hardware - Capacity planning for an enterprise java application

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If you had to do capacity planning and
hardware sizing "BEFORE" you had a chance to actually code and test the application
(typically while you are defining solution architecture), how would you do
it?

I know this can not be known accurately
beforehand but the point is to present the approach at an early stage (including
questions you need to ask, assumptions you need to
make).

All you know, it will be an
enterprise java application with App server, Web Server, Database. Business has given
some number of concurrent "USERES" say 1000. Also assume that you will get a chance to
fine tune your numbers after load testing the application but you can't be far off from
the original estimates.

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class="normal">Answer

All you
can base it on is experience with "similar" applications. And that's not likely to give
you good estimates. If you have no such experience, or cannot get numbers from a
"comparable" system in production elsewhere, you're up a
creek.

It's mostly a nonsense requirement. So
try to budget for an enormous amount of hardware and supporting infrastructure, and then
look like a hero when you use 25% of that budget. Just remember you need N+1 redundancy,
so at least two of everything, including databases, load balancers,
etc.