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With KIP-74, we now have a good way to limit the size of Fetch responses, but it may still be difficult for users to control overall memory since the consumer will send fetches in parallel to all the brokers which own partitions that it is subscribed to. Currently we have:

-max.fetch.bytes: This enabled to control how much data will be returned by the broker for one fetch

-max.partition.fetch.bytes: This enables to control how much data per partition will be returned by the broker

None of these settings take into account that the consumer will be sending requests to multiple brokers in parallel so in practice the memory usage is as stated in KIP-74: min(num brokers * max.fetch.bytes,  max.partition.fetch.bytes * num_partitions)

To give users finer simpler control, it might make sense to add a new setting to properly limit the memory used by Fetch responses in the consumer in a similar fashion than what we already have on the producer.

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The following option will be added for consumers to configure (in ConsumerConfigs.java):

1. buffer.memory (Long) : Long, Priority High:

   The total bytes of memory the consumer can use to buffer

   fetch responses

   records received from the server and waiting to be

   read after being received from the server.

   processed (decompressed and deserialized).

   This setting differs from the total memory the consumer will use because some additional memory will be used for decompression (if compression is enabled), deserialization as well as for maintaining in-flight requests.

Alongside, we will set the priority of max.partition.fetch.bytes to LowNo other changes of APIs.

Proposed Changes

This KIP reuses the MemoryPool interface from KIP-72.

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4) Once messages are returned to the user, messages are deleted from the MemoryPool so new messages can be stored.

Caveats:

There is a risk using the MemoryPool that, after we fill up the memory with fetch data, we can starve the coordinator's connection.

For example, if we send a bunch of pre-fetches right before returning to the user, these fetches might return before the next call to poll(), in which case we might not have enough memory to receive heartbeats, which would block us from sending additional heartbeats until the next call to poll(). Because heartbeats are tiny, this is unlikely to be a significant issue. In any case, KAFKA-4137 (separate network client) suggests a possible way to alleviate this issue.

Compatibility, Deprecation, and Migration Plan

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