When the server responds to a DCP stream request (opcode 0x53) with a rollback error (PROTOCOL_BINARY_RESPONSE_ROLLBACK (0x23)), it would be nice if the response included a brief human-readable message in addition to the rollback seqno.
Motivation: This will aid in diagnosing issues related to rollbacks in the connectors. The connectors will be able to log the rollback cause, so users won't have to struggle with capturing and digging through the corresponding memcached logs.
The body of a rollback response currently consists of the 8 byte sequence number to roll back to. One way to implement the improvement would be to follow the sequence number with the message as a string. Perhaps the message should be prefixed by an unsigned LEB128 integer specifying the length of the message, so additional "fields" could be tacked onto the response in the future.
It's unknown whether this would break existing DCP clients. It might be good to add a new DCP control that enables including the message in the response.