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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: 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).

  • reaping_frequency: frequency in seconds to periodically run the Reaper, which attempts to find and recover connections from dead threads, which can occur if a programmer forgets to close a connection at the end of a thread or a thread dies unexpectedly. Regardless of this setting, the Reaper will be invoked before every blocking wait. (Default nil, which means don't schedule the Reaper).

Namespace
Methods
A
C
D
N
R
W
Included Modules
Attributes
[RW] automatic_reconnect
[RW] checkout_timeout
[R] connections
[R] reaper
[RW] schema_cache
[R] size
[R] spec
Class Public methods
new(spec)

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 318
def initialize(spec)
  super()

  @spec = spec

  @checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5
  @reaper = Reaper.new(self, (spec.config[:reaping_frequency] && spec.config[:reaping_frequency].to_f))
  @reaper.run

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

  # The cache of threads mapped to reserved connections, the sole purpose
  # of the cache is to speed-up +connection+ method, it is not the authoritative
  # registry of which thread owns which connection, that is tracked by
  # +connection.owner+ attr on each +connection+ instance.
  # The invariant works like this: if there is mapping of +thread => conn+,
  # 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 +@thread_cached_conns+ does not require
  # synchronization.
  @thread_cached_conns = ThreadSafe::Cache.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

  # A boolean toggle that allows/disallows new connections.
  @new_cons_enabled = true

  @available = ConnectionLeasingQueue.new self
end
Instance Public methods
active_connection?()

Is there an open connection that is being used for the current thread?

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 370
def active_connection?
  @thread_cached_conns[connection_cache_key(Thread.current)]
end
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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 507
def checkin(conn)
  synchronize do
    remove_connection_from_thread_cache conn

    conn.run_callbacks :checkin do
      conn.expire
    end

    @available.add conn
  end
end
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:

  • ConnectionTimeoutError: no connection can be obtained from the pool.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 498
def checkout(checkout_timeout = @checkout_timeout)
  checkout_and_verify(acquire_connection(checkout_timeout))
end
clear_reloadable_connections(raise_on_acquisition_timeout = true)

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

Raises:

  • ExclusiveConnectionTimeoutError if unable to gain ownership of all connections in the pool within a timeout interval (default duration is +spec.config * 2+ seconds).

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 442
def clear_reloadable_connections(raise_on_acquisition_timeout = true)
  num_new_conns_required = 0

  with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
    synchronize do
      @connections.each do |conn|
        checkin conn
        conn.disconnect! if conn.requires_reloading?
      end
      @connections.delete_if(&:requires_reloading?)

      @available.clear

      if @connections.size < @size
        # because of the pruning done by this method, we might be running
        # low on connections, while threads stuck in queue are helpless
        # (not being able to establish new connections for themselves),
        # see also more detailed explanation in +remove+
        num_new_conns_required = num_waiting_in_queue - @connections.size
      end

      @connections.each do |conn|
        @available.add conn
      end
    end
  end

  bulk_make_new_connections(num_new_conns_required) if num_new_conns_required > 0
end
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.config * 2+ seconds), the pool forcefully clears the cache and reloads connections without any regard for other connection owning threads.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 480
def clear_reloadable_connections!
  clear_reloadable_connections(false)
end
connected?()

Returns true if a connection has already been opened.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 402
def connected?
  synchronize { @connections.any? }
end
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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 361
def connection
  @thread_cached_conns[connection_cache_key(Thread.current)] ||= checkout
end
disconnect(raise_on_acquisition_timeout = true)

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

Raises:

  • ExclusiveConnectionTimeoutError if unable to gain ownership of all connections in the pool within a timeout interval (default duration is +spec.config * 2+ seconds).

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 412
def disconnect(raise_on_acquisition_timeout = true)
  with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
    synchronize do
      @connections.each do |conn|
        checkin conn
        conn.disconnect!
      end
      @connections = []
      @available.clear
    end
  end
end
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.config * 2+ seconds), the pool is forcefully disconneted wihout any regard for other connection owning threads.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 431
def disconnect!
  disconnect(false)
end
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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 553
def reap
  stale_connections = synchronize do
    @connections.select do |conn|
      conn.in_use? && !conn.owner.alive?
    end
  end

  stale_connections.each do |conn|
    synchronize do
      if conn.active?
        conn.reset!
        checkin conn
      else
        remove conn
      end
    end
  end
end
release_connection(owner_thread = Thread.current)

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 381
def release_connection(owner_thread = Thread.current)
  if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
    checkin conn
  end
end
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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 521
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
with_connection()

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 391
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