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Merge pull request #6492 from pmahoney/fair-connection-pool2

Fair connection pool2

Conflicts:
	activerecord/test/cases/associations/eager_test.rb
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commit 174f36a0778dd4ed26663f22e62d6b010cf7d216 2 parents 2d1ca38 + 02b2335
@rafaelfranca rafaelfranca authored
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211 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -2,7 +2,6 @@
require 'monitor'
require 'set'
require 'active_support/core_ext/module/deprecation'
-require 'timeout'
module ActiveRecord
# Raised when a connection could not be obtained within the connection
@@ -70,6 +69,131 @@ module ConnectionAdapters
# after which the Reaper will consider a connection reapable. (default
# 5 seconds).
class 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
+
+ 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 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.
@@ -100,21 +224,6 @@ def run
attr_accessor :automatic_reconnect, :checkout_timeout, :dead_connection_timeout
attr_reader :spec, :connections, :size, :reaper
- class Latch # :nodoc:
- def initialize
- @mutex = Mutex.new
- @cond = ConditionVariable.new
- end
-
- def release
- @mutex.synchronize { @cond.broadcast }
- end
-
- def await
- @mutex.synchronize { @cond.wait @mutex }
- end
- end
-
# Creates a new ConnectionPool object. +spec+ is a ConnectionSpecification
# object which describes database connection information (e.g. adapter,
# host name, username, password, etc), as well as the maximum size for
@@ -137,9 +246,18 @@ def initialize(spec)
# default max pool size to 5
@size = (spec.config[:pool] && spec.config[:pool].to_i) || 5
- @latch = Latch.new
@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
@@ -197,6 +315,7 @@ def disconnect!
conn.disconnect!
end
@connections = []
+ @available.clear
end
end
@@ -211,6 +330,10 @@ def clear_reloadable_connections!
@connections.delete_if do |conn|
conn.requires_reloading?
end
+ @available.clear
+ @connections.each do |conn|
+ @available.add conn
+ end
end
end
@@ -234,23 +357,10 @@ def clear_stale_cached_connections! # :nodoc:
# Raises:
# - PoolFullError: no connection can be obtained from the pool.
def checkout
- loop do
- # Checkout an available connection
- synchronize do
- # Try to find a connection that hasn't been leased, and lease it
- conn = connections.find { |c| c.lease }
-
- # If all connections were leased, and we have room to expand,
- # create a new connection and lease it.
- if !conn && connections.size < size
- conn = checkout_new_connection
- conn.lease
- end
-
- return checkout_and_verify(conn) if conn
- end
-
- Timeout.timeout(@checkout_timeout, PoolFullError) { @latch.await }
+ synchronize do
+ conn = acquire_connection
+ conn.lease
+ checkout_and_verify(conn)
end
end
@@ -266,8 +376,9 @@ def checkin(conn)
end
release conn
+
+ @available.add conn
end
- @latch.release
end
# Remove a connection from the connection pool. The connection will
@@ -275,12 +386,14 @@ def checkin(conn)
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
- @latch.release
end
# Removes dead connections from the pool. A dead connection can occur
@@ -293,11 +406,35 @@ def reap
remove conn if conn.in_use? && stale > conn.last_use && !conn.active?
end
end
- @latch.release
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
+ t0 = Time.now
+ begin
+ @available.poll(@checkout_timeout)
+ rescue ConnectionTimeoutError
+ msg = 'could not obtain a database connection within %0.3f seconds (waited %0.3f seconds)' %
+ [@checkout_timeout, Time.now - t0]
+ raise PoolFullError, msg
+ end
+ end
+ end
+
def release(conn)
thread_id = if @reserved_connections[current_connection_id] == conn
current_connection_id
View
4 activerecord/test/cases/associations/eager_test.rb
@@ -962,6 +962,10 @@ def test_eager_loading_with_order_on_joined_table_preloads
end
def test_eager_loading_with_conditions_on_joined_table_preloads
+ # cache metadata in advance to avoid extra sql statements executed while testing
+ Tagging.first
+ Tag.first
+
posts = assert_queries(2) do
Post.scoped(:select => 'distinct posts.*', :includes => :author, :joins => [:comments], :where => "comments.body like 'Thank you%'", :order => 'posts.id').all
end
View
3  activerecord/test/cases/associations/has_many_associations_test.rb
@@ -1353,6 +1353,9 @@ def test_custom_primary_key_on_new_record_should_fetch_with_query
author = Author.new(:name => "David")
assert !author.essays.loaded?
