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)

  • +checkout _timeout+: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds). ('wait_timeout' supported for backwards compatibility, but conflicts with key used for different purpose by mysql2 adapter).

Methods
A
C
D
N
P
R
W
Included Modules
Attributes
[RW] automatic_reconnect
[R] connections
[R] spec
Class Public methods
new(spec)

Creates a new ConnectionPool object. specis 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 74
def initialize(spec)
  super()

  @spec = spec

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

  @queue = new_cond
  # 'wait_timeout', the backward-compatible key, conflicts with spec key 
  # used by mysql2 for something entirely different, checkout_timeout
  # preferred to avoid conflict and allow independent values. 
  @timeout = spec.config[:checkout_timeout] || spec.config[:wait_timeout] || 5

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

  @connections         = []
  @automatic_reconnect = true
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 107
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 checkouton this pool.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 285
def checkin(conn)
  synchronize do
    conn.run_callbacks :checkin do
      conn.expire
      @queue.signal
    end

    release 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 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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 238
def checkout
  synchronize do
    waited_time = 0

    loop do
      conn = @connections.find { |c| c.lease }

      unless conn
        if @connections.size < @size
          conn = checkout_new_connection
          conn.lease
        end
      end

      if conn
        checkout_and_verify conn
        return conn
      end

      if waited_time >= @timeout
        raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout} (waited #{waited_time} seconds). The max pool size is currently #{@size}; consider increasing it."
      end

      # Sometimes our wait can end because a connection is available,
      # but another thread can snatch it up first. If timeout hasn't
      # passed but no connection is avail, looks like that happened --
      # loop and wait again, for the time remaining on our timeout. 
      before_wait = Time.now
      @queue.wait( [@timeout - waited_time, 0].max )
      waited_time += (Time.now - before_wait)

      # Will go away in Rails 4, when we don't clean up
      # after leaked connections automatically anymore. Right now, clean
      # up after we've returned from a 'wait' if it looks like it's
      # needed, then loop and try again. 
      if(active_connections.size >= @connections.size)
        clear_stale_cached_connections!
      end
    end
  end
end
clear_cache!()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 197
def clear_cache!
  with_connection do |c|
    c.schema_cache.clear!
  end
end
clear_reloadable_connections!()

Clears the cache which maps classes.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 152
def clear_reloadable_connections!
  synchronize do
    @reserved_connections = {}
    @connections.each do |conn|
      checkin conn
      conn.disconnect! if conn.requires_reloading?
    end
    @connections.delete_if do |conn|
      conn.requires_reloading?
    end
  end
end
clear_stale_cached_connections!()

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 206
      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|
          conn = @reserved_connections[key]
          ActiveSupport::Deprecation.warn("Database connections will not be closed automatically, please close your
database connection at the end of the thread by calling `close` on your
connection.  For example: ActiveRecord::Base.connection.close
") if conn.in_use?
          checkin conn
          @reserved_connections.delete(key)
        end
      end
columns()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 176
def columns
  with_connection do |c|
    c.schema_cache.columns
  end
end
columns_hash()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 183
def columns_hash
  with_connection do |c|
    c.schema_cache.columns_hash
  end
end
connected?()

Returns true if a connection has already been opened.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 135
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 100
def connection
  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 140
def disconnect!
  synchronize do
    @reserved_connections = {}
    @connections.each do |conn|
      checkin conn
      conn.disconnect!
    end
    @connections = []
  end
end
primary_keys()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 190
def primary_keys
  with_connection do |c|
    c.schema_cache.primary_keys
  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 118
def release_connection(with_id = current_connection_id)
  conn = synchronize { @reserved_connections.delete(with_id) }
  checkin conn if conn
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 126
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