class 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:
-
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.
-
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).
-
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: maximum number of connections the pool may manage (default 5). -
idle_timeout: number of seconds that a connection will be kept unused in the pool before it is automatically disconnected (default 300 seconds). Set this to zero to keep connections forever. -
checkout_timeout: number of seconds to wait for a connection to become available before giving up and raising a timeout error (default 5 seconds).
Attributes
Public Class Methods
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 370
def initialize(spec)
super()
@spec = spec
@checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5
if @idle_timeout = spec.config.fetch(:idle_timeout, 300)
@idle_timeout = @idle_timeout.to_f
@idle_timeout = nil if @idle_timeout <= 0
end
# 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 <tt>@thread_cached_conns</tt> 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
# +reaping_frequency+ is configurable mostly for historical reasons, but it could
# also be useful if someone wants a very low +idle_timeout+.
reaping_frequency = spec.config.fetch(:reaping_frequency, 60)
@reaper = Reaper.new(self, reaping_frequency && reaping_frequency.to_f)
@reaper.run
end 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.
Public Instance Methods
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 439 def active_connection? @thread_cached_conns[connection_cache_key(current_thread)] end
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?
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 595
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 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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 586 def checkout(checkout_timeout = @checkout_timeout) checkout_and_verify(acquire_connection(checkout_timeout)) end
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 544
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 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] * 2seconds).
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 568 def clear_reloadable_connections! clear_reloadable_connections(false) end
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 471
def connected?
synchronize { @connections.any? }
end Returns true if a connection has already been opened.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 430 def connection @thread_cached_conns[connection_cache_key(current_thread)] ||= checkout end
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 486
def connections
synchronize { @connections.dup }
end Returns an array containing the connections currently in the pool. Access to the array does not require synchronization on the pool because the array is newly created and not retained by the pool.
However; this method bypasses the ConnectionPool's thread-safe connection access pattern. A returned connection may be owned by another thread, unowned, or by happen-stance owned by the calling thread.
Calling methods on a connection without ownership is subject to the thread-safety guarantees of the underlying method. Many of the methods on connection adapter classes are inherently multi-thread unsafe.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 496
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 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] * 2seconds).
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 518 def disconnect! disconnect(false) end
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 665
def flush(minimum_idle = @idle_timeout)
return if minimum_idle.nil?
idle_connections = synchronize do
@connections.select do |conn|
!conn.in_use? && conn.seconds_idle >= minimum_idle
end.each do |conn|
conn.lease
@available.delete conn
@connections.delete conn
end
end
idle_connections.each do |conn|
conn.disconnect!
end
end Disconnect all connections that have been idle for at least minimum_idle seconds. Connections currently checked out, or that were checked in less than minimum_idle seconds ago, are unaffected.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 686 def flush! reap flush(-1) end
Disconnect all currently idle connections. Connections currently checked out are unaffected.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 417
def lock_thread=(lock_thread)
if lock_thread
@lock_thread = Thread.current
else
@lock_thread = nil
end
end # File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 643
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 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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 450
def release_connection(owner_thread = Thread.current)
if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
checkin conn
end
end 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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 611
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 Remove a connection from the connection pool. The connection will remain open and active but will no longer be managed by this pool.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 699
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 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 }
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 460
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 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.
© 2004–2019 David Heinemeier Hansson
Licensed under the MIT License.