Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Inherits:
Object
  • Object
show all
Defined in:
lib/active_record/connection_adapters/abstract/connection_pool.rb

Overview

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).

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(spec) ⇒ ConnectionPool

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.



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# 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

Instance Attribute Details

#connections ⇒ Object (readonly)

Returns the value of attribute connections.



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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 60

def connections
  @connections
end

#spec ⇒ Object (readonly)

Returns the value of attribute spec.



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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 60

def spec
  @spec
end

Instance Method Details

#checkin(conn) ⇒ Object

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.



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# 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 ⇒ Object

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.



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# 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! ⇒ Object

Clears the cache which maps classes



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# 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! ⇒ Object

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



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# 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? ⇒ Boolean

Returns true if a connection has already been opened.

Returns:

  • (Boolean) 


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 117

def connected?
  !@connections.empty?
end

#connection ⇒ Object

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.



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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 93

def connection
  @reserved_connections[current_connection_id] ||= checkout
end

#disconnect! ⇒ Object

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



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# 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) ⇒ Object

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



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# 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

#verify_active_connections! ⇒ Object

Verify active connections and remove and disconnect connections associated with stale threads.



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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 149

def verify_active_connections! #:nodoc:
  clear_stale_cached_connections!
  @connections.each do |connection|
    connection.verify!
  end
end

#with_connection ⇒ Object

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.



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# 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