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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.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.

  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:

  • #checkout_timeout: number of seconds to wait for a connection to become available before giving up and raising a timeout error (default 5 seconds).

  • 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.

  • #keepalive: number of seconds between keepalive checks if the connection has been idle (default 600 seconds).

  • #max_age: number of seconds the pool will allow the connection to exist before retiring it at next checkin. (default Float::INFINITY).

  • #max_connections: maximum number of connections the pool may manage (default 5).

  • #min_connections: minimum number of connections the pool will open and maintain (default 0).

  • pool_jitter: maximum reduction factor to apply to #max_age and #keepalive intervals (default 0.2; range 0.0-1.0).

Class Method Summary

Instance Attribute Summary

Instance Method Summary

QueryCache::ConnectionPoolConfiguration - Included

#checkout_and_verify, #clear_query_cache, #dirties_query_cache,
#disable_query_cache

Disable the query cache within the block.
#disable_query_cache!, #enable_query_cache, #enable_query_cache!, #initialize, #query_cache, #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 246



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
  @max_connections = db_config.max_connections
  @min_connections = db_config.min_connections
  @max_age = db_config.max_age
  @keepalive = db_config.keepalive

  # 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

  # Sometimes otherwise-idle connections are temporarily held by the Reaper for
  # maintenance. This variable tracks the number of connections currently in that
  # state -- if a thread requests a connection and there are none available, it
  # will await any in-maintenance connections in preference to creating a new one.
  @maintaining = 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

  @activated = false
  @original_context = ActiveSupport::IsolatedExecutionState.context

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


  







Class Method Details



  

    .install_executor_hooks(executor = ActiveSupport::Executor)  
  

  [ GitHub ]


  

  


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



def install_executor_hooks(executor = ActiveSupport::Executor)
  executor.register_hook(ExecutorHooks)
end


  







Instance Attribute Details



  

    #activated?  ⇒ Boolean  (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 341



def activated?
  @activated
end


  








  

    #active_connection   (readonly)
  

  

    This method is for internal use only.
  



  

    

Alias for #active_connection?.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 414



alias_method :active_connection, :active_connection? # :nodoc:


  








  

    #async_executor   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #automatic_reconnect   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 234



attr_accessor :automatic_reconnect, :checkout_timeout


  








  

    #checkout_timeout   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 234



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 482



def connected?
  synchronize { @connections.any?(&:connected?) }
end


  








  

    #db_config   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #discarded?  ⇒ Boolean  (readonly)
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 559



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


  








  

    #keepalive   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #maintainable?  ⇒ Boolean  (readonly)
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 563



def maintainable? # :nodoc:
  synchronize do
    @connections&.size&.> 0 || (activated? && @min_connections > 0)
  end
end


  








  

    #max_age   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #max_connections   (readonly)
    Also known as: #size
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #min_connections   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #permanent_lease?  ⇒ Boolean  (readonly)
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 356



def permanent_lease? # :nodoc:
  connection_lease.sticky.nil?
end


  








  

    #pool_config   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #pool_transaction_isolation_level   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 886



def pool_transaction_isolation_level
  isolation_level_key = "activerecord_pool_transaction_isolation_level_#{db_config.name}"
  ActiveSupport::IsolatedExecutionState[isolation_level_key]
end


  








  

    #pool_transaction_isolation_level=(isolation_level)   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 891



def pool_transaction_isolation_level=(isolation_level)
  isolation_level_key = "activerecord_pool_transaction_isolation_level_#{db_config.name}"
  ActiveSupport::IsolatedExecutionState[isolation_level_key] = isolation_level
end


  








  

    #reaper   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #role   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #schema_reflection   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 238



delegate :schema_reflection, :server_version, to: :pool_config


  








  

    #schema_reflection=(schema_reflection)   (rw)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 316



def schema_reflection=(schema_reflection)
  pool_config.schema_reflection = schema_reflection
  @schema_cache = nil
end


  








  

    #server_version   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 238



delegate :schema_reflection, :server_version, to: :pool_config


  








  

