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Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Overview

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:

  1. Simply use {ActiveRecord::Base.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.

  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).

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 Method Summary

Instance Attribute Summary

Instance Method Summary

QueryCache::ConnectionPoolConfiguration - Included

#disable_query_cache!, #enable_query_cache!, #initialize, #query_cache_enabled

Constructor Details

.new(pool_config) ⇒ ConnectionPool

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.

[ GitHub ] 

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 120



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>@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

  @async_executor = build_async_executor

  @reaper = Reaper.new(self, db_config.reaping_frequency)
  @reaper.run
end


  







Instance Attribute Details



  

    #active_connection?  ⇒ Boolean  (readonly)
  



  

    

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?


  





  


  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 196



def active_connection?
  @thread_cached_conns[connection_cache_key(current_thread)]
end


  








  

    #async_executor   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #automatic_reconnect   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 109



attr_accessor :automatic_reconnect, :checkout_timeout


  








  

    #checkout_timeout   (rw)
  

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 109



attr_accessor :automatic_reconnect, :checkout_timeout


  








  

    #connected?  ⇒ Boolean  (readonly)
  



  

    

Returns true if a connection has already been opened.


  





  


  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 233



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


  








  

    #db_config   (readonly)
  

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #discarded?  ⇒ Boolean  (readonly)
  

  

    This method is for internal use only.
  

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 299



def discarded? # :nodoc:
  @connections.nil?
end


  








  

    #lock_thread=(lock_thread)   (writeonly)
  

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 164



def lock_thread=(lock_thread)
  if lock_thread
    @lock_thread = ActiveSupport::IsolatedExecutionState.context
  else
    @lock_thread = nil
  end

  if (active_connection = @thread_cached_conns[connection_cache_key(current_thread)])
    active_connection.lock_thread = @lock_thread
  end
end


  








  

    #pool_config   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #reaper   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #role   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #schema_reflection   (rw)
  

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# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 112



delegate :schema_reflection, :schema_reflection=, to: :pool_config


  








  

    #shard   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #size   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 110



attr_reader :db_config, :size, :reaper, :pool_config, :async_executor, :role, :shard


  







Instance Method Details



  

    #acquire_connection(checkout_timeout)   (private)
  



  

    

Acquire a connection by one of 1) immediately removing one from the queue of available connections, 2) creating a new connection if the pool is not at capacity, 3) waiting on the queue for a connection to become available.



Raises:



  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 647



def acquire_connection(checkout_timeout)
  # NOTE: we rely on <tt>@available.poll</tt> and try_to_checkout_new_connection to
  # conn.lease the returned connection (and to do this in a synchronized
  # section). This is not the cleanest implementation, as ideally we would
  # <tt>synchronize { conn.lease }</tt> in this method, but by leaving it to <tt>@available.poll</tt>
  # and try_to_checkout_new_connection we can piggyback on synchronize sections
  # of the said methods and avoid an additional synchronize overhead.
  if conn = @available.poll || try_to_checkout_new_connection
    conn
  else
    reap
    # Retry after reaping, which may return an available connection,
    # remove an inactive connection, or both
    if conn = @available.poll || try_to_checkout_new_connection
      conn
    else
      @available.poll(checkout_timeout)
    end
  end
end


  








  

    #adopt_connection(conn)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 716



def adopt_connection(conn)
  conn.pool = self
  @connections << conn
end


  








  

    #attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout = true)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 541



def attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout = true)
  collected_conns = synchronize do
    # account for our own connections
    @connections.select { |conn| conn.owner == ActiveSupport::IsolatedExecutionState.context }
  end

  newly_checked_out = []
  timeout_time      = Process.clock_gettime(Process::CLOCK_MONOTONIC) + (@checkout_timeout * 2)

  @available.with_a_bias_for(ActiveSupport::IsolatedExecutionState.context) do
    loop do
      synchronize do
        return if collected_conns.size == @connections.size && @now_connecting == 0
        remaining_timeout = timeout_time - Process.clock_gettime(Process::CLOCK_MONOTONIC)
        remaining_timeout = 0 if remaining_timeout < 0
        conn = checkout_for_exclusive_access(remaining_timeout)
        collected_conns   << conn
        newly_checked_out << conn
      end
    end
  end
rescue ExclusiveConnectionTimeoutError
  # <tt>raise_on_acquisition_timeout == false</tt> means we are directed to ignore any
  # timeouts and are expected to just give up: we've obtained as many connections
  # as possible, note that in a case like that we don't return any of the
  # newly_checked_out connections.

