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
This code defines a join between the sitevisits
and webpage.adclicks
streams based on customerId
.
expression It matches all
customerId
values fromsitevisits
towebpage.adclicks
.merge events Matching events are merged.
left policy
sitevisits
events expire after 30 minutes.right policy Matched
webpage.adclicks
events are deleted after the join.
New sitevisits
events trigger with a new customerId
initial processing and subsequent matching webpage.adclicks
events continue until expiration, with cleanup after joining.
Outer joins are set by *=
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