Among the problems that can occur in a data processing system are failures with:
All these problems are potentially more severe in an online system than in a system that performs only batch processing.
In batch systems, input data is usually prepared before processing begins, and jobs can be rerun, either from the start of the job or from some intermediate checkpoint.
In online systems, input is usually created dynamically by terminal operators, and arrives in an unpredictable sequence from many different sources. If a failure occurs, it is generally not possible simply to rerun the application, because the content and sequence of the input data is unknown. And, even if it is known, it is usually impractical for operators to reenter a day's work.
Online applications therefore require a system with special mechanisms for recovery and restart which batch systems do not require. These mechanisms ensure that each resource associated with an interrupted online application returns to a known state so that processing can restart safely.
In mixed systems, where both batch and online processing can occur against data at the same time, the recovery requirements for batch processing and online systems are similar.
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