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Irena · Posted: Sun Mar 10, 2002 12:41 am Post subject:

Is that correct that message order is not guaranteed within a message flow ?

Here is an abstract from the programming manual:
"as with all WebSphere MQ messages, it is possible for messages to be delivered out of order. This could happen:
1. If additional instances of the message flow are running.
2. If a link in the network goes down and subsequent messages are rerouted along
another link.
3. If a queue becomes temporarily full, or put-inhibited. In this case a message is put to a dead-letter queue and therefore delayed, while subsequent messages
might pass straight through."

It sounds like an out of order delivery happens only in some exceptional situations (like 1, 2, 3).
But what about parallel calls to evaluate function ? Here is another abstract:
"Threading considerations
The message broker runs on multiple threads, and it is possible that two threads
might be executing cniEvaluate on the same node object at the same time to process different messages. Therefore the code contained within this implementation function, and any that it calls, must be thread safe and fully re-entrant."

When does broker call evaluate simultaneously ?
Isn't it an additional reason for out of order message delivery ?

Should/could evaluate be synchronized ?

Regarding the recommended message ordering techniques (sequence number, timestamp), should each node type implement them itself ? How can nodes coordinate their "message ordering efforts" ?

Thank you,
Irena.