require 'monitor'
require 'set'
module ActiveRecord
# Raised when a connection could not be obtained within the connection
# acquisition timeout period.
class ConnectionTimeoutError < ConnectionNotEstablished
end
module ConnectionAdapters
# Connection pool base class for managing ActiveRecord 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.
#
# == Obtaining (checking out) a connection
#
# Connections can be obtained and used from a connection pool in several
# ways:
#
# 1. Simply use ActiveRecord::Base.connection as with ActiveRecord 2.1 and
# earlier (pre-connection-pooling). Eventually, when you're done with
# the connection(s) and wish it to be returned to the pool, you call
# ActiveRecord::Base.clear_active_connections!. This will be the
# default behavior for ActiveRecord when used in conjunction with
# ActionPack'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).
#
# == Options
#
# There are two connection-pooling-related options that you can add to
# your database connection configuration:
#
# * +pool+: number indicating size of connection pool (default 5)
# * +wait_timeout+: number of seconds to block and wait for a connection
# before giving up and raising a timeout error (default 5 seconds).
class ConnectionPool
attr_reader :spec
# 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)
@spec = spec
# The cache of reserved connections mapped to threads
@reserved_connections = {}
# The mutex used to synchronize pool access
@connection_mutex = Monitor.new
@queue = @connection_mutex.new_cond
# default 5 second timeout
@timeout = spec.config[:wait_timeout] || 5
# default max pool size to 5
@size = (spec.config[:pool] && spec.config[:pool].to_i) || 5
@connections = []
@checked_out = []
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
if conn = @reserved_connections[current_connection_id]
conn
else
@reserved_connections[current_connection_id] = checkout
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
conn = @reserved_connections.delete(current_connection_id)
checkin conn if conn
end
# Reserve a connection, and yield it to a block. Ensure the connection is
# checked back in when finished.
def with_connection
conn = checkout
yield conn
ensure
checkin conn
end
# Returns true if a connection has already been opened.
def connected?
!@connections.empty?
end
# Disconnects all connections in the pool, and clears the pool.
def disconnect!
@reserved_connections.each do |name,conn|
checkin conn
end
@reserved_connections = {}
@connections.each do |conn|
conn.disconnect!
end
@connections = []
end
# Clears the cache which maps classes
def clear_reloadable_connections!
@reserved_connections.each do |name, conn|
checkin conn
end
@reserved_connections = {}
@connections.each do |conn|
conn.disconnect! if conn.requires_reloading?
end
@connections = []
end
# Verify active connections and remove and disconnect connections
# associated with stale threads.
def verify_active_connections! #:nodoc:
clear_stale_cached_connections!
@connections.each do |connection|
connection.verify!
end
end
# Return any checked-out connections back to the pool by threads that
# are no longer alive.
def clear_stale_cached_connections!
remove_stale_cached_threads!(@reserved_connections) do |name, conn|
checkin conn
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 an existing connection, or by creating
# a new connection. If the maximum number of connections for this pool has
# already been reached, but the pool is empty (i.e. they're all being used),
# then this method will wait until a thread has checked in a connection.
# The wait time is bounded however: if no connection can be checked out
# within the timeout specified for this pool, then a ConnectionTimeoutError
# exception will be raised.
#
# Returns: an AbstractAdapter object.
#
# Raises:
# - ConnectionTimeoutError: no connection can be obtained from the pool
# within the timeout period.
def checkout
# Checkout an available connection
@connection_mutex.synchronize do
loop do
conn = if @checked_out.size < @connections.size
checkout_existing_connection
elsif @connections.size < @size
checkout_new_connection
end
return conn if conn
# No connections available; wait for one
if @queue.wait(@timeout)
next
else
# try looting dead threads
clear_stale_cached_connections!
if @size == @checked_out.size
raise ConnectionTimeoutError, "could not obtain a database connection within #{@timeout} seconds. The pool size is currently #{@size}, perhaps you need to increase it?"
