class ActiveRecord::ConnectionAdapters::ConnectionPool - activerecord-4.2.4 Documentation

# class ActiveRecord::ConnectionAdapters::ConnectionPool

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).

### Attributes

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

### Public Class Methods

new(spec) click to toggle source

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

    # File 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] && 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 reserved connections mapped to threads
      @reserved_connections = ThreadSafe::Cache.new(:initial_capacity => @size)

      @connections         = []
      @automatic_reconnect = true

      @available = Queue.new self
    end

### Public Instance Methods

active_connection?() click to toggle source

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

    # File 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) click to toggle source

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 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 conn, owner

        @available.add conn
      end
    end

checkout() click to toggle source

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 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!() click to toggle source

Clears the cache which maps classes.

    # File 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?() click to toggle source

Returns true if a connection has already been opened.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 298
    def connected?
      synchronize { @connections.any? }
    end

connection() click to toggle source

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 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!() click to toggle source

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

    # File 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() click to toggle source

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 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) click to toggle source

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 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) click to toggle source

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

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 376
    def remove(conn)
      synchronize do
        @connections.delete conn
        @available.delete conn

        release conn, conn.owner

        @available.add checkout_new_connection if @available.any_waiting?
      end
    end

with_connection() { |connection| ... } click to toggle source

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 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

### Private Instance Methods

acquire_connection() click to toggle source

Acquire a connection by one of 1) immediately removing one from the queue of available connections, 2) creating a new connection if the pool is not at capacity, 3) waiting on the queue for a connection to become available.

Raises:

- ConnectionTimeoutError if a connection could not be acquired

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 418
    def acquire_connection
      if conn = @available.poll
        conn
      elsif @connections.size < @size
        checkout_new_connection
      else
        reap
        @available.poll(@checkout_timeout)
      end
    end

checkout_and_verify(c) click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 454
    def checkout_and_verify(c)
      c._run_checkout_callbacks do
        c.verify!
      end
      c
    rescue
      remove c
      c.disconnect!
      raise
    end

checkout_new_connection() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 445
    def checkout_new_connection
      raise ConnectionNotEstablished unless @automatic_reconnect

      c = new_connection
      c.pool = self
      @connections << c
      c
    end

new_connection() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 437
    def new_connection
      Base.send(spec.adapter_method, spec.config)
    end

release(conn, owner) click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 429
    def release(conn, owner)
      thread_id = owner.object_id

      if @reserved_connections[thread_id] == conn
        @reserved_connections.delete thread_id
      end
    end