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From Flavio Junqueira <>
Subject Re: [Discussion] [Hedwig] Add queue semantic support for Hedwig
Date Sat, 23 Feb 2013 10:25:58 GMT

On Feb 23, 2013, at 11:15 AM, Jiannan Wang <> wrote:

> I see Flavio's reply to Yannick which suggests using ZooKeeper to coordinate the actions
of publisher and subscriber. But it's a client-side solution, I would prefer solution 1) in
Sijie's proposal which requires no special works in client side.

Yannick's message seems to imply that a subscriber can't subscribe before an external event
causing a race between a publisher and the subscriber. If you solve it, for example, using
bookkeeper directly as one of the options Sijie suggests, you'd end up coordinating using
zookeeper or using some other coordination scheme. Yannick's case seems to be a special one.


> From: Sijie Guo <>
> Reply-To: "" <>
> Date: Thursday, February 21, 2013 4:50 PM
> To: "" <>
> Cc: "" <>,
Hang Qi <>, Hongjian Chen <>, Bizhu
Qiu <>, Fangmin Lv <>, Lin Shen <>
> Subject: Re: [Discussion] [Hedwig] Add queue semantic support for Hedwig
> Thanks Jiannan for raising the discussion of queue semantic. There was some other guys
in the mail list asked for queue semantic before.
> Basically, topic (pub/sub) is quite different from queue in messaging concepts. In pub/sub
model, when a publisher publish a message, it goes to all the consumers (subscribers) who
are interested; while a queue model implements a load balancer semantic. A single message
would be consumed almost exactly by one consumer. It means that a queue has many consumers
with messages load balanced across the available consumers.
> If the application requires all consumers seen same view of published messages, a topic
is better for it. If the application doesn't matter who would receive and consume the published
messages, a queue is better. But these two concepts become similar when there are only one
consumer. It might make you confused on using a queue or a topic.
> for your case, it is still a pub/sub application. so your first question is how to handle
this case gracefully in a pub/sub model. two ideas could be proceed to resolve it (similar
as what kafka did):
> 1) have a subscription option to indicate subscribe starting from the latest sequence
id or the oldest sequence id.
> 2) let subscriber managed its consumed ptr and passed the consumed ptr back when subscribe
to tell hub server where to start delivery. this subscriber could be a special subscriber
distinguished by a subscription option.
> several benefits could be made by 2):
> a) eliminate the storage and access of subscription metadata.
> b) provided the mechanism to rewind the subscription back for replaying already consumed
messages again.
> for the garbage collection stuff you mentioned on how long to keep the messages, we already
have messageBound to limit the length of a topic. We don't need to worry about it.
> for your second question, it might be nice to have the queue semantic in Hedwig, since
JMS implementation needs it. But implementing the queue semantic is totally a different story
than pub/sub.
> -Sijie
> On Wed, Feb 20, 2013 at 6:58 PM, Jiannan Wang <> wrote:
>> 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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