An Interactive Intro to CRDTs (2023)

I've spent a lot of time studying CRDTs & order theory in the last year & will publish an article too. Local-first apps are not easy to build, complex data structures (say, calendar repetitions with exceptions) become harder to model. Everything must converge automatically while clients can branch off.

In general, you don't really get to compact tombstones meaningfully without consensus so you really are pushing at least remnants of the entire log around to each client indefinitely. You also can't easily upgrade your db you're stuck looking after legacy data structures indefinitely - or you have to impose arbitrary cut off points.

List CRDTs - which text CRDTs are built from are probably unavoidable except for really really simple applications. Over the last 15 years they have evolved, roughly: WOOT (paper) -> RGA (paper) -> YATA (paper) / YJS (js + later rust port) -> Peritext (paper) / Automerge (rust/js/swift) -> Loro (Rust/js). Cola (rust) is another recent one. The big libs (yjs, automerge, loro) offer full doc models.

Mostly the later ones improve on space, time & intent capture (not interleaving concurrent requests).

The same few guys (Martin Kleppman, Kevin Jahns, Joseph Gentle, probably others) pop up all over the more recent optimisations.

Last-Write-Win CRDTs are nice, but I wish the article talked about where CRDT really shine, which is when the state truly converge in a non-destructive way, for example:

1) Counters

While not really useful, they demonstrate this well:

  - mutations are +n and -n
  - their order do not matter
  - converging the state is a matter of applying the operations of remote peers locally

2) Append-only data structures

Useful for accounting, or replication of time-series/logs with no master/slave relationship between nodes (where writes would be accepted only on a "master" node).

  - the only mutation is "append"
  - converging the state is applying the peers operations then sorting by timestamp

EDIT: add more

3) Multi Value registers (and maps)

Similar to Last-Write-Win registers (and maps), but all writes are kept, the value becomes a set of concurrent values.

4) Many more...

Each is useful for specific use cases. And since not everybody is making collaborative tools, but many are working on distributed systems, I think it's worth it to mention this.

On another note, the article talks about state based CRDTs, where you need to share the whole state. In the examples I gave above, they are operation based CRDTs, where you need to share the operations done on the state and recompute it when needed.

For example, in the Elixir ecosystem, we have Horde ( https://hexdocs.pm/horde/readme.html ) which allows distributing a worker pool over multiple nodes, it's backed by DeltaCrdt ( https://hexdocs.pm/delta_crdt/DeltaCrdt.html ).

Delta-CRDTs are an optimization over state based CRDTs where you share state diffs instead of the whole state (described in this paper: https://arxiv.org/pdf/1603.01529 ).

My absolute favorite kind of blog post and the same structure/style I use.

Also a really well written piece.

I haven't dug into this deeply, but to me CRDTs look like a P2P data structure abstracted to the programming language variable level. The article says they shine when you don't want a central server. But most communication libraries already handle authentication and multiple peers well — and if you designate one peer as canonical (via leader election), conflict resolution is solved. I'm curious what use cases make avoiding a central server worth the paradigm shift. That said, it's a choice of approach — some may prefer the CRDT paradig

Previously...

An interactive intro to CRDTs - https://news.ycombinator.com/item?id=37764581 - Oct 2023 (130 comments)

Enjoyed the article! As someone who's worked on bits of collaborative software (and is currently helping build one at work), I'd caution people from using CRDTs in general. A central server is the right tool for the job in most cases; there are certain things that are difficult to handle with CRDTs, like permissions and acquiring locks on objects.

Edit: I had an excerpt here which I completely misread. Sorry.

I tried vibe-coding a CRDT app last year in the pre-Sonnet 4.5 era (based partly on this write-up), and it really struggled with timing issues.

It would be interesting to try again now.