Proper system testing is a three-step process. Each component must be tested individually, then the system as a whole should be tested to ensure that the pieces work together. Finally, it should be subjected to "live" testing" that simulates real usage, with a similar workload and with a distribution of users and processing volumes similar to what will occur on a typical day.

Please see these Hints for successful performance testing

Testing the System
There are three important steps to conduct when your system is in place. The opinion "It works well enough" is not sufficient.

Hardware and software testing - Technical team members who are developing, integrating or customizing your system must include hardware and software testing as part of their routine work to verify that each product does what it was designed to do. Whether the project is a custom development process or implementation of a product, each product must be tested as it is brought online. In any situation, there should be a pre-approved objective specification to which testers can refer. This reference point could be a Functional Specification (see Chapter 2), product documentation supplied by a product vendor, or a design document prepared earlier in the implementation process.

Integration testing - The fact that each product works in isolation doesn't mean the entire system will work. The functioning of the overall system generally requires the set of components and software applications to work together. Computers that are networked need to be able to exchange information with one another or share access to software programs and peripherals. The only way to verify this is to enter information into one part of the system and check to see if it is properly dealt with and reflected in other parts, or check to be sure all networked computers have access to all programs and information. For example, in a simple software package that provides for storage and retrieval of student information, enter student information in the data entry screens, and verify that you can limit access to the data as needed and generate a report from it. In transaction processing systems (such as financial packages), try carrying out each of the types of transactions the system is supposed to handle. With instructional applications, make sure all computers used by teachers and students can access the software and use it as planned. For example, check to be sure that all student computers on the network can access a particular CD-ROM, such as one containing an encyclopedia. This integration testing often must be done repeatedly in order to verify that the system handles information properly.

Performance testing is the final stage. Here, you may need to enlist some volunteers to bang away at the system, simulating "normal" usage levels to see if the software system itself holds up under pressure and to make sure other components (e.g., the network, desktop workstations, and printers) can carry the load. Be careful not to confuse user approval with system functionality.

Testing the Software Interface
If your new system involves interfaces to other external systems, the importance of planning, testing and carefully verifying that the new interface works cannot be overstated. A new or revised software application may not communicate with other software applications in the same way that the old application did. Your new system may need specifications for how other software applications will interact with it. Keeping all of the applications used by an organization compatible is a major challenge, just as is keeping all hardware compatible with the requirements of new software. Changes to existing applications should be identified, specified, and included in the implementation process for your new application.