[mtdowling]

Smithy today doesn't support any serialization formats that require fixed ordering. We do recommend that any additional members added to structures are added at the end to help with C++ and Rust codegen though.

That said, traits can be used in Smithy to enforce constraints on structures, so if you ever needed explicit indexing like that, it could be done via traits and protocols (Smithy's nomenclature for describing how clients and servers communicate). In fact, protocols are defined by traits, and traits can enforce requirements on the rest of the model using a DSL called selectors... Probably way too much info other than -- it's possible and easy to support this in Smithy if it's ever needed.

As for required vs optional -- today it's treated as server-side validation only and not used in client codegen. This allows service teams to remove the required trait from members if something ends up needing to be optional in future without breaking clients. We're working on some ideas too to see if we can generate even better code for SDKs in languages like Rust where optionality is very explicit, but without sacrificing the ability of being able to remove the required trait.

And, in general, backward compatibility issues are caught with Smithy diff, which also supports custom rules: https://github.com/awslabs/smithy/tree/main/smithy-diff