Design thoughts: header/trailer advanced usage

[mgravell]

Note ties into #284

Consider:

SomeType Method1(SomeType arg, [Header] string x, ...)
SomeType Method2(SomeType arg, [Header] out string x, ...)
SomeType Method3(SomeType arg, [Header] ref string x, ...)
SomeType Method1(SomeType arg, [Trailer] string x, ...)
SomeType Method2(SomeType arg, [Trailer] out string x, ...)
SomeType Method3(SomeType arg, [Trailer] ref string x, ...)

• [Header] also interchangeable with aspnet [FromHeader]
• optional name, otherwise uses arg name
• types: limited; ala Convert? always invariant; suggest string, unsigned+signed integers 8/16/32/64, byte[], bool, char, DateTime; nullable-of-same
• Directionality:
  • nothing/in: client-to server
  • out: server-to-client
  • ref: both directions
• [Trailer] not allowed for C2S/both for client-streaming/duplex
• [Trailer] not allowed for S2C/both for server-streaming/duplex

Thoughts?

[menaheme]

Does this open up the way to have more than one input parameter (excluding context)?

[mgravell]

Yes, although I also want to allow full body binding as protobuf implied models. But I have definitely missed an important category above - binary headers that are serialized object payloads. I'm conflicted as to whether to add this one, as header sizes are bounded. Open to thoughts, though... i.e.

..., [Header] SomeType arg, ...
..., [Header] in SomeType arg, ...
..., [Header] ref SomeType arg, ...
..., [Header] out SomeType arg, ...

(and [Trailer])

which should only work if SomeType is recognized by the configured marshaller

[menaheme]

Hmm, the difference between method parameter types and header types, is that the former are contracted in the interface signature , the latter are not documented anywhere, which is risky

[mgravell]

Partly. Also no streaming for headers. And headers have more limited sizes. And the marshaller API is different (although I've suggested some improvements for that over on gRPC.Core)

[menaheme]

How does one communicate to comsumer, the header type expected at the server?

[mgravell]

Well, in code-first land: like the above :)

In contract-first: "you tell them"