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file's data structure can be planned,
How should the file be organized? That depends on how it will be accessed. For example, in some applications, data are processed at regular, predictable intervals. Typically, the data are collected over time and processed together, as a batch. If batch processing is acceptable, a sequential file organization is probably best.
It is not always possible to wait until a batch of transactions is collected, however. For example, consider an air defense early warning system. If an unidentified aircraft is spotted it must be identified immediately the idea of waiting until 5 _ 00 p.m. because \need for quick response, each transaction must be processed as it occurs. Generally such transaction processing systems call for direct access file.
Our inventory system has two programs. One processes transactions. A direct access inventory file seems a reasonable choice. The other allows management to study inventory data occasionally; batch processing would certainly do. Should the inventory file be organized sequentially or directly? Faced with such a choice a good analyst considers both options. One possible system might accept transactions and process them as they occur. As an alternative, sales slips might be collected throughout the day and processed as a batch after the store closes. In the first system, the two programs would deal with direct access files; in the second system, they would be linked to sequential files. A program to process direct access data is different from a program to process sequential data. The data drive the system. The choice of a data structure determines the program’s structure. Note that the program is defined and planned in the context of the system.
2- 4 Implementation
Once the system's major components have been identified .we can begin to develop them. Our system includes two programs, several pieces of equipment, and a number of data structures. During implementation, each program is planned and written using the techniques described in Chapter 7. Files are created, and their contents checked. New hardware is purchased, installed, and tested. Additionally, operating procedures are written and evaluated. Once all the component parts are ready, the system is tested. Assuming the user is satisfied, the finished system is released.
2- 5 Maintenance