    #shard   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 235



attr_reader :db_config, :max_connections, :min_connections, :max_age, :keepalive, :reaper, :pool_config, :async_executor, :role, :shard


  








  

    #size   (readonly)
  



  

    

Alias for #max_connections.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 236



alias :size :max_connections


  







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 1151



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_queue_for_background_connection(checkout_timeout) || 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_queue_for_background_connection(checkout_timeout) || try_to_checkout_new_connection
      conn
    else
      @available.poll(checkout_timeout)
    end
  end
rescue ConnectionTimeoutError => ex
  raise ex.set_pool(self)
end


  








  

    #activate  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 337



def activate
  @activated = true
end


  








  

    #active_connection?  ⇒ Boolean  (readonly)
    Also known as: #active_connection
  



  

    

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?


  





  


  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 411



def active_connection?
  connection_lease.connection
end


  








  

    #adopt_connection(conn)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1253



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

  # We just created the first connection, it's time to load the schema
  # cache if that wasn't eagerly done before
  if @schema_cache.nil? && ActiveRecord.lazily_load_schema_cache
    schema_cache.load!
  end
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 1042



def attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout = true)
  collected_conns = synchronize do
    reap # No need to wait for dead owners

    # 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 901



def build_async_executor
  case ActiveRecord.async_query_executor
  when :multi_thread_pool
    if @db_config.max_threads > 0
      name_with_shard = [name_inspect, shard_inspect].join("-").tr("_", "-")
      Concurrent::ThreadPoolExecutor.new(
        name: "ActiveRecord-#{name_with_shard}-async-query-executor",
        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 1019



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 @max_connections 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 650



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)  
  



  

    

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 622



def checkout(checkout_timeout = @checkout_timeout)
  return checkout_and_verify(acquire_connection(checkout_timeout)) unless @pinned_connection

  @pinned_connection.lock.synchronize do
    synchronize do
      # The pinned connection may have been cleaned up before we synchronized, so check if it is still present
      if @pinned_connection
        @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

        @pinned_connection
      else
        checkout_and_verify(acquire_connection(checkout_timeout))
      end
    end
  end
end


  








  

    #checkout_and_verify(c)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1269



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 1091



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.new(msg, connection_pool: self)
end


  








  

    #checkout_for_maintenance(conn)   (private)
  



  

    

Directly check a specific connection out of the pool. Skips callbacks.



The connection must later either #return_from_maintenance or #remove_from_maintenance, or the pool will hang.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 982



def checkout_for_maintenance(conn)
  synchronize do
    @maintaining += 1
    @available.delete(conn)
    conn.lease
    conn
  end
end


  








  

    #checkout_new_connection   (private)
  



  

    
  





  
Raises:





  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1264



def checkout_new_connection
  raise ConnectionNotEstablished unless @automatic_reconnect
  new_connection
end


  








  

    #clear_reloadable_connections(raise_on_acquisition_timeout = true)  
  



  

    

Clears reloadable connections from the pool and re-connects connections that require reloading.



Raises:



  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 580



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 reloadable connections from the pool 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 604



def clear_reloadable_connections!
  clear_reloadable_connections(false)
end


  








  

    #connection_descriptor  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 402



def connection_descriptor # :nodoc:
  pool_config.connection_descriptor
end


  








  

    #connection_lease   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 897



def connection_lease
  @leases[ActiveSupport::IsolatedExecutionState.context]
end


  








  

    #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 497



def connections
  synchronize { @connections.dup }
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 547



def discard! # :nodoc:
  @reaper_lock.synchronize do
    synchronize do
      return if self.discarded?
      @connections.each do |conn|
        conn.discard!
      end
      @connections = @available = @leases = nil
    end
  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 507



def disconnect(raise_on_acquisition_timeout = true)
  @reaper_lock.synchronize do
    return if self.discarded?

