⚡️ Cache latest op version when broadcasting presence #679
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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
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ericyhwang
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Aug 27, 2024
| }; | ||
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| Agent.prototype._unsubscribeDocVersion = function(collection, id, callback) { | ||
| var stream = util.dig(this.latestDocVersionStreams, collection, id); |
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This doesn't clean up the this.latestDocVersions, which means a long-lived client that goes through many docs could have an agent with tons of old entries in that map.
Per discussion, immediately cleaning up this.latestDocVersions may make the case of multiple null presences worse. Perhaps a delayed cleanup or something.
- 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
ericyhwang
approved these changes
Aug 27, 2024
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At the moment, when sending a presence update to other subscribers, we call
transformPresenceToLatestVersion()for every presence update which internally callsgetOps()for every presence update.Calls to
getOps()can be expensive, and rapid presence updates may cause undue load on the server, even when theDochas not been updated.This change tries to mitigate this by subscribing to a pubsub stream for any
Docs that anAgenttries 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
Agentwill internally handle its stream subscription state by:nullpresence update is broadcastnullpresence update is broadcastThis means that rapid changes in presence being
nullor 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.