by camgunz | 2025-07-30 15:45:55+00:00
Oh good, another CBOR thread. Disclaimer: I wrote and maintain a MessagePack implementation. I've also bird dogged this for a while, HN search me.

Mostly, I just want to offer a gentle critique of this book's comparison with MessagePack [0].

> Encoding Details: CBOR supports indefinite-length arrays and maps (beneficial for streaming when total size is unknown), while MessagePack typically requires fixed collection counts.

This refers to CBOR's indefinite length types, but awkwardly, streaming is a protocol level feature, not a data format level feature. As a result, there's many better options, ranging from "use HTTP" to "simply send more than 1 message". Crucially, CBOR provides no facility for re-syncing a stream in the event of an error, whether that's network or simply a bad encoding. "More features" is not necessarily better.

> Standardization: CBOR is a formal IETF standard (RFC 8949) developed through consensus, whereas MessagePack uses a community-maintained specification. Many view CBOR as a more rigorous standard inspired by MessagePack.

Well, CBOR is MessagePack. Carsten Bormann forked MessagePack, changed some of the tag values, wrote a standard around it, and submitted it to the IETF against the wishes of MessagePack's creators.

> Extensibility: CBOR employs a standardized semantic tag system with an IANA registry for extended types (dates, URIs, bignums). MessagePack uses a simpler but less structured ext type where applications define tag meanings.

Warning: I have a big rant about the tag registry.

The facilities are the same (well, the tag is 8 bytes instead of 1 byte, but w/e); it's TLV all the way down (Bormann ripped this also). Bormann's contribution is the registry, which is bonkers [1]. There's... dozens of extensions there? Hundreds? No CBOR implementation supports anywhere near all this stuff. "Universal Geographical Area Description (GAD) description of velocity"? "ur:request, Transaction Request identifier"?

The registry isn't useful. Here are the possible scenarios:

If something is in high demand and has good support across platforms, then it's a no-brainer to reserve a tag. MP does this with timestamps.

If something is in high demand, but doesn't have good support across platforms, then you're putting extra burden on those platforms. Ex: it's not great if my tiny microcontroller now has to support bignums or 128-bit UUIDs. Maybe you do that, or you make them optional, but that leads us to...

If something isn't in high demand or can't easily be supported across platforms, but you want support for it anyway, there's no need to tell anyone else you're using that thing. You can just use it. That's MP's ext types.

CBOR seems to imagine that there's a hypothetical general purpose decoder out there that you can point to any CBOR API, but there isn't and there never will be. Nothing will support both "Used to mark pointers in PSA Crypto API IPC implementation" and "PlatformV_HAS_PROPERTY" (I just cannot get over this stuff). There is no world where you tell the IETF about your tags, define an API with them, and someone completely independently builds a decoder for them. It will always be a person who cares about your specific tags, in which case, why not just agree on the ext types ahead of time? A COSE decoder doesn't need also need to decode a "RAINS Message".

> Performance and Size: Comparisons vary by implementation and data. CBOR prioritizes small codec size (for constrained devices) alongside message compactness, while MessagePack focuses primarily on message size and speed.

I can't say I fully understand what this means, but CBOR and MP are equivalent here, because CBOR is MP.

> Conceptual Simplicity: MessagePack's shorter specification appears simpler, but CBOR's unification of types under its major type/additional info system and tag mechanism offers conceptual clarity.

Even if there's some subjectivity around "conceptual simplicity/clarity", again CBOR and MP are equivalent here because they're functionally the same format.

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I have some notes about the blurb above too:

> MessagePack delivers greater efficiency than JSON

I think it's probably true that the fastest JSON encoders/decoders are faster than the fastest MP encoders/decoders. Not that JSON performance has a higher ceiling, but it's got gazillions of engineering hours poured into it, and rightly so. JSON is also usually compressed, so space benefits only matter at the perimeters. I'm not saying there's no case for MP/CBOR/etc., just that the efficienty/etc. gap is a lot smaller than one would predict.

> However, MessagePack sacrifices human-readability

This, of course, applies to CBOR as well.

> ext mechanism provides less structure than CBOR's IANA-registered tags

Again the mechanism is the same, only the registry is different.

[0]: https://cborbook.com/introduction/cbor_vs_the_other_guys.htm...

[1]: https://www.iana.org/assignments/cbor-tags/cbor-tags.xhtml