this post was submitted on 27 Sep 2024
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Lemmy federation protocol: How is the "posts from all communities" view sourced? (feddit.org)
submitted 1 month ago by 7EP6vuI@feddit.org to c/programming@programming.dev
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I'm a newbie to ActivityPub so please be patient with me.

All intros into ActivityPub speak about how a user of a server A subscribes to a specific community from server B, and then server A will be informed about changes in that community.

But on lemmy it's possible to look at the posts of all communities. For a single concrete community it would be relatively easy: server A gets the request to serve the top post of a community on server B, so A simple asks B for the posts.

But there is also the "posts from all communities" tab on the lemmy front page. This opens questions:

Does each lemmy instance has a full copy of all posts of all communities? If this is true: How are new Instances discovered? Is each Instance distributing all updates to all other Instances?

If each lemmy instance has only a partial dataset (this theory is backed by [1] "Only if a least one user on your instance subscribes to the remote community, will the community send updates to your instance.") then how is the "all posts" view composed? is it in reality not "all" but only "all posts that at least one user of this instance is subscribed to"?

If this is the case: what happens if a bad actor subscribes to all communities of all servers? Is there a maximum number of subscriptions per user?

The source of those questions is, that I'm looking for a way to subscribe to all events of all lemmy instances, to be able to build statistics about upvotes, new posts, comments etc. There seems to be a similar API endpoint for mastadon [2] but nothing for lemmy?!

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[–] andrew_s@piefed.social 4 points 1 month ago

Yeah. There's no wildcard call. One thing you could do to script it would be pull JSONs from https://data.lemmyverse.net - use one for the initial effort, then subsequent ones to track new communities. You'd definitely want to filter it - as you've noticed the vast majority of that 30k are dead or spam or something you wouldn't want for one reason or another (e.g. communities from instances you've defederated from).

As for what bots do, it depends on how they were programmed I suppose. There's a bonkers one on https://leaf.dance that just seems to crawl comments and subscribe to any ! links it finds, but there are others (I can't remember their names) where it's more of a manual job (the mods of a community submit the details to it).