Swift does have some guarantees around read-after-write consistency, but
for Heat I think the best bet would be the X-Newest[1] header which has
been in swift for a very, very long time. The downside here is that
(IIUC) it queries all storage nodes for that object. It does not provide
a hard guarantee[2] but does at least try *harder* to get the most
recent version.

We could also (assuming it was turned on) use object versioning to
ensure that the most up to date version of the metadata was used, but I
think X-Newest is the way to go.

[1]: https://lists.launchpad.net/openstack/msg06846.html
[2]:
https://ask.openstack.org/en/question/26403/does-x-newest-apply-to-getting-container-lists-and-object-lists-also-dlo/

On 08/27/2014 11:41 AM, Zane Bitter wrote:
> On 27/08/14 11:04, Steven Hardy wrote:
>> On Wed, Aug 27, 2014 at 07:54:41PM +0530, Jyoti Ranjan wrote:
>>>     I am little bit skeptical about using Swift for this use case
>>> because of
>>>     its eventual consistency issue. I am not sure Swift cluster is
>>> good to be
>>>     used for this kind of problem. Please note that Swift cluster may
>>> give you
>>>     old data at some point of time.
>>
>> This is probably not a major problem, but it's certainly worth
>> considering.
>>
>> My assumption is that the latency of making the replicas consistent
>> will be
>> small relative to the timeout for things like SoftwareDeployments, so all
>> we need is to ensure that instances  eventually get the new data, act on
> 
> That part is fine, but if they get the new data and then later get the
> old data back again... that would not be so good.
> 
>> it, and send a signal back to Heat (again, Heat eventually getting it via
>> Swift will be OK provided the replication delay is small relative to the
>> stack timeout, which defaults to one hour)
>>
>> Steve
>>
>> _______________________________________________
>> OpenStack-dev mailing list
>> OpenStack-dev@lists.openstack.org
>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>>
> 
> 
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-- 
Ryan Brown / Software Engineer, Openstack / Red Hat, Inc.

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