  if raise_on_acquisition_timeout
    release_newly_checked_out = true
    raise
  end
rescue Exception # if something else went wrong
  # this can't be a "naked" rescue, because we have should return conns
  # even for non-StandardErrors
  release_newly_checked_out = true
  raise
ensure
  if release_newly_checked_out && newly_checked_out
    # releasing only those conns that were checked out in this method, conns
    # checked outside this method (before it was called) are not for us to release
    newly_checked_out.each { |conn| checkin(conn) }
  end
end


  








  

    #build_async_executor   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 489



def build_async_executor
  case ActiveRecord.async_query_executor
  when :multi_thread_pool
    if @db_config.max_threads > 0
      Concurrent::ThreadPoolExecutor.new(
        min_threads: @db_config.min_threads,
        max_threads: @db_config.max_threads,
        max_queue: @db_config.max_queue,
        fallback_policy: :caller_runs
      )
    end
  when :global_thread_pool
    ActiveRecord.global_thread_pool_async_query_executor
  end
end


  








  

    #bulk_make_new_connections(num_new_conns_needed)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 507



def bulk_make_new_connections(num_new_conns_needed)
  num_new_conns_needed.times do
    # try_to_checkout_new_connection will not exceed pool's @size limit
    if new_conn = try_to_checkout_new_connection
      # make the new_conn available to the starving threads stuck @available Queue
      checkin(new_conn)
    end
  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 #checkout on this pool.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 363



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

      conn._run_checkin_callbacks do
        conn.expire
      end

      conn.lock_thread = nil
      @available.add conn
    end
  end
end


  








  

    #checkout(checkout_timeout = @checkout_timeout)  
  



  

    

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:



  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 352



def checkout(checkout_timeout = @checkout_timeout)
  connection = checkout_and_verify(acquire_connection(checkout_timeout))
  connection.lock_thread = @lock_thread
  connection
end


  








  

    #checkout_and_verify(c)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 726



def checkout_and_verify(c)
  c._run_checkout_callbacks do
    c.clean!
  end
  c
rescue Exception
  remove c
  c.disconnect!
  raise
end


  








  

    #checkout_for_exclusive_access(checkout_timeout)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 587



def checkout_for_exclusive_access(checkout_timeout)
  checkout(checkout_timeout)
rescue ConnectionTimeoutError
  # this block can't be easily moved into attempt_to_checkout_all_existing_connections's
  # rescue block, because doing so would put it outside of synchronize section, without
  # being in a critical section thread_report might become inaccurate
  msg = +"could not obtain ownership of all database connections in #{checkout_timeout} seconds"

  thread_report = []
  @connections.each do |conn|
    unless conn.owner == ActiveSupport::IsolatedExecutionState.context
      thread_report << "#{conn} is owned by #{conn.owner}"
    end
  end

  msg << " (#{thread_report.join(', ')})" if thread_report.any?

  raise ExclusiveConnectionTimeoutError, msg
end


  








  

    #checkout_new_connection   (private)
  



  

    
  





  
Raises:





  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 721



def checkout_new_connection
  raise ConnectionNotEstablished unless @automatic_reconnect
  new_connection
end


  








  

    #clear_reloadable_connections(raise_on_acquisition_timeout = true)  
  



  

    

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



Raises:



  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 310



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!  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 334



def clear_reloadable_connections!
  clear_reloadable_connections(false)
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 cache keyed by a thread.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 181



def connection
  @thread_cached_conns[connection_cache_key(current_thread)] ||= checkout
end


  








  

    #connection_cache_key(thread)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 522



def connection_cache_key(thread)
  thread
end


  








  

    #connection_class  
    Also known as: #connection_klass
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 185



def connection_class # :nodoc:
  pool_config.connection_class
end


  








  

    #connection_klass  
  



  

    

Alias for #connection_class.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 188



alias :connection_klass :connection_class


  








  

    #connections  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 248



def connections
  synchronize { @connections.dup }
end


  








  

    #current_thread   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 526



def current_thread
  @lock_thread || ActiveSupport::IsolatedExecutionState.context
end


  








  

    #discard!  
  

  

    This method is for internal use only.
  



  

    

Discards all connections in the pool (even if they’re currently leased!), along with the pool itself. Any further interaction with the pool (except #spec and #schema_cache) is undefined.



