Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Inherits:
Object
  • Object
show all
Includes:
MonitorMixin
Defined in:
activerecord/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 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).

Defined Under Namespace

Classes: Queue, Reaper

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 '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] && 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

Instance Attribute Details

#automatic_reconnectObject

Returns the value of attribute automatic_reconnect


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 224

def automatic_reconnect
  @automatic_reconnect
end

#checkout_timeoutObject

Returns the value of attribute checkout_timeout


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 224

def checkout_timeout
  @checkout_timeout
end

#connectionsObject (readonly)

Returns the value of attribute connections


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 225

def connections
  @connections
end

#reaperObject (readonly)

Returns the value of attribute reaper


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 225

def reaper
  @reaper
end

#sizeObject (readonly)

Returns the value of attribute size


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 225

def size
  @size
end

#specObject (readonly)

Returns the value of attribute spec


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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 225

def spec
  @spec
end

Instance Method Details

#active_connection?Boolean

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

Returns:

  • (Boolean)

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# 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) ⇒ 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 '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 conn, owner

    @available.add conn
  end
end

#checkoutObject

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.


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# 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!Object

Clears the cache which maps classes.


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# 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?Boolean

Returns true if a connection has already been opened.

Returns:

  • (Boolean)

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 298

def connected?
  synchronize { @connections.any? }
end

#connectionObject

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 '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!Object

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


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# 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

#reapObject

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.


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# 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) ⇒ 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 '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) ⇒ Object

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


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# 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, conn.owner

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

#with_connectionObject

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 '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