Re: [squid-users] CARP question

On 17/03/2009, at 7:44 AM, Chris Woodfield wrote:

> Hi,
>
> Had a question about squid's CARP implementation.
>
> Let's say I have a farm of squids sitting behind an SLB, and behind
> those I have a set of parent caches. If I were to enable CARP on the
> front-end caches, is the hash algorithm deterministic enough to
> result in a URL request seen by more than one edge cache to be
> directed to the same parent cache?

Yes (keeping in mind that they can move around if the set of servers
considered 'up' changes, and of course different FE caches will have a
different idea of what set is 'up' at any particular point in time).

> Or will each front-end cache have its own hash assignments compared
> to the others?
>
> Also, how does CARP handle parent server removals and/or additions
> (i.e. are hash "buckets" reassigned gracefully or are they all
> redistributed)? Is this behavior also deterministic between multiple
> front-end squids?

It is deterministic, and the idea is to cause the least disruption.
Search on 'consistent hashing' for the math; it's the same technique
used in Akamai, Hadoop/BigTable, etc.

Cheers,

--
Mark Nottingham       mnot_at_yahoo-inc.com