Let’s say Andy is on the Andy-instance, Beth is on the Beth-instance, and Charlie is on the Charlie-instance. The Charlie-instance has a community called Something-Cool, and all three people are subscribed to it. Beth makes a new post in Something-Cool. The “real” version of the post is on Charlie-instance because that’s where the community is, but the Lemmy software on Andy-instance and Beth-instance makes a copy of the post locally. Andy comments on the post, which happens on his local instance, but the software synchs up his comment with the “real” version of the post on Charlie-instance.

Now let’s say that Andy-instance defederates from Charlie-instance. That means that the copying and syncing no longer happen between those two instances. There’s already a copy of Beth’s post on Andy-instance, and that doesn’t get removed, but if anyone else comments on that post, Andy will never see it because his instance isn’t connected to the “real” version. He could make another comment on the post, because it’s still on his instance, but his comment won’t get synced to the “real” version, so it will never get synced to any other instances either, because they always take from the “real” version. Other people on Andy-instance will see Andy’s new comment because it’s on their home instance.

Let’s imagine there are 3 magic books, and what someone writes/draws/glues onto the pages of one of them appears in all 3 simultaneously. You can use them to share stories, articles, pictures, have conversations, etc. Those 3 books are federated.

If one of those books gets lost, stolen, damaged, etc. everything still exists in those other 2 books and they can keep talking to each other. and if/when the lost book is found or repaired, it will get backups of everything from the other 2 books.

Now let’s say the owner of books 1 and 3 have a falling out, and they break the spell federating their books together, but each of them remain federated with book 2.

Books 1 and 3 can no longer see what the other is writing in their book, but everything they already wrote is still there, and they can still see what 2 is doing, and for anyone using book 2 nothing much has changed, except that they no longer see 1 and 3 getting into arguments with each other in the pages.

The books are the different Lemmy instances, anything anything written, drawn, or glued onto the pages are all of the comments, posts, up/downvotes, etc. One of the books getting lost or damaged is when that server goes down for any of the various reasons servers go down.

And of course instead of magic, it’s the internet and programming and such.

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