memcached failover question

[Daniel Hutchison]

On Tue, 2016-01-19 at 21:28 +0000, Marvin Addison wrote:
>         So, is this normal behavior, and I read too much into
>         MemcachedStorageService's ability to handle failover?
> 
> 
> I believe the docs need to be updated. We recently discovered during
> an outage that locatorType="CONSISTENT" prevents rehashing the key
> against the reduced pool, which effectively leads to non-deterministic
> failures for storage events. Basically anything that hashes to the
> downed node will fail. We changed locatorType="ARRAY_MOD" and achieved
> the resiliency we were after. Please make that change and report back.
> I will probably update the docs either way; I'm fairly sure it's the
> best setting for most deployments.
> 

Thanks for the response.   At least for my logout test case,
locatorType="ARRAY_MOD" exhibits the same behavior as
locatorType="CONSISTENT".     However, it appears that the ARRAY_MOD
*seems* to handle errors better for my test case when an existing logged
in user tries to access a new SP after one of the memcache servers was
forced out of the pool.   I put an asterisk on that because I wonder if
it matters which node I take down in my simulation.    ARRAY_MOD doesn't
seem to generate all of the errors I was seeing in my original test
case, but I have not had the time to do an exhaustive test.

Anyway, part of my confusion was due to some of the wording on the wiki
that said that there wasn't a need to setup repcached and the keys would
be written/retrieved from the backup node.   After rereading it, I see I
glossed over the part where not all use cases can handle the failover
situation the same.

Thanks,

-daniel

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