What changes to WIT are "compatible" and which are not?

[josh-berry]

Suppose I have a WIT definition, and some old WebAssembly components built/defined using the old definition, and then I want to make a change to my interface without requiring the component to be modified or rebuilt. Or perhaps I want to do the same in the reverse direction—make a change to my WASM component that remains backward-compatible with a host using an older WIT.

What changes to the WIT could be considered "compatible", if any, and which are not?

In essence, I'm looking for guidance similar to what Protobuf offers in their Updating a Message Type documentation.

A few example questions/scenarios I can think of: are the following compatible in one or both directions (old host/new guest vs. new guest/old host)?

1. Removing an argument to a function
2. Adding a new optional argument to a function
3. Adding a new optional field to a struct
4. Removing a field from a struct
5. Adding a new case to a variant
6. Removing a case from a variant

[vados-cosmonic]

Hi @josh-berry have you taken a look at the WIT specification documentation?

That's a good place to start -- it has excerpts like:

Many aspects of the WIT syntax can evolve over time without breaking downstream tooling, similar to what has happened with the Core WebAssembly WAT text format over time.

Note that, at least until subtyping is relaxed in the Component Model, if we had to add a new case to calc-error, this would be a breaking change and require either a new major version or adding a second, distinct variant definition used by new functions.

Feature Gates

I certainly agree that a centralized area for documentation like that would be great -- I personally am not sure whether that should go here or in the component-model repo.

That said, I at the very least would be very happy to accept a PR if you want to get this started and loop in the right people that should be looking at this for a once over (in addition to our review). Would be a great chance to call out a lot of the cases you're discussing/have questions about specifically.