Open Stack

Hi All,

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.
At-Least-Once Delivery

Amazon SQS stores copies of your messages on multiple servers for
redundancy and high availability. On rare occasions, one of the servers
storing a copy of a message might be unavailable when you receive or delete
the message. If that occurs, the copy of the message will not be deleted on
that unavailable server, and you might get that message copy again when you
receive messages. Because of this, you must design your application to be
idempotent (i.e., it must not be adversely affected if it processes the
same message more than once).
Message Sample

The behavior of retrieving messages from the queue depends whether you are
using short (standard) polling, the default behavior, or long polling. For
more information about long polling, see Amazon SQS Long Polling
<http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-long-polling.html>
.

With short polling, when you retrieve messages from the queue, Amazon SQS
samples a subset of the servers (based on a weighted random distribution)
and returns messages from just those servers. This means that a particular
receive request might not return all your messages. Or, if you have a small
number of messages in your queue (less than 1000), it means a particular
request might not return any of your messages, whereas a subsequent request
will. If you keep retrieving from your queues, Amazon SQS will sample all
of the servers, and you will receive all of your messages.

The following figure shows short polling behavior of messages being
returned after one of your system components makes a receive request.
Amazon SQS samples several of the servers (in gray) and returns the
messages from those servers (Message A, C, D, and B). Message E is not
returned to this particular request, but it would be returned to a
subsequent request.



Presumably SQS has these properties because it makes the system scalable,
if so does Zaqar have the same properties (not just making these same
guarantees in the API, but actually having these properties in the
backends)? And if not, why? I looked on the wiki [1] for information on
this, but couldn't find anything.





[0]
http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/DistributedQueues.html
[1] https://wiki.openstack.org/wiki/Zaqar
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