On Thu, Sep 18, 2014 at 7:45 AM, Flavio Percoco <fla...@redhat.com> wrote:

> On 09/18/2014 04:09 PM, Gordon Sim wrote:
> > On 09/18/2014 12:31 PM, Flavio Percoco wrote:
> >> On 09/17/2014 10:36 PM, Joe Gordon wrote:
> >>> My understanding of Zaqar is that it's like SQS. SQS uses distributed
> >>> queues, which have a few unusual properties [0]:
> >>>
> >>>
> >>>      Message Order
> >>>
> >>> Amazon SQS makes a best effort to preserve order in messages, but due
> to
> >>> the distributed nature of the queue, we cannot guarantee you will
> >>> receive messages in the exact order you sent them. If your system
> >>> requires that order be preserved, we recommend you place sequencing
> >>> information in each message so you can reorder the messages upon
> >>> receipt.
> >>>
> >>
> >> Zaqar guarantees FIFO. To be more precise, it does that relying on the
> >> storage backend ability to do so as well. Depending on the storage used,
> >> guaranteeing FIFO may have some performance penalties.
> >
> > Would it be accurate to say that at present Zaqar does not use
> > distributed queues, but holds all queue data in a storage mechanism of
> > some form which may internally distribute that data among servers but
> > provides Zaqar with a consistent data model of some form?
> I think this is accurate. The queue's distribution depends on the
> storage ability to do so and deployers will be able to choose what
> storage works best for them based on this as well. I'm not sure how
> useful this separation is from a user perspective but I do see the
> relevance when it comes to implementation details and deployments.
> > [...]
> >> As of now, Zaqar fully relies on the storage replication/clustering
> >> capabilities to provide data consistency, availability and fault
> >> tolerance.
> >
> > Is the replication synchronous or asynchronous with respect to client
> > calls? E.g. will the response to a post of messages be returned only
> > once the replication of those messages is confirmed? Likewise when
> > deleting a message, is the response only returned when replicas of the
> > message are deleted?
> It depends on the driver implementation and/or storage configuration.
> For example, in the mongodb driver, we use the default write concern
> called "acknowledged". This means that as soon as the message gets to
> the master node (note it's not written on disk yet nor replicated) zaqar
> will receive a confirmation and then send the response back to the
> client. This is also configurable by the deployer by changing the
> default write concern in the mongodb uri using `?w=SOME_WRITE_CONCERN`[0].

This means that by default Zaqar cannot guarantee a message will be
delivered at all. A message can be acknowledged and then the 'master node'
crashes and the message is lost. Zaqar's ability to guarantee delivery is
limited by the reliability of a single node, while something like swift can
only loose a piece of data if 3 machines crash at the same time.

> [0] http://docs.mongodb.org/manual/reference/connection-string/#uri.w
> >
> >> However, as far as consuming messages is concerned, it can
> >> guarantee once-and-only-once and/or at-least-once delivery depending on
> >> the message pattern used to consume messages. Using pop or claims
> >> guarantees the former whereas streaming messages out of Zaqar guarantees
> >> the later.
> >
> > From what I can see, pop provides unreliable delivery (i.e. its similar
> > to no-ack). If the delete call using pop fails while sending back the
> > response, the messages are removed but didn't get to the client.
> Correct, pop works like no-ack. If you want to have pop+ack, it is
> possible to claim just 1 message and then delete it.
> >
> > What do you mean by 'streaming messages'?
> I'm sorry, that went out wrong. I had the "browsability" term in my head
> and went with something even worse. By streaming messages I meant
> polling messages without claiming them. In other words, at-least-once is
> guaranteed by default, whereas once-and-only-once is guaranteed just if
> claims are used.
> >
> > [...]
> >> Based on our short conversation on IRC last night, I understand you're
> >> concerned that FIFO may result in performance issues. That's a valid
> >> concern and I think the right answer is that it depends on the storage.
> >> If the storage has a built-in FIFO guarantee then there's nothing Zaqar
> >> needs to do there. In the other hand, if the storage does not have a
> >> built-in support for FIFO, Zaqar will cover it in the driver
> >> implementation. In the mongodb driver, each message has a marker that is
> >> used to guarantee FIFO.
> >
> > That marker is a sequence number of some kind that is used to provide
> > ordering to queries? Is it generated by the database itself?
> It's a sequence number to provide ordering to queries, correct.
> Depending on the driver, it may be generated by Zaqar or the database.
> In mongodb's case it's generated by Zaqar[0].
> [0]
> https://github.com/openstack/zaqar/blob/master/zaqar/queues/storage/mongodb/queues.py#L103-L185
> --
> @flaper87
> Flavio Percoco
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