How to use connection pool in Rails2.3.5 with WEBrick and MySQL?

Sheile · February 25, 2010, 2:13am

We use Rails 2.3.5, MySQL, and WEBrick.

ActiveRecord call ConnectionPool#checkout and create new connection to
MySQL when receive a first HTTP request.
At this time, register to connection pool its connection in
ConnectionPool#checkout_new_connection.

github.com

rails/rails/blob/v2.3.5/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L181


      
          # 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

github.com

rails/rails/blob/v2.3.5/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L246


      
            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!

but, Rack::Handler::WEBrick#service call body.close in ensure section
when return the HTTP response.
It call another method. after some call, it call
ConnectionPool#clear_reloadable_connections and clear its connection
pool.

github.com

rails/rails/blob/v2.3.5/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L145


      
          
          
# 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

As a result, the connection was created by every request and spend
time for connection.

How to use connection pool?