Lemmy

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How Subscribing Helps Federate (lemmy.directory)

submitted 1 year ago* (last edited 1 year ago) by kiwi@lemmy.directory to c/lemmy@lemmy.ml

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I recently setup a new Lemmy instance and was surprised when my feed was mostly empty. I've since learned that a key part of Lemmy's federation is based on a user from your instance subscribing to communities on other instances. Only then, will your instance pull in posts from the subscribed community to your "All" feed.

This means that subscribing to new communities is especially important if you're on a young Lemmy instance since it helps to build out everyone's feed on that instance.

I've found discovering new communities to subscribe to on other instances can be difficult. To help me search for new communities I may be interested in, I tried aggregating as much of the Lemmy fediverse together into a single feed by subscribing to the widest range of Lemmy communities possible. This offers a Lemmy feed that's kind of like reddit.com/r/all. If it's interesting to anyone else, you can find the instance here: https://lemmy.directory.

Hopefully this offers another way to find new communities to subscribe to on other instances.

Here's a better description of my understanding on how Lemmy federates communities and why you might be interested in checking out lemmy.directory: https://lemmy.directory/post/34207.

Hope this helps ease the orientation to how Lemmy federation and communities work.

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[–] PriorProject@lemmy.world 1 points 1 year ago* (last edited 1 year ago)

So based on your answer, I am still misunderstanding something. What is the purpose of all the duplication then? Is it just for local caching purposes?

Pretty much everything in your summary was wrong. I can't reasonably type out a complete primer on federation here, but in short... When the first user on an instance subscribes to a remote community, the subscribing server tells the community-hosting server "send me future updates about posts, comments, and votes and such for this community". The subscribing server then stores them locally.

Why do this actual replication rather than just an API gateway?

That's what federation is. I don't mean to be glib, but there's more than one way to potentially do this stuff, and some systems designers think federation is a useful approach for a variety of complex reasons. Federation involves federated replication, API proxies aren't federation.
If the community-hosting server goes down temporarily, content replicated via federation remains available on the subscribing nodes. This enabled me to read communities homed on lemmy.ml recently when it was struggling and frequently down due to overload.
It spreads the browse workload, which is generally bigger than the replication workload. For example, lemmy.world has commonly has like 4k active users this week. But it only takes one batch of federated replication messages for the community's instance to serve the browse traffic of all those users... then the reads come out of lemmy.world's db. This spreads the browse workload around the federated network.
There are many other reasons to work this way as well, but as noted... at some point you'll just want to Google primers on federated apps/networks. They definitely don't operate according to your current intuition though, and the differences aren't accidental.

Can federated networks have problems where the replication traffic becomes "too much"? Yes, they must be carefully designed to avoid that problem. And for some apps, it makes single-user instances sometimes anti-efficient as the federation workload for the instance can exceed the browse workload from the user, but for multi-user instances the federation messages are a rounding error compared to the browse work. But it's a problem that federated networks generally weather, and the replication offers benefits that on-balance outweigh the costs.