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Comment by bruth | Hacker News Reader

#1: This isn't a hard requirement to achieve the desired permissions. For the DriftDB use case, my understanding is that all members in the room have full pub/sub k/v permissions, so that could be achieved by declaring a new permission pinned to the room when the room is created or joined (this can be done dynamically without a config file reload).

#2 Publishes do support optimistic concurrency control using the `Nats-Last-Expected-Sequence` (stream level) or `Nats-Last-Expected-Subject-Sequence` for the subject-level. This ensures to concurrent publishes will be serialized and all but one is rejected with "conflict wrong sequence" error. For example headers in Rust[0] and WS[1]

[0]: https://docs.rs/async-nats/latest/async_nats/header/index.ht...

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#1 your understanding of the DriftDB permission model is correct, but I’m not sure what declaring a new permission at runtime entails? Would I be creating a new bearer token for each room, and attaching the room’s permissions to it?

#2 the optimistic concurrency headers don’t solve the problem here, e.g.:

- I increment a counter (seq: 1)

- I increment a counter again (seq: 2, expected last sequence: 1)

- I begin computing a snapshot, resulting in a counter value of 2

- You increment the counter (seq: 3, expected last sequence: 2)

- I complete the snapshot and publish it with a Nats-Rollup header

- Your event has been lost, the counter value is now 2

#1: Correct, I putting together an example this week to show what this looks like. Pretty straightforward.

#2: You can combine rollup and expected last sequence header to prevent this, unless I am missing another subtle detail?

(I am enjoying this thread FWIW :)

Cool, looking forward to the example :)

I actually didn’t realize that the expected last sequence could be combined with nats-rollup. In the example above (as I understand it) if we added that to the snapshot, NATS would throw out the snapshot, so the log would still be correct, but the work of snapshotting it and sending it over the wire would be lost. If messages were frequent enough, and/or snapshots took a while to compute/transfer, you might never have a roll-up succeed.

Our approach is that a roll-up will always succeed, we just preserve any messages in the stream with a sequence number greater than the one provided.

(I’m enjoying it too, and am a user of NATS so I’m happy to learn things I didn’t know about it :)

> If messages were frequent enough, and/or snapshots took a while to compute/transfer, you might never have a roll-up succeed.

Yes, good point. The "snapshotting up to a lagging sequence" could be achieved with two separate subjects to reduce contention, but is a bit more work.

It sounds like, in Drift's case, the snapshot effectively brings up the tail (snapshot), but the head can still be appended to with new events.

> It sounds like, in Drift's case, the snapshot effectively brings up the tail (snapshot), but the head can still be appended to with new events.

Yep, exactly. It’s a subtle feature but it makes it possible for multiple clients to attempt to compact without worrying about races.

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