Stephan Ewen created FLINK-5048:
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Summary: Kafka Consumer (0.9/0.10) threading model leads
problematic cancellation behavior
Key: FLINK-5048
URL: https://issues.apache.org/jira/browse/FLINK-5048
Project: Flink
Issue Type: Bug
Components: Kafka Connector
Affects Versions: 1.1.3
Reporter: Stephan Ewen
Assignee: Stephan Ewen
Fix For: 1.2.0
The {{FLinkKafkaConsumer}} (0.9 / 0.10) spawns a separate thread that operates
the KafkaConsumer. That thread is shielded from interrupts, because the Kafka
Consumer has not been handling thread interrupts well.
Since that thread is also the thread that emits records, it may block in the
network stack (backpressure) or in chained operators. The later case leads to
situations where cancellations get very slow unless that thread would be
interrupted (which it cannot be).
I propose to change the thread model as follows:
- A spawned consumer thread pull from the KafkaConsumer and pushes its pulled
batch of records into a blocking queue (size one)
- The main thread of the task will pull the record batches from the blocking
queue and emit the records.
This allows actually for some additional I/O overlay while limiting the
additional memory consumption - only two batches are ever held, one being
fetched and one being emitted.
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