Today all client actions require a Writeable.Reader<Response> as part of their registration process, but in practice (since the removal of the transport client) we only ever call a very small proportion of these readers. More precisely, we only receive an action response over the wire when executing an action on a remote cluster via RemoteClusterAwareClient - in all other cases today we are using a NodeClient which executes the action, and constructs the response, on the local node.

Similarly, there are several implementations of TransportAction which do not inherit from HandledTransportAction or otherwise register a transport handler since they are always executed on the local node. They generally fill in the required, but uncalled, de/serialization methods with something that simply throws an exception. I suspect there's also a good fraction of HandledTransportAction derivatives which are never invoked via the transport layer and therefore could drop their transport handler registration and all of the associated de/serialization code.

I think it'd make sense to decouple the notions of local-client-accessible actions (only invoked on the local node), remote-client-accessible actions (invoked on remote clusters via a remote-cluster client), and transport actions (invoked directly via the transport service, not using a client).