CBOR: $type is missing?

[dongle-the-gadget]

I'm using PeterO/CBOR to parse the CBOR for com.atproto.sync.subscribeRepos in C# (the CBOR implementation isn't DAG-CBOR, however if I understood correctly there should be interoperability between both). However, the CBOR payloads the Bluesky API returned didn't have $type, but rather a t property with the type info I'm looking for (see picture below).

I'm not sure if I'm missing anything, or is this expected?

[myConsciousness]

Hi @dongle-the-gadget ,

The t stands for the $type of this response. So it means #commit structure below.

atproto/lexicons/com/atproto/sync/subscribeRepos.json

Lines 34 to 61 in 7e0201d

	"commit": {
	"type": "object",
	"required": ["seq", "rebase", "tooBig", "repo", "commit", "prev", "blocks", "ops", "blobs", "time"],
	"nullable": ["prev"],
	"properties": {
	"seq": {"type": "integer"},
	"rebase": {"type": "boolean"},
	"tooBig": {"type": "boolean"},
	"repo": {"type": "string", "format": "did"},
	"commit": {"type": "cid-link"},
	"prev": {"type": "cid-link"},
	"blocks": {
	"type": "bytes",
	"description": "CAR file containing relevant blocks",
	"maxLength": 1000000
	},
	"ops": {
	"type": "array",
	"items": { "type": "ref", "ref": "#repoOp"},
	"maxLength": 200
	},
	"blobs": {
	"type": "array",
	"items": {"type": "cid-link"}
	},
	"time": {"type": "string", "format": "datetime"}
	}
	},

[dongle-the-gadget]

Does that mean $type will be represented as t in CBOR objects?

[bnewbold]

Check out the Event Stream specs, they talks about the use of t in DAG-CBOR objects returned in the frame header DAG-CBOR object: https://atproto.com/specs/event-stream#framing

In short, the t field indicates the union type variant of the payload, as a short ref (like #commit), not a fully-qualified type path like a $type field would have.

[dongle-the-gadget]

Right. However, if there were a union type inside the content, would it use t or $type?

[bnewbold]

If there was a nested union inside the body payload, it would have $type, in both JSON and DAG-CBOR encodings. As another example, top-level records always have a $type, and field name is $type in both JSON and DAG-CBOR.

The Event Stream header use of t in DAG-CBOR, and having the truncated union variant name, is an exception, to make the wire protocol more efficient. The header is not a regular Lexicon object, which is why this is acceptable. The payload/body of the event frame is a regular Lexicon object.

[dongle-the-gadget]

Thanks for the explanation.

[sugyan]

related discussion: #1137