class ActiveRecord::ConnectionAdapters::ConnectionPool

Parent:
Object
Included modules:
ActiveRecord::ConnectionAdapters::QueryCache::ConnectionPoolConfiguration

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:

  1. 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.

  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).

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).

Attributes

automatic_reconnect[RW]
checkout_timeout[RW]
reaper[R]
schema_cache[RW]
size[R]
spec[R]

Public Class Methods

new(spec) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 370
def initialize(spec)
  super()

  @spec = spec

  @checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5
  if @idle_timeout = spec.config.fetch(:idle_timeout, 300)
    @idle_timeout = @idle_timeout.to_f
    @idle_timeout = nil if @idle_timeout <= 0
  end

  # default max pool size to 5
  @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5

  # 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 a 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

  # +reaping_frequency+ is configurable mostly for historical reasons, but it could
  # also be useful if someone wants a very low +idle_timeout+.
  reaping_frequency = spec.config.fetch(:reaping_frequency, 60)
  @reaper = Reaper.new(self, reaping_frequency && reaping_frequency.to_f)
  @reaper.run
end

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.

Calls superclass method ActiveRecord::ConnectionAdapters::QueryCache::ConnectionPoolConfiguration.new

Public Instance Methods

active_connection?() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 439
def active_connection?
  @thread_cached_conns[connection_cache_key(current_thread)]
end

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?

checkin(conn) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 595
def checkin(conn)
  conn.lock.synchronize do
    synchronize do
      remove_connection_from_thread_cache conn

      conn._run_checkin_callbacks do
        conn.expire
      end

      @available.add conn
    end
  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.

checkout(checkout_timeout = @checkout_timeout) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 586
def checkout(checkout_timeout = @checkout_timeout)
  checkout_and_verify(acquire_connection(checkout_timeout))
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:

clear_reloadable_connections(raise_on_acquisition_timeout = true) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 544
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.

Raises:

clear_reloadable_connections!() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 568
def clear_reloadable_connections!
  clear_reloadable_connections(false)
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.config[:checkout_timeout] * 2 seconds), then the pool forcefully clears the cache and reloads connections without any regard for other connection owning threads.

connected?() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 471
def connected?
  synchronize { @connections.any? }
end

Returns true if a connection has already been opened.

connection() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 430
def connection
  @thread_cached_conns[connection_cache_key(current_thread)] ||= checkout
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 cache keyed by a thread.

connections() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 486
def connections
  synchronize { @connections.dup }
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.

disconnect(raise_on_acquisition_timeout = true) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 496
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

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

Raises:

disconnect!() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 518
def disconnect!
  disconnect(false)
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.config[:checkout_timeout] * 2 seconds), then the pool is forcefully disconnected without any regard for other connection owning threads.

flush(minimum_idle = @idle_timeout) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 665
def flush(minimum_idle = @idle_timeout)
  return if minimum_idle.nil?

  idle_connections = synchronize do
    @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 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.

flush!() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 686
def flush!
  reap
  flush(-1)
end

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

lock_thread=(lock_thread) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 417
def lock_thread=(lock_thread)
  if lock_thread
    @lock_thread = Thread.current
  else
    @lock_thread = nil
  end
end
reap() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 643
def reap
  stale_connections = synchronize do
    @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

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.

release_connection(owner_thread = Thread.current) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 450
def release_connection(owner_thread = Thread.current)
  if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
    checkin conn
  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.

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.

remove(conn) Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 611
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

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

stat() Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 699
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

Return connection pool's usage statistic Example:

ActiveRecord::Base.connection_pool.stat # => { size: 15, connections: 1, busy: 1, dead: 0, idle: 0, waiting: 0, checkout_timeout: 5 }
with_connection() { |conn| ... } Show source
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 460
def with_connection
  unless conn = @thread_cached_conns[connection_cache_key(Thread.current)]
    conn = connection
    fresh_connection = true
  end
  yield conn
ensure
  release_connection if fresh_connection
end

If a connection obtained through connection or with_connection methods already exists yield it to the block. If no such connection exists checkout a connection, yield it to the block, and checkin the connection when finished.

© 2004–2019 David Heinemeier Hansson
Licensed under the MIT License.