See AbstractAdapter#discard!


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 289



def discard! # :nodoc:
  synchronize do
    return if self.discarded?
    @connections.each do |conn|
      conn.discard!
    end
    @connections = @available = @thread_cached_conns = nil
  end
end


  








  

    #disconnect(raise_on_acquisition_timeout = true)  
  



  

    

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



Raises:



  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 258



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!  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 280



def disconnect!
  disconnect(false)
end


  








  

    #flush(minimum_idle = @idle_timeout)  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 435



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!  
  



  

    

Disconnect all currently idle connections. Connections currently checked out are unaffected.


  



  [ GitHub ]


  

  


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



def flush!
  reap
  flush(-1)
end


  








  

    #new_connection   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 675



def new_connection
  connection = Base.public_send(db_config.adapter_method, db_config.configuration_hash)
  connection.pool = self
  connection.check_version
  connection
rescue ConnectionNotEstablished => ex
  raise ex.set_pool(self)
end


  








  

    #num_waiting_in_queue  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 462



def num_waiting_in_queue # :nodoc:
  @available.num_waiting
end


  








  

    #reap  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 412



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(conn, owner_thread = conn.owner)   (private)
  



  

    

Alias for #remove_connection_from_thread_cache.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 673



alias_method :release, :remove_connection_from_thread_cache


  








  

    #release_connection(owner_thread = ActiveSupport::IsolatedExecutionState.context)  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 207



def release_connection(owner_thread = ActiveSupport::IsolatedExecutionState.context)
  if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
    checkin conn
  end
end


  








  

    #remove(conn)  
  



  

    

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


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 380



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_connection_from_thread_cache(conn, owner_thread = conn.owner)   (private)
    Also known as: #release
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 670



def remove_connection_from_thread_cache(conn, owner_thread = conn.owner)
  @thread_cached_conns.delete_pair(connection_cache_key(owner_thread), conn)
end


  








  

    #schedule_query(future_result)  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 483



def schedule_query(future_result) # :nodoc:
  @async_executor.post { future_result.execute_or_skip }
  Thread.pass
end


  








  

    #stat  
  



  

    

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 }


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 469



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


  








  

    #try_to_checkout_new_connection   (private)
  



  

    

If the pool is not at a @size limit, establish new connection. Connecting to the DB is done outside main synchronized section.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 689



def try_to_checkout_new_connection
  # first in synchronized section check if establishing new conns is allowed
  # and increment @now_connecting, to prevent overstepping this pool's @size
  # constraint
  do_checkout = synchronize do
    if @threads_blocking_new_connections.zero? && (@connections.size + @now_connecting) < @size
      @now_connecting += 1
    end
  end
  if do_checkout
    begin
      # if successfully incremented @now_connecting establish new connection
      # outside of synchronized section
      conn = checkout_new_connection
    ensure
      synchronize do
        if conn
          adopt_connection(conn)
          # returned conn needs to be already leased
          conn.lease
        end
        @now_connecting -= 1
      end
    end
  end
end


  








  

    #with_connection  
  



  

    

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 #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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 222



def with_connection
  unless conn = @thread_cached_conns[connection_cache_key(ActiveSupport::IsolatedExecutionState.context)]
    conn = connection
    fresh_connection = true
  end
  yield conn
ensure
  release_connection if fresh_connection
end


  








  

    #with_exclusively_acquired_all_connections(raise_on_acquisition_timeout = true)   (private)
  



  

    

Take control of all existing connections so a “group” action such as reload/disconnect can be performed safely. It is no longer enough to wrap it in synchronize because some pool’s actions are allowed to be performed outside of the main synchronize block.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 534



def with_exclusively_acquired_all_connections(raise_on_acquisition_timeout = true)
  with_new_connections_blocked do
    attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout)
    yield
  end
end


  








  

    #with_new_connections_blocked   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 607



def with_new_connections_blocked
  synchronize do
    @threads_blocking_new_connections += 1
  end

  yield
ensure
  num_new_conns_required = 0

  synchronize do
    @threads_blocking_new_connections -= 1

    if @threads_blocking_new_connections.zero?
      @available.clear

      num_new_conns_required = num_waiting_in_queue

      @connections.each do |conn|
        next if conn.in_use?

        @available.add conn
        num_new_conns_required -= 1
      end
    end
  end

  bulk_make_new_connections(num_new_conns_required) if num_new_conns_required > 0
end