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From Kerry Bonin <kerrybo...@gmail.com>
Subject Re: Client broker failover notification/callback?
Date Tue, 09 Aug 2011 16:44:49 GMT
Missed this to reply.

We spin to receive messages, the older API had a callback for a received
message, but we see no equivalent for messaging.  There is an asynchronous
fetch, so we have a receiving thread that spins on those.  I haven't looked
at the recent library (we're currently using 0.8), although I see QPID-2451
is dead (sigh).  In a multithreaded app, I really hate to have a thread
burning CPU in an idiotic spin loop.

On Tue, Jun 28, 2011 at 12:29 PM, Gordon Sim <gsim@redhat.com> wrote:

> On 06/28/2011 04:35 PM, Kerry Bonin wrote:
>> Irregardless of how it is accomplished, more control is needed over
>> failover to prevent network splits.  The current model is
>> essentially to accept the broker as a single point of failure, or
>> deploy Linux clustering.
> Without replication or persistence of the queue state broker failure
> implies potential message loss. In the case of persistence the
> availability of the messages is tied to the availability of a broker
> using that store.
> Assuming you can tolerate message loss, the issue is simply to ensure
> that communication remains possible i.e. that producers and consumers
> always use (or at least gravitate to) the same broker instance.
> You could perhaps use a QMF based approach to this.
> E.g. you could have an application that connected to all the brokers in
> a list, kept track of their availability and retried periodically to
> connect to unavailable brokers in the list. It would then control which
> of these brokers was the 'primary' and would be able to close all other
> connections on the other brokers using QMF commands.
> Of course you would need to ensure that this application didn't itself
> become a single point of failure. However it would be simple enough to
> have a couple of redundant instances waiting to take over.
> The one fly in the ointment at present is that a QMF close of a
> connection will result in a connection exception on the associated
> client rather than triggering failover. However adding another command,
> abort say, that simply disconnected the client with no explicit
> handshake would fix that (e.g. see attached patch).
> Does this approach sound workable for you? The benefit is that it
> doesn't require any client modification and would also provide a quite
> valuable tool for centralised monitoring of general failover behaviour.
>  I'd recommend an ordered broker list with monitoring and automatic
>> fallback, subject to some flap mitigation rules. I also would expose
>> more client state, I understand the desire to hide stuff so people
>> don't use unsupported interfaces, but it is useful for
>> serviceability and diagnostics to have a library expose basic health
>> information.
> Yes, I agree that being able to determine the remote peer address would
> be valuable, as well as perhaps other aspects of retry such as time
> since last connected etc. (You can at present determine whether you are
> currently connected using Connection::isOpen()).
>  Out of honest curiosity, why don't you like callbacks?  We've had to
>> use threading with spin loops to get around the lack of callbacks
>> for the messaging APIs, and we don't like having CPUs loads float
>> high under low load even if the spin loop load drops gracefully
>> under pressure, it feels inelegant and it raises power consumption.
> Yes, I don't like the need to spin either. I want to expose a more
> general notification system to avoid the need for any polling for
> changes (including changes to failover/connection related state). I prefer
> that to callbacks at the level the API is operating at. Callback based
> approaches could then be built on top of this.
> Out of curiosity, what is it you are spinning for?
> ---------------------------------------------------------------------
> Apache Qpid - AMQP Messaging Implementation
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