Class: ActiveRecord::ConnectionAdapters::ConnectionPool

Inherits:
Object
  • Object
show all
Includes:
MonitorMixin
Defined in:
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)

  • checkout _timeout: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds). ('wait_timeout' supported for backwards compatibility, but conflicts with key used for different purpose by mysql2 adapter).

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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 74

def initialize(spec)
  super()

  @spec = spec

  # The cache of reserved connections mapped to threads
  @reserved_connections = {}

  @queue = new_cond
  # 'wait_timeout', the backward-compatible key, conflicts with spec key 
  # used by mysql2 for something entirely different, checkout_timeout
  # preferred to avoid conflict and allow independent values. 
  @timeout = spec.config[:checkout_timeout] || spec.config[:wait_timeout] || 5

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

  @connections         = []
  @automatic_reconnect = true
end

Instance Attribute Details

#automatic_reconnectObject

Returns the value of attribute automatic_reconnect


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

def automatic_reconnect
  @automatic_reconnect
end

#connectionsObject (readonly)

Returns the value of attribute connections


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 66

def connections
  @connections
end

#specObject (readonly)

Returns the value of attribute spec


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 66

def spec
  @spec
end

Instance Method Details

#active_connection?Boolean

Is there an open connection that is being used for the current thread?

Returns:

  • (Boolean)

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

def active_connection?
  synchronize do
    @reserved_connections.fetch(current_connection_id) {
      return false
    }.in_use?
  end
end

#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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 285

def checkin(conn)
  synchronize do
    conn.run_callbacks :checkin do
      conn.expire
      @queue.signal
    end

    release conn
  end
end

#checkoutObject

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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 238

def checkout
  synchronize do
    waited_time = 0

    loop do
      conn = @connections.find { |c| c.lease }

      unless conn
        if @connections.size < @size
          conn = checkout_new_connection
          conn.lease
        end
      end

      if conn
        checkout_and_verify conn
        return conn
      end

      if waited_time >= @timeout
        raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout} (waited #{waited_time} seconds). The max pool size is currently #{@size}; consider increasing it."
      end

      # Sometimes our wait can end because a connection is available,
      # but another thread can snatch it up first. If timeout hasn't
      # passed but no connection is avail, looks like that happened --
      # loop and wait again, for the time remaining on our timeout. 
      before_wait = Time.now
      @queue.wait( [@timeout - waited_time, 0].max )
      waited_time += (Time.now - before_wait)

      # Will go away in Rails 4, when we don't clean up
      # after leaked connections automatically anymore. Right now, clean
      # up after we've returned from a 'wait' if it looks like it's
      # needed, then loop and try again. 
      if(active_connections.size >= @connections.size)
        clear_stale_cached_connections!
      end
    end
  end
end

#clear_cache!Object


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 197

def clear_cache!
  with_connection do |c|
    c.schema_cache.clear!
  end
end

#clear_reloadable_connections!Object

Clears the cache which maps classes.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 152

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
  end
end

#clear_stale_cached_connections!Object

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


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 206

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

#columnsObject


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 176

def columns
  with_connection do |c|
    c.schema_cache.columns
  end
end

#columns_hashObject


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 183

def columns_hash
  with_connection do |c|
    c.schema_cache.columns_hash
  end
end

#connected?Boolean

Returns true if a connection has already been opened.

Returns:

  • (Boolean)

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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 135

def connected?
  synchronize { @connections.any? }
end

#connectionObject

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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 100

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

#disconnect!Object

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


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 140

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

#primary_keysObject


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 190

def primary_keys
  with_connection do |c|
    c.schema_cache.primary_keys
  end
end

#release_connection(with_id = current_connection_id) ⇒ 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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 118

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

#verify_active_connections!Object

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


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

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

#with_connectionObject

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.


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# File 'lib/active_record/connection_adapters/abstract/connection_pool.rb', line 126

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