Agile Enterprise Application Development with Flex

Guaranteed Delivery of Server Messages

A server can send various types of messages. When a client receives them, we want the client to be able to send back the heartbeat object with delivery confirmations for only some of them, similar to a special treatment that letters with delivery confirmation get in the post office.

Our special messages will be represented by the class ReliableServerMessage; only these types of messages will be acknowledged. Java and ActionScript versions of such a class are shown in Examples 5-7 and 5-8.

Example 5-7. ReliableServerMessage.java

package com.farata.messaging.messages;

import flex.messaging.messages.AsyncMessage;
import flex.messaging.util.UUIDUtils;

public class ReliableServerMessage extends AsyncMessage {

   public ReliableServerMessage(Object body) {
      super();
      this.body = body;
      setMessageId(UUIDUtils.createUUID(false));
      timestamp = System.currentTimeMillis();
   }
}

Example 5-8. ReliableServerMessage.as

package com.farata.messaging.messages {
   import mx.messaging.messages.AsyncMessage;

   [RemoteClass(alias="com.farata.messaging.messages.ReliableServerMessage")]
   public class ReliableServerMessage extends AsyncMessage{
   }
}

The easiest way to identify the server-side outgoing message is by assigning some unique sequence number to its header. Just as a reminder, each AsyncMessage object has a message body and a message header and you are allowed to attach any key/value pairs to its header:

message = new ReliableServerMessage("Server message #" + number); message.setHeader("seqNo", ...