> Do you write a separate serialization function for every single struct and sub-struct you send over a network? And then continue maintaining every single one to keep it consistent whenever a field is added/removed/modified in any struct?

Yes? Of course you do that? Or pick one of a hundred libraries that do that for you - protobuf, cap'n proto, flatbuffers, etc...

That's the protobuf approach, but things like Capnp are different, right? Isn't its wire format its in-memory format? I thought it was closer to the "straight memcpy" approach.

Protobuf serialization functions can turn into a lot of generated code too (read: big binaries) and stomp all over your icache, because they're different for each type. More "data-driven" approaches using type-generic functions with some runtime type-specific data (like proto descriptors&reflection) can be a good tradeoff for code that isn't as "hot". Typically more branchy than a memcpy or a type-specific function though.

There are dozens, if not hundreds of serialization libraries. None of them are going to compare to something like sending a raw struct on the wire, if you need absolute performance. Probably the biggest advantage of things like grpc, flatbuffers, capnproto, etc, are they provide serialization libraries for many different languages. Sending a raw struct will be up to the implementer to get the other side correct.

And by sticking with the serialization libraries with higher performance, like flatbuffers and capnproto, you also lose a little bit of future flexibility, since its typically harder to change the format of these structures compared to higher level libraries.

Author of Cap'n Proto here...

> None of them are going to compare to something like sending a raw struct on the wire, if you need absolute performance.

I don't know about that.

For simple flat structs containing integers only (no pointers), Cap'n Proto is nearly identical to raw structs.

For complex structures with pointers, sending raw structs doesn't work.

Admittedly you may suffer unwanted code bloat linking in the Cap'n Proto library if all you really want to do is send a flat, raw struct.

> And by sticking with the serialization libraries with higher performance, like flatbuffers and capnproto, you also lose a little bit of future flexibility, since its typically harder to change the format of these structures compared to higher level libraries.

I disagree with this. Cap'n Proto's compatibility story is essentially the same as Protobuf and JSON: You can add new fields and old programs will ignore them.

(I suspect Flatbuffers is also similar but I haven't looked at it in a long time.)