[Discuss] Redundant array of inexpensive servers: clustering?

[Richard Pieri]

Derek Martin wrote:
> It really depends, but mostly it isn't.  One of my earliest gigs was
> managing just such an environment, which mostly included a few custom
> applications which were not designed to be clustered.  With the right
> hardware, it's fairly trivial.  But the right hardware is expensive.

Clustering non-HA apps can be trivial if those apps have few 
dependencies. For example, clustering Apache by itself is easy. You just 
need a storage resource, a network resource, and the startup and 
shutdown scripts.

The complexity arises when service groups depend on other service 
groups. In a three-tier web app you have the web server, the application 
server and the database server. There are dependency chains that have to 
kept intact for the whole thing to work. Service migration needs to be 
planned so that you don't end up with all three service groups running 
on a single node. That's just one point in a simple three-node cluster. 
Consider expanding that to a half-dozen nodes with several dozen service 
groups with various dependencies to each other. Planning that out is a 
lot of work and there are few short cuts to be had.

"The right hardware"? That just costs money. It's one of the few cases 
where throwing money at a problem actually is part of the solution.

-- 
Rich P.