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

        # Stop maintaining the minimum size until reactivated
        @activated = false
      end
    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 538



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 723



def flush(minimum_idle = @idle_timeout)
  return if minimum_idle.nil?

  removed_connections = synchronize do
    return if self.discarded?

    idle_connections = @connections.select do |conn|
      !conn.in_use? && conn.seconds_idle >= minimum_idle
    end.sort_by { |conn| -conn.seconds_idle } # sort longest idle first

    # Don't go below our configured pool minimum unless we're flushing
    # everything
    idles_to_retain =
      if minimum_idle > 0
        @min_connections - (@connections.size - idle_connections.size)
      else
        0
      end

    if idles_to_retain > 0
      idle_connections.pop idles_to_retain
    end

    idle_connections.each do |conn|
      conn.lease

      @available.delete conn
      @connections.delete conn
    end
  end

  removed_connections.each do |conn|
    conn.disconnect!
  end
end


  








  

    #flush!  
  



  

    

Disconnect all currently idle connections. Connections currently checked out are unaffected. The pool will stop maintaining its minimum size until it is reactivated (such as by a subsequent checkout).


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 762



def flush!
  reap
  flush(-1)

  # Stop maintaining the minimum size until reactivated
  @activated = false
end


  








  

    #inspect  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 305



def inspect # :nodoc:
  name_field = " name=#{name_inspect}" if name_inspect
  shard_field = " shard=#{shard_inspect}" if shard_inspect

  "#<#{self.class.name} env_name=#{db_config.env_name.inspect}#{name_field} role=#{role.inspect}#{shard_field}>"
end


  








  

    #internal_metadata  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 333



def internal_metadata # :nodoc:
  InternalMetadata.new(self)
end


  








  

    #keep_alive(threshold = @keepalive)  
  



  

    

Prod any connections that have been idle for longer than the configured keepalive time. This will incidentally verify the connection is still alive, but the main purpose is to show the server (and any intermediate network hops) that we’re still here and using the connection.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 820



def keep_alive(threshold = @keepalive)
  return if threshold.nil?

  sequential_maintenance -> c { (c.seconds_since_last_activity || 0) > c.pool_jitter(threshold) } do |conn|
    # conn.active? will cause some amount of network activity, which is all
    # we need to provide a keepalive signal.
    #
    # If it returns false, the connection is already broken; disconnect,
    # so it can be found and repaired.
    conn.disconnect! unless conn.active?
  end
end


  








  

    #lease_connection  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 350



def lease_connection
  lease = connection_lease
  lease.sticky = true
  lease.connection ||= checkout
end


  








  

    #migration_context  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 321



def migration_context # :nodoc:
  MigrationContext.new(migrations_paths, schema_migration, internal_metadata)
end


  








  

    #migrations_paths  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 325



def migrations_paths # :nodoc:
  db_config.migrations_paths || Migrator.migrations_paths
end


  








  

    #name_inspect   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1280



def name_inspect
  db_config.name.inspect unless db_config.name == "primary"
end


  








  

    #new_connection  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 878



def new_connection # :nodoc:
  connection = db_config.new_connection
  connection.pool = self
  connection
rescue ConnectionNotEstablished => ex
  raise ex.set_pool(self)
end


  








  

    #num_available_in_queue  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 852



def num_available_in_queue # :nodoc:
  @available.size
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 848



def num_waiting_in_queue # :nodoc:
  @available.num_waiting
end


  








  

    #pin_connection!(lock_thread)  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


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



def pin_connection!(lock_thread) # :nodoc:
  @pinned_connection ||= (connection_lease&.connection || checkout)
  @pinned_connections_depth += 1

  # 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

  @pinned_connection.lock_thread = ActiveSupport::IsolatedExecutionState.context if lock_thread
  @pinned_connection.verify! # eagerly validate the connection
  @pinned_connection.begin_transaction joinable: false, _lazy: false
end


  








  

    #preconnect  
  



  

    

Preconnect all connections in the pool. This saves pool users from having to wait for a connection to be established when first using it after checkout.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 805



def preconnect
  sequential_maintenance -> c { (!c.connected? || !c.verified?) && c.allow_preconnect } do |conn|
    conn.connect!
  rescue
    # Wholesale rescue: there's nothing we can do but move on. The
    # connection will go back to the pool, and the next consumer will
    # presumably try to connect again -- which will either work, or
    # fail and they'll be able to report the exception.
  end
end


  








  

    #prepopulate  
  



  

    

Ensure that the pool contains at least the configured minimum number of connections.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 772



def prepopulate
  need_new_connections = nil

  synchronize do
    return if self.discarded?

