Tuesday, June 23, 2009

a fairly generalized request/reply model - MOM

background -- req/rep is indispensable in trading as it replaces synchronous call.

I feel the scheme below is overly complex.

Note "server" is an vague term in req/rep. I use "broker" and "service" instead.

* Initiator sends request to message broker.
* broker delivers it to the service provider. I feel async allows an overloaded service to control message rate. Sync could overwhelm the service.
* depending on the acknowledgment mode, service might send an REQ-ACK right away[1], since the reply might take a long time. REQ-ACK can give initiator some useful assurance so it doesn't need to guess "why no reply?".
---
* x seconds/hours later, service sends the "value" to a private queue (initiator's mailbox).
* broker delivers it to the initiator, async or sync. See post on initiator's mailbox.
* depending on the acknowledgment mode, initiator might send a VALUE-ACK.

Generalized, request vs value can be considered 2 independent JMS flows. Perhaps 2 independent brokers, 2 independent queues. Zed used this model as the ringtone/joke/horoscope/.... might get generated after a long delay. If the request is a write operation, then more control is needed.

[1] Further decoupled, an explicit, custom ACK message can go into yet another queue. So the request/reply would use 4 queues. However, i think this is rare. ACK could be a builtin in the broker infrastructure, but perhaps the actual receivers (initiator and server) can issue the ACK.

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