Say you decide to self-host a Lemmy instance. When you create that instance, do you immediately need to download and store all the data that has ever been posted to all federated Lemmy instances? Or perhaps you only need to download and store everything that is posted to the federated Lemmy instances from that point forward? Or better yet, do you only store what the users on that instance do (i.e. their posts, and posts to the communities hosted on that instance)?
My understanding is that it will pull all existing text content (posts/links and their comments) but NOT media (images/video); those are always hosted from the originating instance.
It will only pull/cache the subscribed community, not the entire instance.
It will also pull in new posts and comments as they are created from that point forward, and interactions that occur on your instance will be relayed back to the originating instance as long as your instance remains federated with it.
Also, IIRC, federation is a two-way street. If an instance defederates from yours, it will no longer accept anything from your instance; but you can still see it. Likewise, if you instance defederates from the other, yours won’t pull new content from it at all. Federation, unlike the “cache once subscribed” described above, includes all communities that originate on that instance.
If users interact with a community that originates on a defederated instance from your instance, the original instance will NOT see those changes. I’m not clear on if they will re-sync if re-federation occurs at a later time.