> Here clients HAVE to choose between 'reflect' and 'specific' Avro engine, but that should be ok for now.
That would work if you only transition protocols to Specific and never wish to rely on Reflect. Since some parameter types are used by multiple protocols, this requires you to transition more (all?) protocols at once, whereas, if some parameters could use Reflect, you could transition things from Writable to Avro protocol-by-protocol, which might be easier to debug. But AvroSpecificRpcEngine should be easy to write, and, long-term, when all protocols use Specific, we'll want this, so it'd be a good addition. +1