Sending Individual ACK for multiple messages in one command.

Shared subscription can only employ Individual ACK, which may limit the throughput. However, from PulsarApi.proto we find that Pulsar supports acknowledging multiple messages in one command even in Individual mode.

Java client supports this feature by adding acknowledgeGroupTime setting in ConsumerBuilder, but C++ client does not.

So the questions are:

1. In Individual ACK mode, does sending multiple message IDs in one CommandAck really help achieving higher throughput?
2. If we implement this feature in C++ client (and later contribute back to community), shall it be implemented as an ACK interface, or as a setting like in Java. (Seems the previous one is easier.)
3. If it’s implemented as an ACK interface, what should it be called? acknowledgeGroup or groupAcknowledge? We prefer the term group, because the C++ client has a data structure called BatchAcknowledgeTracker, which may confuse the code reader when we have two similar names for different purpose.

BTW

Does acknowledge interface in C++ client really send ACK synchronously?

It actually calls the acknowledgeAsync and waits for triggering of the callback. But the doAcknowledge calls returns immediately anyway with ResultOk, because sending command is actually an asynchronous operation? Is this a known feature/issue? Or I just misunderstand the implementation? Is there any opened issues related to this?

[tongsucn]

FYI. The feature is almost done, For now, it improves the throughput bottleneck in shared subscription in our scenario, I’ll firstly try it in production envrionment to verify more. After that, I’ll add related necessary building and testing code and submit a new pull request.

[tongsucn]

PR is created. However, the Backwards Compatibility CI checking failed because of docker storage. Retry later.