An Interactive Intro to CRDTs

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

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 which allows distributing a worker pool over multiple nodes, it's backed by DeltaCrdt.

Delta-CRDTs are an optimization over state based CRDTs where you share state diffs instead of the whole state (described in this paper).