In addition to being sensitive to changes in the underlying database, Multi-User applications need to be able to protect users and application processes from the effects of one another when the same record or collection of records are being manipulated at the same time. The process by which these issue are addressed is known as Concurrency Control.
Concurrency control occurs in one of two ways, Optimistic or Pessimistic control.
Optimistic Concurrency Control
Presumes that probability and frequency of multiple users and processes instigating changes to the same database records is low. As result when an end-user or process attempts to change records it first of all determines if the record values at the point of change are still the same as what they were at the time of retrieval. If they are unchanged at the point of change then the change occurs otherwise the change process is rejected and then re-attempted. Although this reduces concurrent user latency, it does have the knock on effect of reducing data integrity if changes rejections aren't managed carefully.
Pessimistic Concurrency Control
Presumes that the probability and frequency of multiple user processing and instigating changes to the same records is high.
As a result an end-user or process attempts to changes records it first of all secures Exclusive Locks on the records in question, performs the changes, and then releases the locks.
Although this increases and preserves data integrity it does introduce concurrent use latency , which is perceived as performance degradation by the end-user or application developer.
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