Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Inherits:
Object
  • Object
show all
Includes:
QueryCache::ConnectionPoolConfiguration, MonitorMixin
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 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).

– Synchronization policy:

  • all public methods can be called outside synchronize

  • access to these instance variables needs to be in synchronize:

    • @connections

    • @now_connecting

  • private methods that require being called in a synchronize blocks are now explicitly documented

Defined Under Namespace

Modules: BiasableQueue Classes: ConnectionLeasingQueue, Queue, Reaper

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from QueryCache::ConnectionPoolConfiguration

#disable_query_cache!, #enable_query_cache!, #query_cache_enabled

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 321

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

  # This variable tracks the cache of threads mapped to reserved connections, with the
  # sole purpose of speeding up the +connection+ method. It is not the authoritative
  # registry of which thread owns which connection. Connection ownership is tracked by
  # the +connection.owner+ attr on each +connection+ instance.
  # The invariant works like this: if there is mapping of <tt>thread => conn</tt>,
  # then that +thread+ does indeed own that +conn+. However, an absence of a such
  # mapping does not mean that the +thread+ doesn't own the said connection. In
  # that case +conn.owner+ attr should be consulted.
  # Access and modification of [email protected]_cached_conns+ does not require
  # synchronization.
  @thread_cached_conns = Concurrent::Map.new(initial_capacity: @size)

  @connections         = []
  @automatic_reconnect = true

  # Connection pool allows for concurrent (outside the main +synchronize+ section)
  # establishment of new connections. This variable tracks the number of threads
  # currently in the process of independently establishing connections to the DB.
  @now_connecting = 0

  @threads_blocking_new_connections = 0

  @available = ConnectionLeasingQueue.new self

  @lock_thread = false
end

Instance Attribute Details

#automatic_reconnectObject

Returns the value of attribute automatic_reconnect


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 312

def automatic_reconnect
  @automatic_reconnect
end

#checkout_timeoutObject

Returns the value of attribute checkout_timeout


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 312

def checkout_timeout
  @checkout_timeout
end

#connectionsObject (readonly)

Returns the value of attribute connections


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 313

def connections
  @connections
end

#reaperObject (readonly)

Returns the value of attribute reaper


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 313

def reaper
  @reaper
end

#schema_cacheObject

Returns the value of attribute schema_cache


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 312

def schema_cache
  @schema_cache
end

#sizeObject (readonly)

Returns the value of attribute size


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 313

def size
  @size
end

#specObject (readonly)

Returns the value of attribute spec


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 313

def spec
  @spec
end

Instance Method Details

#active_connection?Boolean

Returns true if there is an open connection being used for the current thread.

This method only works for connections that have been obtained through #connection or #with_connection methods. Connections obtained through #checkout will not be detected by #active_connection?

Returns:

  • (Boolean)

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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 382

def active_connection?
  @thread_cached_conns[connection_cache_key(Thread.current)]
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 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 508

def checkin(conn)
  conn.lock.synchronize do
    synchronize do
      remove_connection_from_thread_cache conn

      conn._run_checkin_callbacks do
        conn.expire
      end

      @available.add conn
    end
  end
end

#checkout(checkout_timeout = @checkout_timeout) ⇒ 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 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:

  • ActiveRecord::ConnectionTimeoutError no connection can be obtained from the pool.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 499

def checkout(checkout_timeout = @checkout_timeout)
  checkout_and_verify(acquire_connection(checkout_timeout))
end

#clear_reloadable_connections(raise_on_acquisition_timeout = true) ⇒ Object

Clears the cache which maps classes and re-connects connections that require reloading.

Raises:

  • ActiveRecord::ExclusiveConnectionTimeoutError if unable to gain ownership of all connections in the pool within a timeout interval (default duration is spec.config[:checkout_timeout] * 2 seconds).


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 457

def clear_reloadable_connections(raise_on_acquisition_timeout = true)
  with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
    synchronize do
      @connections.each do |conn|
        if conn.in_use?
          conn.steal!
          checkin conn
        end
        conn.disconnect! if conn.requires_reloading?
      end
      @connections.delete_if(&:requires_reloading?)
      @available.clear
    end
  end
end

#clear_reloadable_connections!Object

Clears the cache which maps classes and re-connects connections that require reloading.

