Receivers Implementation

The receivers implementation used by Queues to record who is waiting for a message

(and in which order they should be processed) is pluggable. You just need to implement

the

org.jboss.mq.server.Receivers

interface:

Implementations

• org.jboss.mq.server.ReceiversImpl

  - the default implementation using a

  HashSet

   

• org.jboss.mq.server.ReceiversImplArrayList

  - implemented with an

  ArrayList

   

• org.jboss.mq.server.ReceiversImplLinkedList

  - implemented with n

  LinkedList

   

Config

• You specify it as an attribute on the DestinationManager, Queue or Topic (

  e.g.: <attribute name= "ReceiversImpl">org.jboss.mq.server.ReceiversImplArrayList</attribute>

  ).

• If the destination has no configuration it uses the Destination Manager config.

• If the destination manager has no configuration it uses the default.

Notes

• The default implementation is there for backwards compatibility.

• It has been reported that the

  List

  implementations provide better load balancing when there are competing receivers. Neither

  List

  implementation has been as thoroughly tested as the

  HashSet

  version.

• The default implementation will only use other nodes if its own load is too high. Either of the List-based implementations can be used to get "round-robin" (perhaps "fair queueing" is a better term) distribution.

• The Topic configuration is largely irrelevent since only one receiver can be listening on a subscription, but it is anticipated that something more performant than a

  HashSet

  of 1 could be implemented.

• The receivers implementation should have "Set" semantics, i.e. if the subscription is already present it shouldn't add it twice.

Usage

<mbean code="org.jboss.mq.server.jmx.DestinationManager" name="jboss.mq:service=DestinationManager">
    <depends optional-attribute-name="MessageCache">jboss.mq:service=MessageCache</depends>
    <depends optional-attribute-name="PersistenceManager">jboss.mq:service=PersistenceManager</depends>
    <depends optional-attribute-name="StateManager">jboss.mq:service=StateManager</depends>
    <attribute name="ReceiversImpl">
      org.jboss.mq.server.ReceiversImplArrayList
    </attribute>
  </mbean>

The Receivers interface

/*
* JBoss, Home of Professional Open Source
* Copyright 2005, JBoss Inc., and individual contributors as indicated
* by the @authors tag. See the copyright.txt in the distribution for a
* full listing of individual contributors.
*
* This is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* This software is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this software; if not, write to the Free
* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301 USA, or see the FSF site: http://www.fsf.org.
*/
package org.jboss.mq.server;

import java.util.ArrayList;
import java.util.Iterator;

import org.jboss.mq.Subscription;

/**
 * Interface to be implemented by a receivers implementation.
 * The implementation should also have a default constructor.<p>
 * 
 * NOTE: There is no need to internally synchronize the caller
 * handles that.<p>
 * 
 * NOTE: This datastructure should have Set semantics. 
 *       i.e. attempts to add a subscriber that is already
 *       present should be ignored. Or more explicitly
 *       a subscriber should present zero or once. 
 *
 * @author <a href="mailto:adrian@jboss.com">Adrian Brock</a>
 * @version $Revision: 1.4 $
 */
public interface Receivers
{

   /**
    * @return Get the number of receivers
    */
   int size();
   
   /**
    * @return the subscriptions as an array list, this must be a
    * clone of any internal datastructure
    */
   ArrayList listReceivers();
   
   
   /**
    * Add a receiver, ignored if the receiver is already present.
    * 
    * @param sub the receiver to add
    */
   void add(Subscription sub);
   
   /**
    * Remove a receiver
    * 
    * @param sub the receiver to remove
    */
   void remove(Subscription sub);
   
   /**
    * Get an iterator to loop over all receivers
    * 
    * @return the iterator
    */
   Iterator iterator();
}

For a useful discussion of this subject

See the JBoss Forums (JBoss.com -> JBoss User -> Messaging, JMS & JBossMQ) topic JMS Queue residing in a cluster