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class ActiveRecord::ConnectionAdapters::ConnectionPool

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.lease_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).

Inherits From

Attributes

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

Public class methods

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 220
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>@leases</tt> does not require
  # synchronization.
  @leases = LeaseRegistry.new

  @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
  @pinned_connection = nil

  @async_executor = build_async_executor

  @schema_cache = nil

  @reaper = Reaper.new(self, db_config.reaping_frequency)
  @reaper.run
end

Public instance methods

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 lease_connection or with_connection methods. Connections obtained through checkout will not be detected by active_connection?

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 357
def active_connection?
  connection_lease.connection
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.

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 549
def checkin(conn)
  return if @pinned_connection.equal?(conn)

  conn.lock.synchronize do
    synchronize do
      connection_lease.clear(conn)

      conn._run_checkin_callbacks do
        conn.expire
      end

      @available.add conn
    end
  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:

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 528
def checkout(checkout_timeout = @checkout_timeout)
  if @pinned_connection
    synchronize do
      @pinned_connection.verify!
      # Any leased connection must be in @connections otherwise
      # some methods like #connected? won't behave correctly
      unless @connections.include?(@pinned_connection)
        @connections << @pinned_connection
      end
    end
    @pinned_connection
  else
    checkout_and_verify(acquire_connection(checkout_timeout))
  end
end

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

Raises:

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 486
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

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 510
def clear_reloadable_connections!
  clear_reloadable_connections(false)
end

Returns true if a connection has already been opened.

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 408
def connected?
  synchronize { @connections.any?(&:connected?) }
end
Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 305
      def connection
        ActiveRecord.deprecator.warn(<<~MSG)
          ActiveRecord::ConnectionAdapters::ConnectionPool#connection is deprecated
          and will be removed in Rails 7.3. Use #lease_connection instead.
        MSG
        lease_connection
      end

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 423
def connections
  synchronize { @connections.dup }
end

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

Raises:

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 433
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 = []
      @leases.clear
      @available.clear
    end
  end
end

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 456
def disconnect!
  disconnect(false)
end

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 624
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

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

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 646
def flush!
  reap
  flush(-1)
end

Retrieve the connection associated with the current thread, or call checkout to obtain one if necessary.

lease_connection can be called any number of times; the connection is held in a cache keyed by a thread.

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 295
def lease_connection
  lease = connection_lease
  lease.sticky = true
  lease.connection ||= checkout
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.

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 599
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

  prune_thread_cache
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.

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

Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 369
def release_connection(existing_lease = nil)
  if conn = connection_lease.release
    checkin conn
    return true
  end
  false
end

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 567
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
Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 265
def schema_cache
  @schema_cache ||= BoundSchemaReflection.new(schema_reflection, self)
end
Source code GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 269
def schema_reflection=(schema_reflection)
  pool_config.schema_reflection = schema_reflection
  @schema_cache = nil
end

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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 658
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

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 lease_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 GitHub
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 386
def with_connection(prevent_permanent_checkout: false)
  lease = connection_lease
  sticky_was = lease.sticky
  lease.sticky = false if prevent_permanent_checkout

  if lease.connection
    begin
      yield lease.connection
    ensure
      lease.sticky = sticky_was if prevent_permanent_checkout && !sticky_was
    end
  else
    begin
      yield lease.connection = checkout
    ensure
      lease.sticky = sticky_was if prevent_permanent_checkout && !sticky_was
      release_connection(lease) unless lease.sticky
    end
  end
end

Namespace

Definition files