Chris Curtin created KAFKA-966:
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Summary: Allow high level consumer to 'nak' a message and force
Kafka to close the KafkaStream without losing that message
Key: KAFKA-966
URL: https://issues.apache.org/jira/browse/KAFKA-966
Project: Kafka
Issue Type: Improvement
Components: consumer
Affects Versions: 0.8
Reporter: Chris Curtin
Assignee: Neha Narkhede
Priority: Minor
Enhancement request.
The high level consumer is very close to handling a lot of situations a
'typical' client would need. Except for when the message received from Kafka is
valid, but the business logic that wants to consume it has a problem.
For example if I want to write the value to a MongoDB or Cassandra database and
the database is not available. I won't know until I go to do the write that the
database isn't available, but by then it is too late to NOT read the message
from Kafka. Thus if I call shutdown() to stop reading, that message is lost
since the offset Kafka writes to ZooKeeper is the next offset.
Ideally I'd like to be able to tell Kafka: close the KafkaStream but set the
next offset to read for this partition to this message when I start up again.
And if there are any messages in the BlockingQueue for other partitions, find
the lowest # and use it for that partitions offset since I haven't consumed
them yet.
Thus I can cleanly shutdown my processing, resolve whatever the issue is and
restart the process.
Another idea might be to allow a 'peek' into the next message and if I succeed
in writing to the database call 'next' to remove it from the queue.
I understand this won't deal with a 'kill -9' or hard failure of the JVM
leading to the latest offsets not being written to ZooKeeper but it addresses a
likely common scenario for consumers. Nor will it add true transactional
support since the ZK update could fail.
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