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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
[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 233
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]
  @reaper.run

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

  # The cache of reserved connections mapped to threads
  @reserved_connections = ThreadSafe::Cache.new(:initial_capacity => @size)

  @connections         = []
  @automatic_reconnect = true

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

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 268
def active_connection?
  synchronize do
    @reserved_connections.fetch(current_connection_id) {
      return false
    }.in_use?
  end
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 360
def checkin(conn)
  synchronize do
    owner = conn.owner

    conn._run_checkin_callbacks do
      conn.expire
    end

    release owner

    @available.add conn
  end
end
checkout()

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 347
def checkout
  synchronize do
    conn = acquire_connection
    conn.lease
    checkout_and_verify(conn)
  end
end
clear_reloadable_connections!()

Clears the cache which maps classes.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 316
def clear_reloadable_connections!
  synchronize do
    @reserved_connections.clear
    @connections.each do |conn|
      checkin conn
      conn.disconnect! if conn.requires_reloading?
    end
    @connections.delete_if do |conn|
      conn.requires_reloading?
    end
    @available.clear
    @connections.each do |conn|
      @available.add conn
    end
  end
end
connected?()

Returns true if a connection has already been opened.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 298
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 hash keyed by the thread id.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 259
def connection
  # this is correctly done double-checked locking
  # (ThreadSafe::Cache's lookups have volatile semantics)
  @reserved_connections[current_connection_id] || synchronize do
    @reserved_connections[current_connection_id] ||= checkout
  end
end
disconnect!()

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 303
def disconnect!
  synchronize do
    @reserved_connections.clear
    @connections.each do |conn|
      checkin conn
      conn.disconnect!
    end
    @connections = []
    @available.clear
  end
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 390
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(with_id = current_connection_id)

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 279
def release_connection(with_id = current_connection_id)
  synchronize do
    conn = @reserved_connections.delete(with_id)
    checkin conn if 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 376
def remove(conn)
  synchronize do
    @connections.delete conn
    @available.delete conn

    release conn.owner

    @available.add checkout_new_connection if @available.any_waiting?
  end
end
with_connection()

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 289
def with_connection
  connection_id = current_connection_id
  fresh_connection = true unless active_connection?
  yield connection
ensure
  release_connection(connection_id) if fresh_connection
end