ProtobufCodecBuilder<DocApiT, ProtoT> encoder(Function<DocApiT, ProtoT> fn) {
            if (encoderFn != null) {
                throw new IllegalArgumentException("Encoder already set");
            }
            encoderFn = fn;
            return this;
        }
Implementation of MessageBus message request/response serialization built around Protocol Buffers.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/RoutableFactories80.java