Class: ActiveRecord::ConnectionAdapters::ConnectionPool

### Home Classes Methods

### In Files

- lib/active_record/connection_adapters/abstract/connection_pool.rb

### Parent

Object

### Methods

- ::new
- #checkin
- #checkout
- #clear_reloadable_connections!
- #clear_stale_cached_connections!
- #connected?
- #connection
- #disconnect!
- #release_connection
- #with_connection

### Files

- README.rdoc

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# 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 two connection-pooling-related options that you can add to your database connection configuration:

- `pool`: number indicating size of connection pool (default 5)

- `wait_timeout`: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds).

### Attributes

connections[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.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 68
    def initialize(spec)
      @spec = spec

      # The cache of reserved connections mapped to threads
      @reserved_connections = {}

      # The mutex used to synchronize pool access
      @connection_mutex = Monitor.new
      @queue = @connection_mutex.new_cond

      # default 5 second timeout unless on ruby 1.9
      @timeout = spec.config[:wait_timeout] || 5

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

      @connections = []
      @checked_out = []
    end

### Public Instance Methods

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 215
    def checkin(conn)
      @connection_mutex.synchronize do
        conn.send(:_run_checkin_callbacks) do
          @checked_out.delete conn
          @queue.signal
        end
      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 an existing connection, or by creating a new connection. If the maximum number of connections for this pool has already been reached, but the pool is empty (i.e. they’re all being used), then this method will wait until a thread has checked in a connection. The wait time is bounded however: if no connection can be checked out within the timeout specified for this pool, then a ConnectionTimeoutError exception will be raised.

Returns: an AbstractAdapter object.

Raises:

- ConnectionTimeoutError: no connection can be obtained from the pool within the timeout period.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 184
    def checkout
      # Checkout an available connection
      @connection_mutex.synchronize do
        loop do
          conn = if @checked_out.size < @connections.size
                   checkout_existing_connection
                 elsif @connections.size < @size
                   checkout_new_connection
                 end
          return conn if conn

          @queue.wait(@timeout)

          if(@checked_out.size < @connections.size)
            next
          else
            clear_stale_cached_connections!
            if @size == @checked_out.size
              raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout}.  The max pool size is currently #{@size}; consider increasing it."
            end
          end

        end
      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 134
    def clear_reloadable_connections!
      @reserved_connections.each do |name, conn|
        checkin conn
      end
      @reserved_connections = {}
      @connections.each do |conn|
        conn.disconnect! if conn.requires_reloading?
      end
      @connections.delete_if do |conn|
        conn.requires_reloading?
      end
    end

clear_stale_cached_connections!() click to toggle source

Return any checked-out connections back to the pool by threads that are no longer alive.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 158
    def clear_stale_cached_connections!
      keys = @reserved_connections.keys - Thread.list.find_all { |t|
        t.alive?
      }.map { |thread| thread.object_id }
      keys.each do |key|
        checkin @reserved_connections[key]
        @reserved_connections.delete(key)
      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 117
    def connected?
      !@connections.empty?
    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 93
    def connection
      @reserved_connections[current_connection_id] ||= checkout
    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 122
    def disconnect!
      @reserved_connections.each do |name,conn|
        checkin conn
      end
      @reserved_connections = {}
      @connections.each do |conn|
        conn.disconnect!
      end
      @connections = []
    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 100
    def release_connection(with_id = current_connection_id)
      conn = @reserved_connections.delete(with_id)
      checkin conn if conn
    end

with_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 108
    def with_connection
      connection_id = current_connection_id
      fresh_connection = true unless @reserved_connections[connection_id]
      yield connection
    ensure
      release_connection(connection_id) if fresh_connection
    end

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