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⚡️ Cache latest op version when broadcasting presence #679

Merged
merged 2 commits into from
Aug 27, 2024
Merged

⚡️ Cache latest op version when broadcasting presence #679

merged 2 commits into from
Aug 27, 2024

Commits on Aug 27, 2024

  1. ⚡️ Cache latest op version when broadcasting presence 

    At the moment, when sending a presence update to other subscribers, we
[call `transformPresenceToLatestVersion()`][1] for every presence update
which internally [calls `getOps()`][2] for every presence update.

Calls to `getOps()` can be expensive, and rapid presence updates may
cause undue load on the server, even when the `Doc` has not been
updated.

This change tries to mitigate this by subscribing to a pubsub stream for
any `Doc`s that an `Agent` tries to broadcast presence on. We keep an
in-memory cache of the latest snapshot version sent over this stream,
which lets us quickly check if a presence broadcast is already current
without needing to query the database at all.

To avoid leaking streams, the `Agent` will internally handle its stream
subscription state by:

 - subscribing whenever a non-`null` presence update is broadcast
 - unsubscribing whenever a `null` presence update is broadcast

This means that rapid changes in presence being `null` or not can still
result in database calls, but even in this case they should be less bad
than before, because we only perform a snapshot fetch instead of ops.

[1]: https://github.com/share/sharedb/blob/297ce5dc66563a5955311793a475768d73ac8b87/lib/agent.js#L804
[2]: https://github.com/share/sharedb/blob/297ce5dc66563a5955311793a475768d73ac8b87/lib/backend.js#L919
    @alecgibson
    alecgibson committed Aug 27, 2024
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  2. Review markups 

    - Clean up `latestDocVersions` when unsubscribing
- Fix not setting version if the object already exists
- Removes `null` presence check, which won't work if the object has been
  destroyed
    @alecgibson
    alecgibson committed Aug 27, 2024
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