Outer stream joins
Immediately pass the first event received and initialised downstream processors
Objective
Outer joins are different from inner joins by immediately passing the first event received on the outer stream into the processing pipeline, which can be useful for initialising downstream processors.
Any additional events that meet the join criteria are also emitted as they arrive.
Outer stream joins are extremely useful to tackle the cold start problem in stream recommendation engines.
Uses
It is extremely useful for ideal to prime processing.
Priming can improve performance by reducing the latency to get initial results, which are then refined or updated as more data becomes available.
Example
When a new customerId
event is generated by the siteVisits
stream, the join processor emits it for initial processing.
Subsequent matching events trigger additional join outputs, continuing until the join expiration policy takes effect.
Outer joins are set by *=
.
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