Evgeny created BEAM-6036:
----------------------------
Summary: How to periodically refresh side inputs.
Key: BEAM-6036
URL: https://issues.apache.org/jira/browse/BEAM-6036
Project: Beam
Issue Type: Bug
Components: beam-model
Reporter: Evgeny
Assignee: Kenneth Knowles
I have followed the example provided here
[https://cloud.google.com/blog/products/gcp/guide-to-common-cloud-dataflow-use-case-patterns-part-1]
in the "Pattern: Slowly-changing lookup cache" section. I've converted the
pseudo-code from the article into this Java code:
return p
.apply("GenerateSequence", GenerateSequence.from(0).withRate(1,
Duration.standardHours(1)))
.apply("GenerateSequenceWindow",
Window.<Long>into(new GlobalWindows()).triggering(
Repeatedly.forever(AfterProcessingTime.pastFirstElementInPane()))
.discardingFiredPanes())
.apply("ConvertToKVs",
ParDo.of(new RetrieveToKVs()))
.apply("ToMap", View.asMap());
RetrieveToKVs() queries BigQuery table and outputs KVs.
The issue here is that the resulting map mixes up KVs from different periods
(i.e. the sequence is generated every 1 hour, the resulting map includes KVs
from 2 adjacent hours).
In an attempt to solve it I tried using View.asSingleton() instead.
return p
.apply("GenerateSequence", GenerateSequence.from(0).withRate(1,
Duration.standardHours(1)))
.apply("GenerateSequenceWindow",
Window.<Long>into(new GlobalWindows()).triggering(
Repeatedly.forever(AfterProcessingTime.pastFirstElementInPane()))
.discardingFiredPanes())
.apply("ConvertToKVs",
ParDo.of(new RetrieveMap()))
.apply("ToMap", View.asSingleton());
RetrieveMap queries data from BigQuery and outputs the complete map. The issue
with this is it not only results in flaky tests with the exception 1 times out
of 10:
Caused by: java.lang.IllegalArgumentException: Empty PCollection accessed as a
singleton view. Consider setting withDefault to provide a def
ault value
but also it doesn't seem to work. In the logs I see the RetrieveMap is called
every hour, but the pipeline using the side input get stale data.
Is there a real working example for how to make a side input refresh
periodically?
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