
      end
🔎 See on GitHub

clear_reloadable_connections!()

Clears the cache which maps classes and re-connects connections that require reloading.

The pool first tries to gain ownership of all connections. If unable to do so within a timeout interval (default duration is spec.db_config.checkout_timeout * 2 seconds), then the pool forcefully clears the cache and reloads connections without any regard for other connection owning threads.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 334
      def clear_reloadable_connections!
        clear_reloadable_connections(false)
      end
🔎 See on GitHub

connected?()

Returns true if a connection has already been opened.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 233
      def connected?
        synchronize { @connections.any? }
      end
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connection()

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 cache keyed by a thread.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 181
      def connection
        @thread_cached_conns[connection_cache_key(current_thread)] ||= checkout
      end
🔎 See on GitHub

connections()

Returns an array containing the connections currently in the pool. Access to the array does not require synchronization on the pool because the array is newly created and not retained by the pool.

However; this method bypasses the ConnectionPool’s thread-safe connection access pattern. A returned connection may be owned by another thread, unowned, or by happen-stance owned by the calling thread.

Calling methods on a connection without ownership is subject to the thread-safety guarantees of the underlying method. Many of the methods on connection adapter classes are inherently multi-thread unsafe.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 248
      def connections
        synchronize { @connections.dup }
      end
🔎 See on GitHub

disconnect(raise_on_acquisition_timeout = true)

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

Raises:

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 258
      def disconnect(raise_on_acquisition_timeout = true)
        with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
          synchronize do
            @connections.each do |conn|
              if conn.in_use?
                conn.steal!
                checkin conn
              end
              conn.disconnect!
            end
            @connections = []
            @available.clear
          end
        end
      end
🔎 See on GitHub

disconnect!()

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

The pool first tries to gain ownership of all connections. If unable to do so within a timeout interval (default duration is spec.db_config.checkout_timeout * 2 seconds), then the pool is forcefully disconnected without any regard for other connection owning threads.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 280
      def disconnect!
        disconnect(false)
      end
🔎 See on GitHub

flush(minimum_idle = @idle_timeout)

Disconnect all connections that have been idle for at least minimum_idle seconds. Connections currently checked out, or that were checked in less than minimum_idle seconds ago, are unaffected.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 435
      def flush(minimum_idle = @idle_timeout)
        return if minimum_idle.nil?

        idle_connections = synchronize do
          return if self.discarded?
          @connections.select do |conn|
            !conn.in_use? && conn.seconds_idle >= minimum_idle
          end.each do |conn|
            conn.lease

            @available.delete conn
            @connections.delete conn
          end
        end

        idle_connections.each do |conn|
          conn.disconnect!
        end
      end
🔎 See on GitHub

flush!()

Disconnect all currently idle connections. Connections currently checked out are unaffected.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 457
      def flush!
        reap
        flush(-1)
      end
🔎 See on GitHub

lock_thread=(lock_thread)

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 164
      def lock_thread=(lock_thread)
        if lock_thread
          @lock_thread = ActiveSupport::IsolatedExecutionState.context
        else
          @lock_thread = nil
        end

        if (active_connection = @thread_cached_conns[connection_cache_key(current_thread)])
          active_connection.lock_thread = @lock_thread
        end
      end
🔎 See on GitHub

reap()

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.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 412
      def reap
        stale_connections = synchronize do
          return if self.discarded?
          @connections.select do |conn|
            conn.in_use? && !conn.owner.alive?
          end.each do |conn|
            conn.steal!
          end
        end

        stale_connections.each do |conn|
          if conn.active?
            conn.reset!
            checkin conn
          else
            remove conn
          end
        end
      end
🔎 See on GitHub

release_connection(owner_thread = ActiveSupport::IsolatedExecutionState.context)

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

This method only works for connections that have been obtained through connection or with_connection methods, connections obtained through checkout will not be automatically released.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 207
      def release_connection(owner_thread = ActiveSupport::IsolatedExecutionState.context)
        if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
          checkin conn
        end
      end
🔎 See on GitHub

remove(conn)

Remove a connection from the connection pool. The connection will remain open and active but will no longer be managed by this pool.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 380
      def remove(conn)
        needs_new_connection = false

        synchronize do
          remove_connection_from_thread_cache conn

          @connections.delete conn
          @available.delete conn

          # @available.any_waiting? => true means that prior to removing this
          # conn, the pool was at its max size (@connections.size == @size).
          # This would mean that any threads stuck waiting in the queue wouldn't
          # know they could checkout_new_connection, so let's do it for them.
          # Because condition-wait loop is encapsulated in the Queue class
          # (that in turn is oblivious to ConnectionPool implementation), threads
          # that are "stuck" there are helpless. They have no way of creating
          # new connections and are completely reliant on us feeding available
          # connections into the Queue.
          needs_new_connection = @available.any_waiting?
        end

        # This is intentionally done outside of the synchronized section as we
        # would like not to hold the main mutex while checking out new connections.
        # Thus there is some chance that needs_new_connection information is now
        # stale, we can live with that (bulk_make_new_connections will make
        # sure not to exceed the pool's @size limit).
        bulk_make_new_connections(1) if needs_new_connection
      end
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stat()

Returns the connection pool’s usage statistic.

ActiveRecord::Base.connection_pool.stat # => { size: 15, connections: 1, busy: 1, dead: 0, idle: 0, waiting: 0, checkout_timeout: 5 }
📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 469
      def stat
        synchronize do
          {
            size: size,
            connections: @connections.size,
            busy: @connections.count { |c| c.in_use? && c.owner.alive? },
            dead: @connections.count { |c| c.in_use? && !c.owner.alive? },
            idle: @connections.count { |c| !c.in_use? },
            waiting: num_waiting_in_queue,
            checkout_timeout: checkout_timeout
          }
        end
      end
🔎 See on GitHub

with_connection()

Yields a connection from the connection pool to the block. If no connection is already checked out by the current thread, a connection will be checked out from the pool, yielded to the block, and then returned to the pool when the block is finished. If a connection has already been checked out on the current thread, such as via connection or with_connection, that existing connection will be the one yielded and it will not be returned to the pool automatically at the end of the block; it is expected that such an existing connection will be properly returned to the pool by the code that checked it out.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 222
      def with_connection
        unless conn = @thread_cached_conns[connection_cache_key(ActiveSupport::IsolatedExecutionState.context)]
          conn = connection
          fresh_connection = true
        end
        yield conn
      ensure
        release_connection if fresh_connection
      end
🔎 See on GitHub