    # We don't want to start prepopulating until we know the pool is wanted,
    # so we can avoid maintaining full pools in one-off scripts etc.
    return unless @activated

    need_new_connections = @connections.size < @min_connections
  end

  if need_new_connections
    while new_conn = try_to_checkout_new_connection { @connections.size < @min_connections }
      checkin(new_conn)
    end
  end
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 700



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


  








  

    #reaper_lock(&block)  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 569



def reaper_lock(&block) # :nodoc:
  @reaper_lock.synchronize(&block)
end


  








  

    #recycle!  
  



  

    

Immediately mark all current connections as due for replacement, equivalent to them having reached #max_age – even if there is no #max_age configured.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 836



def recycle!
  synchronize do
    return if self.discarded?

    @connections.each do |conn|
      conn.force_retirement
    end
  end

  retire_old_connections
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 1206



alias_method :release, :remove_connection_from_thread_cache


  








  

    #release_connection(existing_lease = nil)  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 423



def release_connection(existing_lease = nil)
  return if self.discarded?

  if conn = connection_lease.release
    checkin conn
    return true
  end
  false
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 668



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 == @max_connections).
    # 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.num_waiting > @maintaining
  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 @max_connections 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 1201



def remove_connection_from_thread_cache(conn, owner_thread = conn.owner)
  if owner_thread
    @leases[owner_thread].clear(conn)
  end
end


  








  

    #remove_from_maintenance(conn)   (private)
  



  

    

Remove a connection from the pool after it has been checked out for maintenance. It will be automatically replaced with a new connection if necessary.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1010



def remove_from_maintenance(conn)
  synchronize do
    @maintaining -= 1
    remove conn
  end
end


  








  

    #retire_old_connections(max_age = @max_age)  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 792



def retire_old_connections(max_age = @max_age)
  max_age ||= Float::INFINITY

  sequential_maintenance -> c { c.connection_age&.>= c.pool_jitter(max_age) } do |conn|
    # Disconnect, then return the adapter to the pool. Preconnect will
    # handle the rest.
    conn.disconnect!
  end
end


  








  

    #return_from_maintenance(conn)   (private)
  



  

    

Return a connection to the pool after it has been checked out for maintenance. Does not update the connection’s idle time, and skips callbacks.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 999



def return_from_maintenance(conn)
  synchronize do
    conn.expire(false)
    @available.add_back(conn)
    @maintaining -= 1
  end
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 873



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


  








  

    #schema_cache  
  

  [ GitHub ]


  

  


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



def schema_cache
  @schema_cache ||= BoundSchemaReflection.new(schema_reflection, self)
end


  








  

    #schema_migration  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 329



def schema_migration # :nodoc:
  SchemaMigration.new(self)
end


  








  

    #sequential_maintenance(candidate_selector, &maintenance_work)   (private)
  



  

    

Perform maintenance work on pool connections. This method will select a connection to work on by calling the candidate_selector proc while holding the pool lock. If a connection is selected, it will be checked out for maintenance and passed to the maintenance_work proc. The connection will always be returned to the pool after the proc completes.



If the pool has async threads, all work will be scheduled there. Otherwise, this method will block until all work is complete.



Each connection will only be processed once per call to this method, but (particularly in the async case) there is no protection against a second call to this method starting to work through the list before the first call has completed. (Though regular pool behaviour will prevent two instances from working on the same specific connection at the same time.)