+ # cache metadata in advance to avoid extra sql statements executed while testing
+ Essay.first
+
assert_queries 1 do
assert_equal 1, author.essays.size
end
View
2  activerecord/test/cases/connection_adapters/abstract_adapter_test.rb
@@ -36,7 +36,7 @@ def test_expire_mutates_in_use
def test_close
pool = ConnectionPool.new(ConnectionSpecification.new({}, nil))
- pool.connections << adapter
+ pool.insert_connection_for_test! adapter
adapter.pool = pool
# Make sure the pool marks the connection in use
View
104 activerecord/test/cases/connection_pool_test.rb
@@ -200,6 +200,110 @@ def test_checkout_behaviour
end.join
end
+ # The connection pool is "fair" if threads waiting for
+ # connections receive them the order in which they began
+ # waiting. This ensures that we don't timeout one HTTP request
+ # even while well under capacity in a multi-threaded environment
+ # such as a Java servlet container.
+ #
+ # We don't need strict fairness: if two connections become
+ # available at the same time, it's fine of two threads that were
+ # waiting acquire the connections out of order.
+ #
+ # Thus this test prepares waiting threads and then trickles in
+ # available connections slowly, ensuring the wakeup order is
+ # correct in this case.
+ def test_checkout_fairness
+ @pool.instance_variable_set(:@size, 10)
+ expected = (1..@pool.size).to_a.freeze
+ # check out all connections so our threads start out waiting
+ conns = expected.map { @pool.checkout }
+ mutex = Mutex.new
+ order = []
+ errors = []
+
+ threads = expected.map do |i|
+ t = Thread.new {
+ begin
+ conn = @pool.checkout # never checked back in
+ mutex.synchronize { order << i }
+ rescue => e
+ mutex.synchronize { errors << e }
+ end
+ }
+ Thread.pass until t.status == "sleep"
+ t
+ end
+
+ # this should wake up the waiting threads one by one in order
+ conns.each { |conn| @pool.checkin(conn); sleep 0.1 }
+
+ threads.each(&:join)
+
+ raise errors.first if errors.any?
+
+ assert_equal(expected, order)
+ end
+
+ # As mentioned in #test_checkout_fairness, we don't care about
+ # strict fairness. This test creates two groups of threads:
+ # group1 whose members all start waiting before any thread in
+ # group2. Enough connections are checked in to wakeup all
+ # group1 threads, and the fact that only group1 and no group2
+ # threads acquired a connection is enforced.
+ def test_checkout_fairness_by_group
+ @pool.instance_variable_set(:@size, 10)
+ # take all the connections
+ conns = (1..10).map { @pool.checkout }
+ mutex = Mutex.new
+ successes = [] # threads that successfully got a connection
+ errors = []
+
+ make_thread = proc do |i|
+ t = Thread.new {
+ begin
+ conn = @pool.checkout # never checked back in
+ mutex.synchronize { successes << i }
+ rescue => e
+ mutex.synchronize { errors << e }
+ end
+ }
+ Thread.pass until t.status == "sleep"
+ t
+ end
+
+ # all group1 threads start waiting before any in group2
+ group1 = (1..5).map(&make_thread)
+ group2 = (6..10).map(&make_thread)
+
+ # checkin n connections back to the pool
+ checkin = proc do |n|
+ n.times do
+ c = conns.pop
+ @pool.checkin(c)
+ end
+ end
+
+ checkin.call(group1.size) # should wake up all group1
+
+ loop do
+ sleep 0.1
+ break if mutex.synchronize { (successes.size + errors.size) == group1.size }
+ end
+
+ winners = mutex.synchronize { successes.dup }
+ checkin.call(group2.size) # should wake up everyone remaining
+
+ group1.each(&:join)
+ group2.each(&:join)
+
+ assert_equal((1..group1.size).to_a, winners.sort)
+
+ if errors.any?
+ raise errors.first
+ end
+ end
+
def test_automatic_reconnect=
pool = ConnectionPool.new ActiveRecord::Base.connection_pool.spec
assert pool.automatic_reconnect
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