Apama 10.3 | Apama Documentation | Developing Apama Applications | Developing Apama Applications in EPL | Defining Queries | Introduction to queries | Delayed and out of order events
Delayed and out of order events
In many of the typical applications envisaged for Apama queries, the input events may be either delayed or out of order. For example, cars and other mobile sources of events such as smart phones and tablet computers might normally send regular streams of events, but when such devices are out of network coverage, these events will have to be batched and sent when back in range. Many older generation factory robots store events and only send periodic batches by design. And in other cases, events may be sent out of order. Television set top boxes, for example, often employ distinct channels for tuning information and diagnostics. This means that a "channel changed" event may be received before a "set top box crashed" event, and so may be thought to have caused it, even though the event in fact happened after it, and was causally unconnected.
Delayed or out of order events can create problems for the query runtime because it assumes that events should be treated as being in the order in which they are processed, and the time of each event is the correlator's time at the point the event is processed. However, provided that the input events contain a timestamp recording the time that the event was created at the source, these problems can be overcome by using the Apama queries source timestamp functionality. This allows the queries runtime to wait for specified periods before processing events, and then to process those events on the basis of their source timestamps rather than the time they were received by the correlator. (For out of order events, the Apama event definitions must have the appropriate annotation; for more information, see Out of order events).
Events can also be supplemented by heartbeat events with timestamps from data sources to inform the query runtime when communication with the data source is working correctly, which avoids long delays waiting for events to occur in case they are delayed.
See Using source timestamps of events for details on how to configure Apama queries to use source timestamps.

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