######################
#
# ActiveRecord's ConnectionPool in Rails 3.2.3 allows threads to 'steal'
# connections from each other, so some threads get starved out.
#
# This monkey patch uses an implementation from https://github.com/rails/rails/pull/6492
# that ensures 'fair' queue in ConnectionPool.
#
# It's actually a weird hybrid which ALSO maintains the clear_stale_cached_connections!
# behavior to reclaim leaked orphaned connections, and calls that method
# in checkout when pool has no avail connections.
#
# This file referenced from an initializer in our main engine
# class, that loads it to monkey patch AR.
#
##########################
# give a backdoor to disable this patch
unless ENV["NO_AR_PATCH"]
# make sure it's there so we can monkey patch
require 'active_record'
ActiveRecord::ConnectionAdapters::ConnectionPool
# Unload it so we can redefine it completely
ActiveRecord::ConnectionAdapters.send(:remove_const, :ConnectionPool)
# Some require's our new definition will need
require 'thread'
require 'monitor'
require 'set'
require 'active_support/core_ext/module/deprecation'
# And completely redefine ConnectionPool
class ActiveRecord::ConnectionAdapters::ConnectionPool
# Threadsafe, fair, FIFO queue. Meant to be used by ConnectionPool
# with which it shares a Monitor. But could be a generic Queue.
#
# The Queue in stdlib's 'thread' could replace this class except
# stdlib's doesn't support waiting with a timeout.
class Queue
def initialize(lock = Monitor.new)
@lock = lock
@cond = @lock.new_cond
@num_waiting = 0
@queue = []
end
# Test if any threads are currently waiting on the queue.
def any_waiting?
synchronize do
@num_waiting > 0
end
end
# Return the number of threads currently waiting on this
# queue.
def num_waiting
synchronize do
@num_waiting
end
end
# Add +element+ to the queue. Never blocks.
def add(element)
synchronize do
@queue.push element
@cond.signal
end
end
# If +element+ is in the queue, remove and return it, or nil.
def delete(element)
synchronize do
@queue.delete(element)
end
end
# Remove all elements from the queue.
def clear
synchronize do
@queue.clear
end
end
# Remove the head of the queue.
#
# If +timeout+ is not given, remove and return the head the
# queue if the number of available elements is strictly
# greater than the number of threads currently waiting (that
# is, don't jump ahead in line). Otherwise, return nil.
#
# If +timeout+ is given, block if it there is no element
# available, waiting up to +timeout+ seconds for an element to
# become available.
#
# Raises:
# - ConnectionTimeoutError if +timeout+ is given and no element
# becomes available after +timeout+ seconds,
def poll(timeout = nil)
synchronize do
if timeout
no_wait_poll || wait_poll(timeout)
else
no_wait_poll
end
end
end
def num_available
synchronize do
@queue.size
end
end
private
def synchronize(&block)
@lock.synchronize(&block)
end
# Test if the queue currently contains any elements.
def any?
!@queue.empty?
end
# A thread can remove an element from the queue without
# waiting if an only if the number of currently available
# connections is strictly greater than the number of waiting
# threads.
def can_remove_no_wait?
@queue.size > @num_waiting
end
# Removes and returns the head of the queue if possible, or nil.
def remove
@queue.shift
end
# Remove and return the head the queue if the number of
# available elements is strictly greater than the number of
# threads currently waiting. Otherwise, return nil.
def no_wait_poll
remove if can_remove_no_wait?
end
# Waits on the queue up to +timeout+ seconds, then removes and
# returns the head of the queue.
def wait_poll(timeout)
@num_waiting += 1
t0 = Time.now
elapsed = 0
loop do
@cond.wait(timeout - elapsed)
return remove if any?
elapsed = Time.now - t0
raise ActiveRecord::ConnectionTimeoutError if elapsed >= timeout
end
ensure
@num_waiting -= 1
end
end
# Every +frequency+ seconds, the reaper will call +reap+ on +pool+.
# A reaper instantiated with a nil frequency will never reap the
# connection pool.
#
# Configure the frequency by setting "reaping_frequency" in your
# database yaml file.
class Reaper
attr_reader :pool, :frequency
def initialize(pool, frequency)
@pool = pool
@frequency = frequency
end
def run
return unless frequency
Thread.new(frequency, pool) { |t, p|
while true
sleep t
p.reap
end
}
end
end
include MonitorMixin
attr_accessor :automatic_reconnect, :checkout_timeout, :dead_connection_timeout
attr_reader :spec, :connections, :size, :reaper
# Creates a new ConnectionPool object. +spec+ is a ConnectionSpecification
# object which describes database connection information (e.g. adapter,
# host name, username, password, etc), as well as the maximum size for
# this ConnectionPool.
#
# The default ConnectionPool maximum size is 5.
def initialize(spec)
super()
@spec = spec
# The cache of reserved connections mapped to threads
@reserved_connections = {}
@checkout_timeout = spec.config[:checkout_timeout] || 5
@dead_connection_timeout = spec.config[:dead_connection_timeout]
@reaper = Reaper.new self, spec.config[:reaping_frequency]
@reaper.run
# default max pool size to 5
@size = (spec.config[:pool] && spec.config[:pool].to_i) || 5
@connections = []
@automatic_reconnect = true
@available = Queue.new self
end
# Hack for tests to be able to add connections. Do not call outside of tests
def insert_connection_for_test!(c) #:nodoc:
synchronize do
@connections << c
@available.add c
end
end
# Retrieve the connection associated with the current thread, or call
# #checkout to obtain one if necessary.
#
# #connection can be called any number of times; the connection is
# held in a hash keyed by the thread id.
def connection
synchronize do
@reserved_connections[current_connection_id] ||= checkout
end
end
# Is there an open connection that is being used for the current thread?
def active_connection?
