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James Munns
@jamesmunns.com

Does anyone have opinions on non-cryptographic hashes that are relatively simple, can be done streaming (don't know the length apriori), have good avalanche characteristics, and produce a 64-bit output? Currently using fnv1a64 for postcard-rpc schema hashing, wondering if there's something better.

Jamie Donaldson
@jamiedonaldson.bsky.social

I don’t have an answer to your question, but is there a “design doc” for postcard-rpc that explains why design decisions were made? I’m wondering why a small, serially allocated integer is insufficient, when both sides know the full set of possible message types?

James Munns
@jamesmunns.com

I've started working on it at postcard.rs, but the short answer is that postcard is a non self describing binary format, and so if you change the schema on one side but not the other, you could misinterpret or fail to decode messages. Using a hash of the schema means the ID changes if the type does.

postcard.rspostcard.rs

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James Munns
@jamesmunns.com

So, the main answer is: because you might change the code for the client and the server at different times, we want to make sure both ends agree, even over time and different firmware/software versions. The versioning is also per-endpoint, so you can also use this to support multiple revs over time