It really depends on the kind of business the application is supposed to fulfil the requirements of.
In my experience most of the time there are two types of organisations:
- The type that roughly know how their users are going to behave
- The type who have no idea what to expect from their users
It's really easy to start to calculate the requirements for the type of organisation that has a captive set of users. Take for example a call centre for an insurance firm. They know they have to be able to deal with X sales enquiries every day. They also know that Y clerks will be connected to the application on a typical or busy day. You then know the concurrent user rate and the transaction rate. Therefore the application must be able to support this business scenario. X users and a Y transaction rate.
You have to do this kind of evaluation for every type of scenario that is deemed high risk or high volume, typically there will be 5-30 scenarios.
For the organisation that knows nothing about their users such as an organisation creating a new web site / application and opening it to a new market then you have to start with what you can do. That might be to find out how many transactions the application can support per hour under a given load, thereby finding the limits of the application – you’ll be surprised how few web apps do actually scale as I’m sure people who use proper tools have found out. *Note: Beware the tools market (open source & cheap) – I’ve seen some real horror stories using these tools relating to false positives. I’m definitely not saying use LoadRunner (in fact I’d actively avoid it) but do look around and run some evaluations.
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