Revisit the early decision regarding "Remove unused RPCs and disable reconfiguration"

[chenlica]

In #2970 , we decided to "Remove unused RPCs and disable reconfiguration."

@Yicong-Huang @shengquan-ni Can we revisit this decision?

[Yicong-Huang]

I agree we need to reenable them. It was temporarily removed as we are changing our rpc to grpc.

[Yicong-Huang]

Those three should be reenabled:
Reconfiguration.
UpdateExecutor.
UpdateMultipleExecutors.

[chenlica]

Do we have any problems if we reenable them?

[Yicong-Huang]

I think we will need to reimplement these methods. They were disabled because, at the time, they were too time-consuming to support under the new gRPC framework. We also changed how DCM is sent through gRPC, which makes the old implementation incompatible.

These methods are more complex than the other control messages, since they require multiple rounds of communication and stricter message-alignment guarantees.

@shengquan-ni can chime in for more details.

[chenlica]

Thanks. @shengquan-ni Can you chime in?