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Why do comments differ across instances? (feddit.uk)

submitted 1 year ago by [email protected] to c/[email protected]

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If I visit a community from Feddit and then another instance, the comments often differ. I understand there are some issues with defederation between some instances, but this doesn't seem related to that.

For example, here's the post-race discussion on the Formula 1 community on Feddit: https://feddit.uk/post/320484

And then the same discussion on lemmy.world: https://lemmy.world/post/950479

I don't understand why a significant number of comments aren't visible on Feddit.

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[–] [email protected] 3 points 1 year ago* (last edited 1 year ago)

The async doesn't mean if the sending server doesn't know. It's not using UDP lol :) It will retry until it gets a RC 200 or 201. Then stop sending. The asynchronous part is that the receiving instance doesn't have to import the event into the DB immediately. The received event sits in a special "to import" queue until processed. Each event has a timestamp attached to it. If it takes too long to process the event in the queue, because the receiving server is overloaded, then its timestamp may be considered expired.

E.g. from https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/

Mind that there is only a ±30 seconds time window when that signature would be considered valid, which is a big reason why it’s quite difficult to do manually.