[Openstack] A single cross-zone database?
glen.campbell at rackspace.com
Wed Mar 16 18:33:35 UTC 2011
Instead of building data-specific caching (which always worries me), you
could simply build the service to return the data directly, then add a
"Cach-Control: max-age=NNN" header to the result. That way, users who
wanted to improve their performance could add a squid layer (or other
caching HTTP proxy) to their endpoints and ensure consistency within the
cache max-age range (for example, max-age=300 would ensure consistency as
of 5 minutes ago). If the user prefers consistency over performance,
simply bypass the cache.
In that manner, the "consistency vs. performance" decision is left up to
the cloud administrator.
On 3/16/11 12:58 PM, "Paul Voccio" <paul.voccio at rackspace.com> wrote:
>I would agree. The caching would go with the design tenet #7: Accept
>eventual consistency and use it where it is appropriate.
>If we're ok with accepting that the list may or may not always be up to
>date and feel its appropriate, we should be good with the caching.
>On 3/16/11 11:45 AM, "Ed Leafe" <ed.leafe at rackspace.com> wrote:
>>On Mar 16, 2011, at 12:23 PM, Paul Voccio wrote:
>>> Not only is this expensive, but there is no way I can see at the moment
>>>to do pagination, which is what makes this really expensive. If someone
>>>asked for an entire list of all their instances and it was > 10,000 then
>>>I would think they're ok with waiting while that response is gathered
>>>and returned. However, since the API spec says we should be able to do
>>>pagination, this is where asking each zone for all its children every
>>>time gets untenable.
>> This gets us into the caching issues that were discussed at the last
>>summit. We could run the query and then cache the results at the
>>endpoint, but this would require accepting some level of staleness of the
>>results. The cache would handle the paging, and some sort of TTL would
>>have to be established as a balance between performance and staleness.
>>-- Ed Leafe
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