Processing : Transaction Processing

For some applications the master file needs to be kept up to date all of the time. For example in a travel agents whenever a seat is booked on a flight the number of seats that remain available on the flight must be reduced by one immediately. If this update was not done until the end of a day (as might happen in a batch processing system) then the flight could become overbooked with the same seat being booked more than once.

Transaction processing systems are used whenever the master file must be kept up to date. A transaction processing system operates like this :

• When a transaction is entered it is placed in a queue of transactions waiting to be carried out. The transactions are processed in the order that they are placed into the queue. If there are many people using the system at the same time then there could be lots of transactions being made from different computer terminals.

• The computer system will process one transaction at a time. Once the system starts processing a transaction it will not process any other transactions until the current transaction is finished. When a transaction is processed the master file is updated immediately. Therefore the master file is always kept up to date.

Under normal circumstances a well designed transaction processing systems will process a transaction within a few seconds of the transaction being inputted. During peak processing times (e.g. Saturday morning in a travel company) the time that it takes for a transaction to be processed will increase. For most systems this is acceptable. When it is not acceptable, a real time processing system must be used instead.

Transaction processing systems need to use direct access files. In a direct access file any record in the file can be updated directly, without having to read through all of the records that come before it in the file. Serial access media such as magnetic tape can not be used. The time required to find the record in the master file that a transaction related to would make processing incredibly slow.

Transaction processing is interactive, i.e. processing takes place as a "conversation" between the user and the computer : The computer responds to the user’s input by outputting some data before the user can input any more data. This means that the user's input can depend on the computer's previous output.

Transaction Processing is also known as Interactive, On-Line or Pseudo Real Time Processing.

GCSE ICT Companion 04 - (C) P Meakin 2004