Gunnar Morling created KAFKA-6551:

             Summary: Unbounded queues in WorkerSourceTask cause 
                 Key: KAFKA-6551
             Project: Kafka
          Issue Type: Bug
          Components: KafkaConnect
            Reporter: Gunnar Morling

A Debezium user reported an {{OutOfMemoryError}} to us, with over 50,000 
messages in the {{WorkerSourceTask#outstandingMessages}} map.

This map is unbounded and I can't see any way of "rate limiting" which would 
control how many records are added to it. Growth can only indirectly be limited 
by reducing the offset flush interval, but as connectors can return large 
amounts of messages in single {{poll()}} calls that's not sufficient in all 
cases. Note the user reported this issue during snapshotting a database, i.e. a 
high number of records arrived in a very short period of time.

To solve the problem I'd suggest to make this map backpressure-aware and thus 
prevent its indefinite growth, so that no further records will be polled from 
the connector until messages have been taken out of the map again.

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