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From Jiannan Wang <jian...@yahoo-inc.com>
Subject [Discussion] [Hedwig] Add queue semantic support for Hedwig
Date Thu, 21 Feb 2013 02:58:52 GMT
Hi guys,
   Under current Hedwig semantic, a subscriber cannot aware of messages published before he
subscribes the topic. So in following example, subscriber A can only receives messages after
seqId 2.
---------------------------------
Topic T: msg1 msg2 msg3 msg4 ...
                     | <- subscriber A subscribe the topic
---------------------------------

   This semantic is very reasonable, but Hedwig client needs to handle this corner case: a
new topic is just to be created, and as topic is lazily created by the first request (generally
it's PUB or SUB), so the client side must coordinate between publisher and subscriber to make
sure the first SUB is handled before the first PUB at this very beginning status (consider
subscriber may have very bad network connection which causes SUB failed and user does not
want to miss any messages). In summary, it requires special works if there is a subscriber
would like to receive all the messages since topic is created, and I think this requirement
is very general.

   Handle this problem in client side is a choice, but I think maybe we can simply resolve
it in server side if Hedwig can support queue semantic (so that we can also extend Hedwig
JMS provider to support JMS queue in BOOKKEEPER-312). And as I known, the major concern on
queue semantic is how long to keep the messages, however:
   1. It is user's responsibility to know about the feature and impact of queue semantic.
   2. On the other hand, we can add a parameter to limit the queue length.

   In a word, here are the two problem I would like to discuss:
   1. How to gracefully resolve the above issue in server side under current semantic.
   2. Whether or not to introduce queue semantic into Hedwig.

Thanks,
Jiannan

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