How to do a conditional write in spaces?

[Moshyfawn]

Im trying out spaces alpha with a simple sync engine for notes that can be updated from more than one device.

I currently have it working on the public API where a conditional write with com.atproto.repo.putRecord and swapRecord yields InvalidSwap if the record's been changed since last read.

I tried the same with com.atproto.space.putRecord with swapRecord but it just replaced the record.

Am I missing another way to do a conditional write in a space?

[Maxaubert]

You're not missing anything: com.atproto.space.putRecord has no compare-and-swap in the current alpha. The lexicon has no swapRecord (or swapCommit) input, so the PDS ignores the extra field and does a plain create-or-update. That's why it just replaced the record instead of returning InvalidSwap.

You can see it in the lexicon, which only takes space, repo, collection, rkey, validate and record:
https://github.com/bluesky-social/atproto/blob/de009e6e83faea80d92d2fc32a8ad07863586b50/lexicons/com/atproto/space/putRecord.json

The handler only reads those same fields:
https://github.com/bluesky-social/atproto/blob/de009e6e83faea80d92d2fc32a8ad07863586b50/packages/pds/src/api/com/atproto/space/putRecord.ts

The storage layer has a swap check, but nothing can reach it yet. There's a comment saying so ("No space lexicon offers a swap yet"):

atproto/packages/pds/src/actor-store/space/transactor.ts

Lines 166 to 169 in de009e6

	// No space lexicon offers a swap yet, so this only fires if one starts to —
	// better than accepting a compare-and-swap and not performing it.
	if (write.swapCid && (!prev || !write.swapCid.equals(prev))) {
	throw new BadRecordSwapError(prev)

So for now a conditional write has to be done client-side: read the record (com.atproto.space.getRecord) and compare its cid before writing. That's still racy between your read and your write, so if you need real conflict detection with several devices, you'll have to serialize writes yourself or wait for a swap parameter to be added to the space lexicons.