this post was submitted on 20 Jun 2025
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So CAP theorem says you can have a distributed system with at most two of Consistent, Available, or Partition tolerant. I haven't looked too closely into the federation implementation of Mastodon but I suspect they opted for Available and Partition tolerant (as Consistent and Partition tolerant would mean the entire network goes down when one node does, while Consistent and Available would mean once any node lost contact with the network it could never again rejoin). Since consistency is not guaranteed (and provably can't be) there is absolutely no way to guarantee that deleting something from one instance will remove it from all instances even allowing for a very generous time span.
TL;DR: You're not just right, you're mathematically right.
Mastodon's federation is not at all consistent even when it could get much closer with a little effort.
Servers don't remote fetch old posts from recent follows for example, nor replies to off-server posts from people on a third server. There's work being done on both, but I'm surprised it wasn't prioritized much earlier. Some other Fediverse software handles these situations better.
Mathematically right, the best kind of right.