From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-02T01:40:37+09:00
Subject: Re: Fault Tolerant DRb?

On Sat, 2 Jul 2005, Kirk Haines wrote:

> Just pondering different things this morning, and my mind came back to
> something I've thought about now and again.
>
> Assume you are using a DRb service for....something.  It doesn't matter what.
> The case is the same whether one is accessing an array via DRb or a Rinda
> Ring.  Is there some reasonably easy way of making a service work in a fault
> tolerant way?  That is, one could have two processes on two different
> machines both offering the same service.  If one process dies, the data is
> still present on the other, and the clients of that service can continue
> operating without data loss?
>
>
> Kirk Haines

i've done tons of ha (high availability) setups before for stateful and
stateless machines.  suffice it to say it is almost un-imaginably complex.
consider:

   * how to you tell if one machine is down vs. the network just being slow?
     for instance on our machines monthly backups might make any machine seem
     dead (can't ping) for 20 minutes or more.  typically this is solved via a
     serial cable between nodes to ping on using real-time priorities.

   * if you have the data on both machines and it can EVER be written to
     (modified) how to you bring the data back in sync when a machine has died
     but is now back up?

these problems are solved - but it's still amazingly hard to get right.  check
out the linux-ha project (google it).

depending on you needs you may be able to code something simple that 'good
enough' but you'll need some sort of distributed transaction capability and
the easist way to get that is via a real rdbms like postgresql.  however, once
you have that setup it's stilly to use drb unless your data is terrible to
model within the relational model.

feel free to contact me offline if you want to setup an ha box(es).

hth.

-a
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