Affects Version/s: None
Fix Version/s: 0.10.1.0
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.