The pool first tries to gain ownership of all connections. If unable to do so within a timeout interval (default duration is spec.config[:checkout_timeout] * 2 seconds), then the pool forcefully clears the cache and reloads connections without any regard for other connection owning threads.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 481

def clear_reloadable_connections!
  clear_reloadable_connections(false)
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 414

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 cache keyed by a thread.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 373

def connection
  @thread_cached_conns[connection_cache_key(@lock_thread || Thread.current)] ||= checkout
end

#disconnect(raise_on_acquisition_timeout = true) ⇒ Object

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

Raises:

  • ActiveRecord::ExclusiveConnectionTimeoutError if unable to gain ownership of all connections in the pool within a timeout interval (default duration is spec.config[:checkout_timeout] * 2 seconds).


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 424

def disconnect(raise_on_acquisition_timeout = true)
  with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
    synchronize do
      @connections.each do |conn|
        if conn.in_use?
          conn.steal!
          checkin conn
        end
        conn.disconnect!
      end
      @connections = []
      @available.clear
    end
  end
end

#disconnect!Object

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

The pool first tries to gain ownership of all connections. If unable to do so within a timeout interval (default duration is spec.config[:checkout_timeout] * 2 seconds), then the pool is forcefully disconnected without any regard for other connection owning threads.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 446

def disconnect!
  disconnect(false)
end

#lock_thread=(lock_thread) ⇒ Object


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 360

def lock_thread=(lock_thread)
  if lock_thread
    @lock_thread = Thread.current
  else
    @lock_thread = nil
  end
end

#num_waiting_in_queueObject

:nodoc:


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 575

def num_waiting_in_queue # :nodoc:
  @available.num_waiting
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 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 556

def reap
  stale_connections = synchronize do
    @connections.select do |conn|
      conn.in_use? && !conn.owner.alive?
    end.each do |conn|
      conn.steal!
    end
  end

  stale_connections.each do |conn|
    if conn.active?
      conn.reset!
      checkin conn
    else
      remove conn
    end
  end
end

#release_connection(owner_thread = Thread.current) ⇒ 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.

This method only works for connections that have been obtained through #connection or #with_connection methods, connections obtained through #checkout will not be automatically released.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 393

def release_connection(owner_thread = Thread.current)
  if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
    checkin 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 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 524

def remove(conn)
  needs_new_connection = false

  synchronize do
    remove_connection_from_thread_cache conn

    @connections.delete conn
    @available.delete conn

    # @available.any_waiting? => true means that prior to removing this
    # conn, the pool was at its max size (@connections.size == @size).
    # This would mean that any threads stuck waiting in the queue wouldn't
    # know they could checkout_new_connection, so let's do it for them.
    # Because condition-wait loop is encapsulated in the Queue class
    # (that in turn is oblivious to ConnectionPool implementation), threads
    # that are "stuck" there are helpless. They have no way of creating
    # new connections and are completely reliant on us feeding available
    # connections into the Queue.
    needs_new_connection = @available.any_waiting?
  end

  # This is intentionally done outside of the synchronized section as we
  # would like not to hold the main mutex while checking out new connections.
  # Thus there is some chance that needs_new_connection information is now
  # stale, we can live with that (bulk_make_new_connections will make
  # sure not to exceed the pool's @size limit).
  bulk_make_new_connections(1) if needs_new_connection
end

#statObject

Return connection pool's usage statistic Example:

ActiveRecord::Base.connection_pool.stat # => { size: 15, connections: 1, busy: 1, dead: 0, idle: 0, waiting: 0, checkout_timeout: 5 }

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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 583

def stat
  synchronize do
    {
      size: size,
      connections: @connections.size,
      busy: @connections.count { |c| c.in_use? && c.owner.alive? },
      dead: @connections.count { |c| c.in_use? && !c.owner.alive? },
      idle: @connections.count { |c| !c.in_use? },
      waiting: num_waiting_in_queue,
      checkout_timeout: checkout_timeout
    }
  end
end

#with_connectionObject

If a connection obtained through #connection or #with_connection methods already exists yield it to the block. If no such 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 403

def with_connection
  unless conn = @thread_cached_conns[connection_cache_key(Thread.current)]
    conn = connection
    fresh_connection = true
  end
  yield conn
ensure
  release_connection if fresh_connection
end