The CRDT Dictionary: A Field Guide to Conflict-Free Replicated Data Types

Summary

Back around 2014, I kept hearing about this cool database called Riak, a distributed database that could survive network partitions and keep accepting writes. Some really interesting companies were using it at massive scale, and I was curious about it. One of the big selling points was that it could handle concurrent writes without any coordination or consensus. I was intrigued, and I started reading about it. Underlying all of this was the concept of CRDTs, Conflict-free Replicated Data Types.1 At the time, I was working on a beer startup called Brewtown with a friend: a beer review social site and delivery subscription service. It failed for other reasons, but I was a little too enamored with shiny tech back then, and CRDTs and Riak fit the bill for shiny tech. I kept trying to find excuses to shoehorn CRDT stuff into our codebase when, honestly, we didn’t need any of it. Postgres would’ve been fine. Live and learn. Anyways, the idea sounded like pure sorcery: data structures that replicate across nodes and merge deterministically, without coordination, without losing information. I got excited, read a few papers, played with some toy implementations… and then we gave up on the beer startup. I didn’t really have a reason to mess with CRDTs for a while. Fast forward to 2025, I’ve just had Thanksgiving dinner, and I’m curious again. What’s the state of the art? What have I forgotten? Which CRDT should I reach for when? So I’m writing this, both as a refresher for myself and a reference for the next time I need to remember why OR-Sets exist or what WOOT stands for. (“WithOut Operational Transformation.” Yes, really.2) So, grab a coffee. Commutative. Replicated. Data Types. VIDEO In isolation, all of the words make sense. But when you look at the literature, it’s overwhelming: Suddenly you’ve moved beyond the simple terms and start seeing things like G-Counters, PN-Counters, LWW-Sets, OR-Sets, 2P-Sets, RGAs, WOOTs, Logoots (wtf?)… Each with subtle tradeoffs. Each pape...