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12.4. Session Clock
Events have strong temporal constraints making it is necessary to use a reference clock. If a rule needs to determine the average price of a given stock over the last sixty minutes, it is necessary to compare the stock price event's timestamp with the current time. The reference clock provides the current time.
Because the rules engine can simultaneously run an array of different scenarios that require different clocks, multiple clock implementations can be used by the engine.
Scenarios that require different clocks include the following:
- Rules testing: Testing always requires a controlled environment, and when the tests include rules with temporal constraints, it is necessary to control the input rules, facts, and the flow of time.
- Regular execution: A rules engine that reacts to events in real time needs a real-time clock.
- Special environments: Specific environments may have specific time control requirements. For instance, clustered environments may require clock synchronization or JEE environments may require you to use an application server-provided clock.
- Rules replay or simulation: In order to replay or simulate scenarios, it is necessary that the application controls the flow of time.
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