Re: Funding Cyrus High Availability

From: Fabian Fagerholm (no email)
Date: Fri Sep 17 2004 - 01:18:10 EDT


On Thu, 2004-09-16 at 18:56 -0400, Ken Murchison wrote:
> I think this would cause performance to suffer greatly. I think what we
> want is "lazy" replication, where the client gets instant results from
> the machine its connected to, and the replication is done in the
> background. I believe this is what David's implementation does.
>
> Question: Are people looking at this as both redundancy and
> performance, or just redundance?

There has to be some balance between the two, of course. What exactly
would that balance be? A while back I had some ideas of lazy replication
between geographically separate nodes in a mail cluster, to solve a
problem that a customer was having. I think I posted something on this
very list back then. There was some research, but the costs involved in
actually implementing the thing were too big, and the time to do it was
too short.

The idea was to get rid of the single-master structure of Murder, and
have an assymetric structure where each node in the mail cluster can act
as "primary" for one or several domains, and as "secondary" for one or
several domains, at the same time. Synchronization could flow in either
direction. Each domain would have one primary server and some number of
secondary servers -- redundancy could be increased by adding slaves and
performance could be increased by placing them close to users in the
network topology. Placing slaves in a geographically remote location
would act as a sort of hot backup -- if one server breaks then you just
replace it and let it synchronize with an available replica. Basically,
think DNS here, and add the ability to inject messages at any node.

Let's say you have five servers and three offices (customers) -- you'd
set up one server in your own facilities, one server in a co-location
facility, and one server in each of your customers' facilities.

You configure the server in your network -- which acts as a kind of
administration point -- and in the co-location network to handle "all
domains" and each server in the customers' facilities to handle mail
only for their domain(s). You then create domains and mailboxes on the
server close to you in the network topology. The mailboxes will be lazy-
replicated to the correct locations. Using suitable DNS records, you can
have mail delivered directly to each customer's server, and it would
lazy-replicate to your servers. Your servers would act as MX backups
when the customer's network is down, and the mail would be lazy-
replicated to them when they reappear on the network. Also, you could
support dial-up users by having them connect to the co-located server
instead of having to open firewalls etc. to the customer's network,
which is potentially behind a much slower link.

So to answer your question, I believe that by selecting a suitable
structure, you could actually address both performance and redundancy at
the same time. (Although I realize I've broadened the terms beyond what
you probably meant originally.)

In any case, I'd be willing to join the fundraising, but before that I'd
like to see an exact specification of what is actually being
implemented. I imagine that the specification could be drafted here on
this list, put somewhere on the web along with the fundraising details,
and we'd go from there.

Cheers,

-- 
Fabian Fagerholm <>

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