Use Cases

The classic definition of a use case is that it describes a sequence of actions that are performed by a system to yield a result of value to a user. In simple terms, use cases describe what the actors do and what they want the system to do. Use cases present another opportunity for measurement and act as a source of activity that can be measured. The previous sections mentioned use cases as a potential basis for both derived and explicit requirements and constraints, as well as a means of establishing the context for a measurable SLR.

The sequence referred to in the definition is a specific flow of events through the system or an instance. Identifying and describing your use cases means identifying and describing a group of related flows of events. Actions are computational or algorithmic procedures, either invoked when the actor provides a signal to the system or when the system gets a time event. An action can imply signal transmissions to either the invoking actor or other actors. An action is atomic, which means it is performed either entirely or not at all. You can put a value on a successfully performed use case. This is very important to determine the correct level of granularity for a use case, to ensure that you are not achieving use cases that are either too minute to be useful or too large in number for the project's scale. In the unified modelling language (UML), actors are external to the system and always start off the use case.

The N1 Grid vision is about unleashing organizational potential, so your N1 Grid strategy tasks should be worked out into specific use cases that will help you illuminate, prepare for, and prove that the people, process, and technology choices you made deliver the solution you are designing.

Some functional use case examples might be:

• Users are properly authenticated and able to access and view properly formatted information from wireless PDA.

• Ten thousand properly authenticated users are simultaneously able to access and view properly formatted information from ten thousand wireless PDAs.

• Systems are properly correlating multiple event and information data points every ten minutes for business SLR reporting.

• Users can browse the repository of automated N1 Grid software installation plans for Enterprise JavaBeans deployment.

• Users can choose an N1 Grid software installation plan for Enterprise JavaBeans based on a business policy for deployment into the production environment.

Operational use case examples to test might include:

• Users are updating Enterprise JavaBeans in application server containers as part of the test-to-production service life cycle update.

• Users or the system are moving additional applications onto compute resources according to a business policy in response to an observation of spare CPU capacity.

• Help desk is appropriately responding to an alert in the management console according to problem management and escalation procedures.

• Users are using the N1 Grid software to quickly reconfigure the test environment to its exact configuration on midnight of June 7 of last year in response to a regulatory demand.