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 935



def sequential_maintenance(candidate_selector, &maintenance_work)
  # This hash doesn't need to be synchronized, because it's only
  # used by one thread at a time: the perform_work block gives
  # up its right to connections_visited when it schedules the
  # next iteration.
  connections_visited = Hash.new(false)
  connections_visited.compare_by_identity

  perform_work = lambda do
    connection_to_maintain = nil

    synchronize do
      unless self.discarded?
        if connection_to_maintain = @connections.select { |conn| !conn.in_use? }.select(&candidate_selector).sort_by(&:seconds_idle).find { |conn| !connections_visited[conn] }
          checkout_for_maintenance connection_to_maintain
        end
      end
    end

    if connection_to_maintain
      connections_visited[connection_to_maintain] = true

      # If we're running async, we can schedule the next round of work
      # as soon as we've grabbed a connection to work on.
      @async_executor&.post(&perform_work)

      begin
        maintenance_work.call connection_to_maintain
      ensure
        return_from_maintenance connection_to_maintain
      end

      true
    end
  end

  if @async_executor
    @async_executor.post(&perform_work)
  else
    nil while perform_work.call
  end
end


  








  

    #shard_inspect   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1284



def shard_inspect
  shard.inspect unless shard == :default
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 859



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 @max_connections limit, establish new connection. Connecting to the DB is done outside main synchronized section.



If a block is supplied, it is an additional constraint (checked while holding the pool lock) on whether a new connection should be established.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1216



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 @max_connections
  # constraint
  do_checkout = synchronize do
    return if self.discarded?

    if @threads_blocking_new_connections.zero? && (@connections.size + @now_connecting) < @max_connections && (!block_given? || yield)
      if @connections.size > 0 || @original_context != ActiveSupport::IsolatedExecutionState.context
        @activated = true
      end

      @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
        @now_connecting -= 1
        if conn
          if self.discarded?
            conn.discard!
          else
            adopt_connection(conn)
            # returned conn needs to be already leased
            conn.lease
          end
        end
      end
    end
  end
end


  








  

    #try_to_queue_for_background_connection(checkout_timeout)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1186



def try_to_queue_for_background_connection(checkout_timeout)
  return unless @maintaining > 0

  synchronize do
    return unless @maintaining > @available.num_waiting

    # We are guaranteed the "maintaining" thread will return its promised
    # connection within one maintenance-unit of time. Thus we can safely
    # do a blocking wait with (functionally) no timeout.
    @available.poll(100)
  end
end


  








  

    #unpin_connection!  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 375



def unpin_connection! # :nodoc:
  raise "There isn't a pinned connection #{object_id}" unless @pinned_connection

  clean = true
  @pinned_connection.lock.synchronize do
    @pinned_connections_depth -= 1
    connection = @pinned_connection
    @pinned_connection = nil if @pinned_connections_depth.zero?

    if connection.transaction_open?
      connection.rollback_transaction
    else
      # Something committed or rolled back the transaction
      clean = false
      connection.reset!
    end

    if @pinned_connection.nil?
      connection.steal!
      connection.lock_thread = nil
      checkin(connection)
    end
  end

  clean
end


  








  

    #with_connection(prevent_permanent_checkout: false)  
  



  

    

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.


  



  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 442



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


  








  

    #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 1033



def with_exclusively_acquired_all_connections(raise_on_acquisition_timeout = true)
  @reaper_lock.synchronize do
    with_new_connections_blocked do
      attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout)
      yield
    end
  end
end


  








  

    #with_new_connections_blocked   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 1111



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


  








  

    #with_pool_transaction_isolation_level(isolation_level, transaction_open, &block)  
  

  

    This method is for internal use only.
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 463



def with_pool_transaction_isolation_level(isolation_level, transaction_open, &block) # :nodoc:
  if !ActiveRecord.default_transaction_isolation_level.nil?
    begin
      if transaction_open && self.pool_transaction_isolation_level != ActiveRecord.default_transaction_isolation_level
        raise ActiveRecord::TransactionIsolationError, "cannot set default isolation level while transaction is open"
      end

      old_level = self.pool_transaction_isolation_level
      self.pool_transaction_isolation_level = isolation_level
      yield
    ensure
      self.pool_transaction_isolation_level = old_level
    end
  else
    yield
  end
end