Class: ActiveRecord::ConnectionAdapters::ConnectionPool

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

Overview

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.

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 Active Record 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 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).

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

- (ConnectionPool) initialize(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.



71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 71

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
  @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         = []
  @automatic_reconnect = true
  @tables              = {}
  @visitor             = nil

  @columns     = Hash.new do |h, table_name|
    h[table_name] = with_connection do |conn|

      # Fetch a list of columns
      conn.columns(table_name, "#{table_name} Columns").tap do |columns|

        # set primary key information
        columns.each do |column|
          column.primary = column.name == primary_keys[table_name]
        end
      end
    end
  end

  @columns_hash = Hash.new do |h, table_name|
    h[table_name] = Hash[columns[table_name].map { |col|
      [col.name, col]
    }]
  end

  @column_defaults = Hash.new do |h, table_name|
    h[table_name] = Hash[columns[table_name].map { |col|
      [col.name, col.default]
    }]
  end

  @primary_keys = Hash.new do |h, table_name|
    h[table_name] = with_connection do |conn|
      table_exists?(table_name) ? conn.primary_key(table_name) : 'id'
    end
  end
end

Instance Attribute Details

- (Object) automatic_reconnect

Returns the value of attribute automatic_reconnect



60
61
62
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 60

def automatic_reconnect
  @automatic_reconnect
end

- (Object) column_defaults (readonly)

Returns the value of attribute column_defaults



63
64
65
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 63

def column_defaults
  @column_defaults
end

- (Object) columns (readonly)

Returns the value of attribute columns



62
63
64
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 62

def columns
  @columns
end

- (Object) columns_hash (readonly)

Returns the value of attribute columns_hash



62
63
64
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 62

def columns_hash
  @columns_hash
end

- (Object) connections (readonly)

Returns the value of attribute connections



61
62
63
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 61

def connections
  @connections
end

- (Object) primary_keys (readonly)

Returns the value of attribute primary_keys



62
63
64
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 62

def primary_keys
  @primary_keys
end

- (Object) spec (readonly)

Returns the value of attribute spec



61
62
63
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 61

def spec
  @spec
end

- (Object) tables (readonly)

Returns the value of attribute tables



62
63
64
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 62

def tables
  @tables
end

Instance Method Details

- (Boolean) active_connection?

Check to see if there is an active connection in this connection pool.

Returns:

  • (Boolean) 


167
168
169
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 167

def active_connection?
  @reserved_connections.key? current_connection_id
end

- (Object) 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.



289
290
291
292
293
294
295
296
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 289

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

- (Object) checkout

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.



258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 258

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

      @queue.wait(@timeout)

      if(@checked_out.size < @connections.size)
        next
      else
        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

- (Object) clear_cache!

Clears out internal caches:

* columns
* columns_hash
* tables


141
142
143
144
145
146
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 141

def clear_cache!
  @columns.clear
  @columns_hash.clear
  @column_defaults.clear
  @tables.clear
end

- (Object) clear_reloadable_connections!

Clears the cache which maps classes.



208
209
210
211
212
213
214
215
216
217
218
219
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 208

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.delete_if do |conn|
    conn.requires_reloading?
  end
end

- (Object) clear_stale_cached_connections!

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



232
233
234
235
236
237
238
239
240
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 232

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|
    checkin @reserved_connections[key]
    @reserved_connections.delete(key)
  end
end

- (Object) clear_table_cache!(table_name)

Clear out internal caches for table with table_name.



149
150
151
152
153
154
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 149

def clear_table_cache!(table_name)
  @columns.delete table_name
  @columns_hash.delete table_name
  @column_defaults.delete table_name
  @primary_keys.delete table_name
end

- (Boolean) connected?

Returns true if a connection has already been opened.

Returns:

  • (Boolean) 


191
192
193
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 191

def connected?
  !@connections.empty?
end

- (Object) connection

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.



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

def connection
  @reserved_connections[current_connection_id] ||= checkout
end

- (Object) disconnect!

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



196
197
198
199
200
201
202
203
204
205
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 196

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

- (Object) release_connection(with_id = current_connection_id)

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



174
175
176
177
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 174

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

- (Boolean) table_exists?(name)

A cached lookup for table existence.

Returns:

  • (Boolean) 


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

def table_exists?(name)
  return true if @tables.key? name

  with_connection do |conn|
    conn.tables.each { |table| @tables[table] = true }
    @tables[name] = true if !@tables.key?(name) && conn.table_exists?(name)
  end

  @tables.key? name
end

- (Object) verify_active_connections!

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



223
224
225
226
227
228
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 223

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

- (Object) with_connection

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.



182
183
184
185
186
187
188
 
# File 'activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb', line 182

def with_connection
  connection_id = current_connection_id
  fresh_connection = true unless @reserved_connections[connection_id]
  yield connection
ensure
  release_connection(connection_id) if fresh_connection
end