Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Inherits:
Object
  • Object
show all
Defined in:
lib/active_record/connection_adapters/abstract/connection_pool.rb

Overview

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).

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(spec) ⇒ ConnectionPool

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.



66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 66

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 unless on ruby 1.9
  @timeout =
    if RUBY_VERSION < '1.9'
      spec.config[:wait_timeout] || 5
    end

  # default max pool size to 5
  @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5

  @connections = []
  @checked_out = []
end

Instance Attribute Details

#spec ⇒ Object (readonly)

Returns the value of attribute spec.



58
59
60
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 58

def spec
  @spec
end

Instance Method Details

#checkin(conn) ⇒ Object

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.



210
211
212
213
214
215
216
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 210

def checkin(conn)
  @connection_mutex.synchronize do
    conn.run_callbacks :checkin
    @checked_out.delete conn
    @queue.signal
  end
end

#checkout ⇒ Object

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.



181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 181

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" if @timeout}.  The max pool size is currently #{@size}; consider increasing it."
        end
      end
    end
  end
end

#clear_reloadable_connections! ⇒ Object

Clears the cache which maps classes



137
138
139
140
141
142
143
144
145
146
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 137

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

#clear_stale_cached_connections! ⇒ Object

Return any checked-out connections back to the pool by threads that are no longer alive.



159
160
161
162
163
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 159

def clear_stale_cached_connections!
  remove_stale_cached_threads!(@reserved_connections) do |name, conn|
    checkin conn
  end
end

#connected? ⇒ Boolean

Returns true if a connection has already been opened.

Returns:

  • (Boolean) 


120
121
122
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 120

def connected?
  !@connections.empty?
end

#connection ⇒ Object

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.



94
95
96
97
98
99
100
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 94

def connection
  if conn = @reserved_connections[current_connection_id]
    conn
  else
    @reserved_connections[current_connection_id] = checkout
  end
end

#disconnect! ⇒ Object

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



125
126
127
128
129
130
131
132
133
134
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 125

def disconnect!
  @reserved_connections.each do |name,conn|
    checkin conn
  end
  @reserved_connections = {}
  @connections.each do |conn|
    conn.disconnect!
  end
  @connections = []
end

#release_connection ⇒ Object

Signal that the thread is finished with the current connection. #release_connection releases the connection-thread association and returns the connection to the pool.



105
106
107
108
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 105

def release_connection
  conn = @reserved_connections.delete(current_connection_id)
  checkin conn if conn
end

#verify_active_connections! ⇒ Object

Verify active connections and remove and disconnect connections associated with stale threads.



150
151
152
153
154
155
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 150

def verify_active_connections! #:nodoc:
  clear_stale_cached_connections!
  @connections.each do |connection|
    connection.verify!
  end
end

#with_connection ⇒ Object

Reserve a connection, and yield it to a block. Ensure the connection is checked back in when finished.



112
113
114
115
116
117
 
# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 112

def with_connection
  conn = checkout
  yield conn
ensure
  checkin conn
end