Fault Tolerance?

Peter Chiu peterchiu at jobsdb.com
Fri Sep 28 09:01:07 UTC 2007

Totally agreed. What about building a HA memory cache based on NDB, running on machines with plenty of RAM (64GB)? That way NDB would cache most of the stuff in memory for fast access, while at the same time provide HA. Ideas?


-----Original Message-----
From: memcached-bounces at lists.danga.com [mailto:memcached-bounces at lists.danga.com] On Behalf Of Mdecandia
Sent: Friday, September 28, 2007 3:50 PM
To: memcached at lists.danga.com
Subject: Re: Fault Tolerance?

On Sep 27, 2007, at 22:16, Dustin Sallings write: 
> On Sep 27, 2007, at 21:56, Paul Scott wrote:
> >> Agreed. Wouldn’t it be great though to have a mem-based HA datastore?
> >
> > I would certainly vote +1 on that idea!
> 	You do realize you wouldn't have anything remotely like the  
> performance of memcached, don't you?  You'd need something along the  
> lines of two-phase-commit if you want any kind of correctness.  If  
> you don't want correctness, then why are you worried about HA?
We use memcached to store items fetched from a slow service, not from
Performance difference between this service and memcached are huge.
May be really convenient to have an HA feature on memcached servers.

> 	If you lose a node, how do you plan on rematerializing?  A complete  
> synchronization would block both nodes in a two-node cluster.

We've introduced libketama consistant hashing to reduce effects on server
faults but 
it will be useful to have a redundant caching system between servers to be
really fault toulerant.

> 	How would you handle conflicts during rematerialization after a  
> netsplit?
> 	Is it acceptable to block all clients during a netsplit (pending  
> some sort of magical synchronization that knows what to do when  
> conflicts occur)?
> 	After you get all of the pieces in place, are you sure you'd have  
> something that would be any faster than any solution that isn't  
> completely in-memory?
> -- 
> Dustin Sallings


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