GraqhQL

[ackwell]

GQL is a very strong contender for a non-REST API surface, at least for sheets.
There's a few stumbling blocks to implementing it, primarily centered around the effectively dynamic nature of the sheet schema at runtime - many (rust) GQL libraries expect a singular static schema.

One of the best bets as of last look into this was juniper, which a community member built an example dynamic schema builder for here.

Further research is required.

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async-graphql has very recently landed dynamic schema generation, and looks like it'll be included in an upcoming v5 of the library.