by crdrost | 2025-06-19 01:43:23+00:00
So, there is a reason that CRDT researchers would not like this response that you have given, but down-thread from you it's not why the author jakelazaroff didn't like it, but it's worth giving this answer too.

The reason CRDT researchers don't like the sync server is, that's the very thing that CRDTs are meant to solve. CRDTs are a building-block for theoretically-correct eventual consistency: that's the goal. Which means our one source-of-truth now exists in N replicas, those replicas are getting updated separately, and now: why choose eventual consistency rather than strong consistency? You always want strong consistency if you can get it, but eventually, the cost of syncing the replicas is too high.

So now we have a sync server like you planned? Well, if we're at the scale where CRDTs make sense then presumably we have data races. Let's assume Alice and Bob both read from the sync server and it's a (synchronous, unencrypted!) last-write-wins register, both Alice and Bob pull down "v1" and Alice writes "v1a" to the register and Bob in parallel writes "v1b" as Alice disconnects and Bob wins because he happens to have the higher user-ID. Sync server acknowledged Alice's write but it got lost until she next comes online. OK so new solution, we need a compare-and-swap register, we need Bob to try to write to the server and get rejected. Well, except in the contention regime that we're anticipating, this means that we're running your sync server as a single-point-of-failure strong consistency node, and we're accepting the occasional loss of availability (CAP theorem) when we can't reach the server.

Even worse, such a sync server _forces_ you into strong consistency even if you're like "well the replicas can lose connection to the sync server and I'll still let them do stuff, I'll just put up a warning sign that says they're not synced yet." Why? Because they use the sync server as if it is one monolithic thing, but under contention we have to internally scale the sync server to contain multiple replicas so that we can survive crashes etc. ... if the stuff happening inside the sync server is not linearizable (aka strongly consistent) then external systems cannot pretend it is one monolithic thing!

So it's like, the sync server is basically a sort of GitHub, right? It's operating at a massive scale and so internally it presumably needs to have many Git-clones of the data so that if the primary replica goes down then we can still serve your repo to you and merge a pull request and whatever else. But then it absolutely sucks to merge a PR and find out that afterwards, it's not merged, so you go into panic mode and try to fix things, only for 5 minutes later to discover that the PR is now merged. And if you've got a really active eventually consistent CRDT system that has a lot of buggy potential.

For the CRDT researcher the idea of "we'll solve this all with a sync server" is a misunderstanding that takes you out of eventual-consistency-land. The CRDT equivalent that lacks this misunderstanding is, "a quorum of nodes will always remain online (or at least will eventually sync up) to make sure that everything eventually gets shared," and your "sync server" is actually just another replica that happens to remain online, but isn't doing anything fundamentally different from any of the other peers in the swarm.