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<div class="moz-cite-prefix">On 26/07/13 17:07, Ceri Storey wrote:<br>
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<blockquote cite="mid:51F29EA7.20807@lshift.net" type="cite">
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<div class="moz-cite-prefix">(26/07/13 16:49), Tom Anderson wrote:<br>
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<blockquote cite="mid:51F29A82.8040302@timgroup.com" type="cite">
Implemented exactly as described there, it yields an infinite
loop for unprocessable messages. You might therefore also want
to keep a count of the number of processing attempts in a header
on the message, and more thoroughly reject messages which reach
some maximum number of attempts. I think you could do the final
rejection by setting a routing key on the message when you
reject it for the last time, and having exchange B be a direct
exchange which routes to either queue B or some final deadletter
queue.<br>
<br>
If you want exponential backoff in the retries, then life gets
more complicated (multiple timeout queues, selected between by a
routing key set by the consumer of A?). We are currently
pussyfooting around this issue at my company. I will report back
here if we ever implement a good solution!<br>
</blockquote>
<br>
I've just written some code to do exactly this; limited retries
with exponential backoff. That said, we're kind of cheating in
that we store the retry state in a secondary datastore and buffer
messages in the client. <br>
<br>
So whenever we receive a message, we:<br>
<ul>
<li>When we post each message, we assign it a unique message_id<br>
</li>
<li>Lookup message's due time by it's message_id property in our
datastore</li>
<li>Stash the message in a heap queue</li>
<li>When the message becomes due, remove it from the heap queue
and pass it to the client code.</li>
<li>If the client code succeeds, then we finally ack the
message. Otherwise, we reject the message. <br>
</li>
</ul>
</blockquote>
<br>
I'm currently writing almost exactly the same thing! The difference
being that i'm putting the due time in a header on the message
rather than in a lookaside store, and that my component moves
messages from a queue to an exchange, rather than from a queue to
client code directly.<br>
<br>
<blockquote cite="mid:51F29EA7.20807@lshift.net" type="cite">
<p>Whilst you can scale this horizontally, you will need enough
buffer space to hold a reasonable proportion of your queue,
although<i> what</i> proportion depends on how much you care
about timeliness.</p>
</blockquote>
<br>
I'm not sure i understand. Don't you need to have enough space to
hold all the messages that could be delayed at any given time? In
our case, that happens to not be all that large, fortunately.<br>
<br>
<blockquote cite="mid:51F29EA7.20807@lshift.net" type="cite">
<p> Also, you are effectively implementing a secondary queue in
your application (retaining rabbit for it's reliability
properties), which seems less than ideal. <br>
</p>
</blockquote>
<br>
Yes. We did kick around the idea of writing a custom exchange to do
this, but that sounded really scary.<br>
<br>
tom<br>
<br>
<div class="moz-signature">-- <br>
<p>Tom Anderson | Developer | +44 20 7826 4312 | <a
href="http://timgroup.com/">timgroup.com</a></p>
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