synchronize do
@reserved_connections.fetch(current_connection_id) {
return false
}.in_use?
end
end
# Signal that the thread is finished with the current connection.
# #release_connection releases the connection-thread association
# and returns the connection to the pool.
def release_connection(with_id = current_connection_id)
synchronize do
conn = @reserved_connections.delete(with_id)
checkin conn if conn
end
end
# If a connection already exists yield it to the block. If no connection
# exists checkout a connection, yield it to the block, and checkin the
# connection when finished.
def with_connection
connection_id = current_connection_id
fresh_connection = true unless active_connection?
yield connection
ensure
release_connection(connection_id) if fresh_connection
end
# Returns true if a connection has already been opened.
def connected?
synchronize { @connections.any? }
end
# Disconnects all connections in the pool, and clears the pool.
def disconnect!
synchronize do
@reserved_connections = {}
@connections.each do |conn|
checkin conn
conn.disconnect!
end
@connections = []
@available.clear
end
end
# Clears the cache which maps classes.
def clear_reloadable_connections!
synchronize do
@reserved_connections = {}
@connections.each do |conn|
checkin conn
conn.disconnect! if conn.requires_reloading?
end
@connections.delete_if do |conn|
conn.requires_reloading?
end
@available.clear
@connections.each do |conn|
@available.add conn
end
end
end
# clear_stale_cached imp from Rails 3.2, still using Threads.
# Yes, we've created a monster.
# Return any checked-out connections back to the pool by threads that
# are no longer alive.
def clear_stale_cached_connections!
keys = @reserved_connections.keys - Thread.list.find_all { |t|
t.alive?
}.map { |thread| thread.object_id }
keys.each do |key|
conn = @reserved_connections[key]
ActiveSupport::Deprecation.warn(<<-eowarn) if conn.in_use?
Database connections will not be closed automatically, please close your
database connection at the end of the thread by calling `close` on your
connection. For example: ActiveRecord::Base.connection.close
eowarn
checkin conn
@reserved_connections.delete(key)
end
end
# Check-out a database connection from the pool, indicating that you want
# to use it. You should call #checkin when you no longer need this.
#
# This is done by either returning and leasing existing connection, or by
# creating a new connection and leasing it.
#
# If all connections are leased and the pool is at capacity (meaning the
# number of currently leased connections is greater than or equal to the
# size limit set), an ActiveRecord::PoolFullError exception will be raised.
#
# Returns: an AbstractAdapter object.
#
# Raises:
# - PoolFullError: no connection can be obtained from the pool.
def checkout
synchronize do
conn = acquire_connection
conn.lease
checkout_and_verify(conn)
end
end
# Check-in a database connection back into the pool, indicating that you
# no longer need this connection.
#
# +conn+: an AbstractAdapter object, which was obtained by earlier by
# calling +checkout+ on this pool.
def checkin(conn)
synchronize do
conn.run_callbacks :checkin do
conn.expire
end
release conn
@available.add conn
end
end
# Remove a connection from the connection pool. The connection will
# remain open and active but will no longer be managed by this pool.
def remove(conn)
synchronize do
@connections.delete conn
@available.delete conn
# FIXME: we might want to store the key on the connection so that removing
# from the reserved hash will be a little easier.
release conn
@available.add checkout_new_connection if @available.any_waiting?
end
end
# Removes dead connections from the pool. A dead connection can occur
# if a programmer forgets to close a connection at the end of a thread
# or a thread dies unexpectedly.
def reap
synchronize do
stale = Time.now - @dead_connection_timeout
connections.dup.each do |conn|
remove conn if conn.in_use? && stale > conn.last_use && !conn.active?
end
end
end
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:
# - PoolFullError if a connection could not be acquired (FIXME:
# why not ConnectionTimeoutError?
def acquire_connection
if conn = @available.poll
conn
elsif @connections.size < @size
checkout_new_connection
else
clear_stale_cached_connections!
t0 = Time.now
Rails.logger.info("POLLED_CHECKOUT: num avail connections: #{@available.num_available}; num waiting: #{@available.num_waiting}; total connections: #{@connections.size}")
begin
@available.poll(@checkout_timeout)
rescue ActiveRecord::ConnectionTimeoutError
msg = 'could not obtain a database connection within %0.3f seconds (waited %0.3f seconds)' %
[@checkout_timeout, Time.now - t0]
raise ActiveRecord::ConnectionTimeoutError, msg
end
end
end
def release(conn)
thread_id = if @reserved_connections[current_connection_id] == conn
current_connection_id
else
@reserved_connections.keys.find { |k|
@reserved_connections[k] == conn
}
end
@reserved_connections.delete thread_id if thread_id
end
def new_connection
ActiveRecord::Base.send(spec.adapter_method, spec.config)
end
def current_connection_id #:nodoc:
ActiveRecord::Base.connection_id ||= Thread.current.object_id
end
def checkout_new_connection
raise ConnectionNotEstablished unless @automatic_reconnect
c = new_connection
c.pool = self
@connections << c
c
end
def checkout_and_verify(c)
c.run_callbacks :checkout do
c.verify!
end
c
end
end
end