end
end
end
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)
@connection_mutex.synchronize do
conn.run_callbacks :checkin
@checked_out.delete conn
@queue.signal
end
end
synchronize :clear_reloadable_connections!, :verify_active_connections!,
:connected?, :disconnect!, :with => :@connection_mutex
private
def new_connection
ActiveRecord::Base.send(spec.adapter_method, spec.config)
end
def current_connection_id #:nodoc:
Thread.current.object_id
end
# Remove stale threads from the cache.
def remove_stale_cached_threads!(cache, &block)
keys = Set.new(cache.keys)
Thread.list.each do |thread|
keys.delete(thread.object_id) if thread.alive?
end
keys.each do |key|
next unless cache.has_key?(key)
block.call(key, cache[key])
cache.delete(key)
end
end
def checkout_new_connection
c = new_connection
@connections << c
checkout_and_verify(c)
end
def checkout_existing_connection
c = (@connections - @checked_out).first
checkout_and_verify(c)
end
def checkout_and_verify(c)
c.verify!
c.run_callbacks :checkout
@checked_out << c
c
end
end
# ConnectionHandler is a collection of ConnectionPool objects. It is used
# for keeping separate connection pools for ActiveRecord models that connect
# to different databases.
#
# For example, suppose that you have 5 models, with the following hierarchy:
#
# |
# +-- Book
# | |
# | +-- ScaryBook
# | +-- GoodBook
# +-- Author
# +-- BankAccount
#
# Suppose that Book is to connect to a separate database (i.e. one other
# than the default database). Then Book, ScaryBook and GoodBook will all use
# the same connection pool. Likewise, Author and BankAccount will use the
# same connection pool. However, the connection pool used by Author/BankAccount
# is not the same as the one used by Book/ScaryBook/GoodBook.
#
# Normally there is only a single ConnectionHandler instance, accessible via
# ActiveRecord::Base.connection_handler. ActiveRecord models use this to
# determine that connection pool that they should use.
class ConnectionHandler
def initialize(pools = {})
@connection_pools = pools
end
def connection_pools
@connection_pools ||= {}
end
def establish_connection(name, spec)
@connection_pools[name] = ConnectionAdapters::ConnectionPool.new(spec)
end
# Returns any connections in use by the current thread back to the pool,
# and also returns connections to the pool cached by threads that are no
# longer alive.
def clear_active_connections!
@connection_pools.each_value {|pool| pool.release_connection }
end
# Clears the cache which maps classes
def clear_reloadable_connections!
@connection_pools.each_value {|pool| pool.clear_reloadable_connections! }
end
def clear_all_connections!
@connection_pools.each_value {|pool| pool.disconnect! }
end
# Verify active connections.
def verify_active_connections! #:nodoc:
@connection_pools.each_value {|pool| pool.verify_active_connections! }
end
# Locate the connection of the nearest super class. This can be an
# active or defined connection: if it is the latter, it will be
# opened and set as the active connection for the class it was defined
# for (not necessarily the current class).
def retrieve_connection(klass) #:nodoc:
pool = retrieve_connection_pool(klass)
(pool && pool.connection) or raise ConnectionNotEstablished
end
# Returns true if a connection that's accessible to this class has
# already been opened.
def connected?(klass)
conn = retrieve_connection_pool(klass)
conn ? conn.connected? : false
end
# Remove the connection for this class. This will close the active
# connection and the defined connection (if they exist). The result
# can be used as an argument for establish_connection, for easily
# re-establishing the connection.
def remove_connection(klass)
pool = @connection_pools[klass.name]
@connection_pools.delete_if { |key, value| value == pool }
pool.disconnect! if pool
pool.spec.config if pool
end
def retrieve_connection_pool(klass)
pool = @connection_pools[klass.name]
return pool if pool
return nil if ActiveRecord::Base == klass
retrieve_connection_pool klass.superclass
end
end
end
end
