Jay Kreps created KAFKA-2063:
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Summary: Bound fetch response size
Key: KAFKA-2063
URL: https://issues.apache.org/jira/browse/KAFKA-2063
Project: Kafka
Issue Type: Improvement
Reporter: Jay Kreps
Currently the only bound on the fetch response size is
max.partition.fetch.bytes * num_partitions. There are two problems:
1. First this bound is often large. You may chose max.partition.fetch.bytes=1MB
to enable messages of up to 1MB. However if you also need to consume 1k
partitions this means you may receive a 1GB response in the worst case!
2. The actual memory usage is unpredictable. Partition assignment changes, and
you only actually get the full fetch amount when you are behind and there is a
full chunk of data ready. This means an application that seems to work fine
will suddenly OOM when partitions shift or when the application falls behind.
We need to decouple the fetch response size from the number of partitions.
The proposal for doing this would be to add a new field to the fetch request,
max_bytes which would control the maximum data bytes we would include in the
response.
The implementation on the server side would grab data from each partition in
the fetch request until it hit this limit, then send back just the data for the
partitions that fit in the response. The implementation would need to start
from a random position in the list of topics included in the fetch request to
ensure that in a case of backlog we fairly balance between partitions (to avoid
first giving just the first partition until that is exhausted, then the next
partition, etc).
This setting will make the max.partition.fetch.bytes field in the fetch request
much less useful and we should discuss just getting rid of it.
I believe this also solves the same thing we were trying to address in
KAFKA-598. The max_bytes setting now becomes the new limit that would need to
be compared to max_message size. This can be much larger--e.g. setting a 50MB
max_bytes setting would be okay, whereas now if you set 50MB you may need to
allocate 50MB*num_partitions.
This will require evolving the fetch request protocol version to add the new
field and we should do a KIP for it.
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