Active Record Connection Pool
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, or the checkout_timeout
has expired.
Obtaining (checking out) a connection
Connections can be obtained and used from a connection pool in several ways:
-
Simply use ActiveRecord::Base.lease_connection. When you’re done with the connection(s) and wish it to be returned to the pool, you call ActiveRecord::Base.connection_handler.clear_active_connections!. This is 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).
While a thread has a connection checked out from the pool using one of the above three methods, that connection will automatically be the one used by ActiveRecord
queries executing on that thread. It is not required to explicitly pass the checked out connection to Rails models or queries, for example.
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).
- CLASS ActiveRecord::ConnectionAdapters::ConnectionPool::Queue
- CLASS ActiveRecord::ConnectionAdapters::ConnectionPool::Reaper
- A
- C
- D
- F
- I
- L
- N
- R
- S
- W
- MonitorMixin
Attributes
[R] | async_executor | |
[RW] | automatic_reconnect | |
[RW] | checkout_timeout | |
[R] | db_config | |
[R] | pool_config | |
[R] | reaper | |
[R] | role | |
[R] | shard | |
[R] | size |
Class Public methods
install_executor_hooks(executor = ActiveSupport::Executor) Link
new(pool_config) Link
Creates a new ConnectionPool
object. pool_config
is a PoolConfig 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 226 def initialize(pool_config) super() @pool_config = pool_config @db_config = pool_config.db_config @role = pool_config.role @shard = pool_config.shard @checkout_timeout = db_config.checkout_timeout @idle_timeout = db_config.idle_timeout @size = db_config.pool # 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 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>@leases</tt> does not require # synchronization. @leases = LeaseRegistry.new @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 @pinned_connection = nil @pinned_connections_depth = 0 @async_executor = build_async_executor @schema_cache = nil @reaper = Reaper.new(self, db_config.reaping_frequency) @reaper.run end
Instance Public methods
active_connection?() Link
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 lease_connection
or with_connection
methods. Connections obtained through checkout
will not be detected by active_connection?
checkin(conn) Link
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 564 def checkin(conn) return if @pinned_connection.equal?(conn) conn.lock.synchronize do synchronize do connection_lease.clear(conn) conn._run_checkin_callbacks do conn.expire end @available.add conn end end end
checkout(checkout_timeout = @checkout_timeout) Link
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 541 def checkout(checkout_timeout = @checkout_timeout) if @pinned_connection @pinned_connection.lock.synchronize do synchronize do @pinned_connection.verify! # Any leased connection must be in @connections otherwise # some methods like #connected? won't behave correctly unless @connections.include?(@pinned_connection) @connections << @pinned_connection end end end @pinned_connection else checkout_and_verify(acquire_connection(checkout_timeout)) end end
clear_reloadable_connections(raise_on_acquisition_timeout = true) Link
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 isspec.db_config.checkout_timeout * 2
seconds).
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 499 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!() Link
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.db_config.checkout_timeout * 2
seconds), then the pool forcefully clears the cache and reloads connections without any regard for other connection owning threads.
connected?() Link
Returns true if a connection has already been opened.
connections() Link
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.
disconnect(raise_on_acquisition_timeout = true) Link
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 isspec.db_config.checkout_timeout * 2
seconds).
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 446 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 = [] @leases.clear @available.clear end end end
disconnect!() Link
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.db_config.checkout_timeout * 2
seconds), then the pool is forcefully disconnected without any regard for other connection owning threads.
flush(minimum_idle = @idle_timeout) Link
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 637 def flush(minimum_idle = @idle_timeout) return if minimum_idle.nil? idle_connections = synchronize do return if self.discarded? @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
flush!() Link
Disconnect all currently idle connections. Connections currently checked out are unaffected.
lease_connection() Link
Retrieve the connection associated with the current thread, or call checkout
to obtain one if necessary.
lease_connection
can be called any number of times; the connection is held in a cache keyed by a thread.
reap() Link
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 614 def reap stale_connections = synchronize do return if self.discarded? @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(existing_lease = nil) Link
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 lease_connection
or with_connection
methods, connections obtained through checkout
will not be automatically released.
remove(conn) Link
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 582 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
schema_cache() Link
schema_reflection=(schema_reflection) Link
stat() Link
Returns the connection pool’s usage statistic.
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 671 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_connection(prevent_permanent_checkout: false) Link
Yields a connection from the connection pool to the block. If no connection is already checked out by the current thread, a connection will be checked out from the pool, yielded to the block, and then returned to the pool when the block is finished. If a connection has already been checked out on the current thread, such as via lease_connection
or with_connection
, that existing connection will be the one yielded and it will not be returned to the pool automatically at the end of the block; it is expected that such an existing connection will be properly returned to the pool by the code that checked it out.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 399 def with_connection(prevent_permanent_checkout: false) lease = connection_lease sticky_was = lease.sticky lease.sticky = false if prevent_permanent_checkout if lease.connection begin yield lease.connection ensure lease.sticky = sticky_was if prevent_permanent_checkout && !sticky_was end else begin yield lease.connection = checkout ensure lease.sticky = sticky_was if prevent_permanent_checkout && !sticky_was release_connection(lease) unless lease.sticky end end end