Nico Habermann created KAFKA-12508:
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Summary: Emit-on-change tables may lose updates on error or
restart in at_least_once
Key: KAFKA-12508
URL: https://issues.apache.org/jira/browse/KAFKA-12508
Project: Kafka
Issue Type: Bug
Components: streams
Affects Versions: 2.6.1, 2.7.0
Reporter: Nico Habermann
[https://cwiki.apache.org/confluence/display/KAFKA/KIP-557%3A+Add+emit+on+change+support+for+Kafka+Streams]
Added emit-on-change semantics to KTables that suppress duplicate values.
However, this may cause data loss in at_least_once topologies when records are
retried from the last commit due to an error / restart / etc.
Consider the following example:
{code:java}
streams.table(source, materialized)
.toStream()
.map(mayThrow())
.to(output){code}
# Record A gets read
# Record A is stored in the table
# The update for record A is forwarded through the topology
# Map() throws (or alternatively, any restart while the forwarded update was
still being processed and not yet produced to the output topic)
# The stream is restarted and "retries" from the last commit
# Record A gets read again
# The table will discard the update for record A because
## The value is the same
## The timestamp is the same
# Eventually the stream will commit
# There is absolutely no output for Record A even though we're running in
at_least_once
This behaviour does not seem intentional. [The emit-on-change logic explicitly
forwards records that have the same value and an older
timestamp.|https://github.com/apache/kafka/blob/367eca083b44261d4e5fa8aa61b7990a8b35f8b0/streams/src/main/java/org/apache/kafka/streams/state/internals/ValueAndTimestampSerializer.java#L50]
This logic should probably be changed to also forward updates that have an
older *or equal* timestamp.
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