CRDT Survey, Part 1: Introduction

Matthew Weidner | Sep 26th, 2023
Home | RSS Feed
Keywords: CRDTs, collaborative apps

This blog post is Part 1 of a series.

What is a CRDT?

Suppose you’re implementing a collaborative app. You’ve heard that Conflict-free Replicated Data Types (CRDTs) are a good fit and you want to know more about them.

If you look up the definition of CRDT, you will find that there are two main kinds, “op-based” and “state-based”, and these are defined using mathematical terms like “commutativity”, “semilattice”, etc. This is probably already more complicated than your mental model of a collaborative app, and I imagine it can be intimidating.

Let’s step back a bit and think about what you’re trying to accomplish.

In a collaborative app, users expect to see their own operations immediately, without waiting for a round-trip to a central server. This is especially true in a local-first app, where users can make edits even when they are offline, or when there is no central server.

Immediate local edits make it possible for users to perform operations concurrently: logically simultaneously, with no agreement on the order of operations. Those users will temporarily see different states. Eventually, though, they will synchronize with each other, combining their operations.

At that point, the collaborative app must decide: What is the state resulting from these concurrent operations? Because the operations were logically simultaneous, we shouldn’t just pretend that they happened in some sequential order. Instead, we need to combine them in a way that matches users’ expectations.

Example: Ingredient List

Here is a simple example. The app is a collaborative recipe editor (Collabs demo), which includes an ingredient list:

A list of ingredients: "Head of broccoli", "Oil", "Salt".

Suppose that:

  1. One user deletes the first ingredient (“Head of broccoli”).
  2. Concurrently, a second user edits “Oil” to read “Olive Oil”.

We could try broadcasting the non-collaborative version of these operations: Delete ingredient 0; Prepend "Olive " to ingredient 1. But if another user applies those operations literally in that order, they’ll end up with “Olive Salt”:

The wrong result: "Oil", "Olive Salt".

Instead, you need to interpret those operations in a concurrency-aware way: Prepend 'Olive' applies to the “Oil” ingredient regardless of its current index.

The intended result: "Olive Oil", "Salt".

Semantics

In the above example, the users’ intended outcome was obvious. You can probably also anticipate how to implement this behavior: identify each ingredient by a unique ID instead of its index.

Other situations can be more interesting. For example, starting from the last recipe above, suppose the two users perform two more operations:

  1. The first user increases the amount of salt to 3 mL.
  2. Concurrently, the second user clicks a “Halve the recipe” button, which halves all amounts.

We’d like to preserve both users’ edits. Also, since it’s a recipe, the ratio of amounts is more important than their absolute values. Thus you should aim for the following result:

An ingredients list starts with 15 mL Olive Oil and 2 mL Salt. One user edits the amount of Salt to 3 mL. Concurrently, another user halves the recipe (7.5 mL Olive Oil, 1 mL Salt). The final state is: 7.5 mL Olive Oil, 1.5 mL Salt.

In general, a collaborative app’s semantics are an abstract description of what the app’s state should be, given the operations that users have performed. This state must be consistent across users: two users who are aware of the same operations must be in the same state.

Choosing an app’s semantics is more difficult than it sounds, because they must be well-defined in arbitrarily complex situations. For example, if a third user performs a bunch of operations concurrently to the four operations above, the collaborative recipe editor must still end up in some consistent state - hopefully one that makes sense to users.

Algorithms

Once you’ve chosen your collaborative app’s semantics, you need to implement them, using algorithms on each user device.

For example, suppose train conductors at different doors of a train count the number of passengers boarding. When a conductor sees a passenger board through their door, they increment the collaborative count. Increments don’t always synchronize immediately due to flaky internet; this is fine as long as all conductors eventually converge to the correct total count.

The app’s semantics are obvious: its state should be the number of increment operations so far, regardless of concurrency. Specifically, an individual conductor’s app will display the number of increment operations that it is aware of.

Op-Based CRDTs

Here is one algorithm that implements these semantics:

Top user performs inc(), changing their state from 0 to 1. They broadcast a "+1" message to other users. Upon receipt, those two users each change their states from 0 to 1.

It is assumed that when a user broadcasts a message, it is eventually received by all collaborators, without duplication (i.e., exactly-once). For example, each user could send their messages to a server; the server stores these messages and forwards them to other users, who filter out duplicates.

Algorithms in this style are called op-based CRDTs. They work even if the network has unbounded delays or delivers messages in different orders to different users. Indeed, the above algorithm matches our chosen semantics even under those conditions.

State-Based CRDTs

Op-based CRDTs can also be used in peer-to-peer networks without any servers. However, usually each user needs to store a complete message history, in case another user requests old messages. (It is a good idea to let users forward each other’s messages, not just their own.) This has a high storage cost: O(total count) in our example app.

A state-based CRDT is a different kind of algorithm that sometimes reduces this storage cost. It consists of:

(Set of +1s mapping to "Current state") plus (set of +1s mapping to "Other state") becomes (set of +1s mapping to "Merged state"). Each state has a box with color-coded +1 ops: Current state has orange, red, green; Other state has orange, red, purple, blue, yellow; Merged state has orange, red, green, purple, blue, yellow.

We’ll see an optimized state-based counter CRDT later in this blog series. But briefly, instead of storing the complete message history, you store a map from each device to the number of inc() operations performed by that device. This encodes the operation history in a way that permits merging (entrywise max), but has storage cost O(# devices) instead of O(total count).

Unless you need state-based merging, the counting app doesn’t really require specialized knowledge - it just counts in the obvious way. But you can still talk about the “op-based counter CRDT” if you want to sound fancy.

Defining CRDTs

One way to define a CRDT is as “either an op-based CRDT or a state-based CRDT”. In practice, we can be more flexible. For example, CRDT libraries usually implement hybrid op-based/state-based CRDTs, which let you use both op-based messaging and state-based merging in the same app.

I like this broader, informal definition: A CRDT is a distributed algorithm that computes a collaborative app’s state from its operation history. This state must depend only on the operations that users have performed: it must be the same for all users that are aware of the same operations, and it must be computable without extra coordination or help from a single source-of-truth. (Message forwarding servers are okay, but you’re not allowed to put a central server in charge of the state like in a traditional web app.)

Of course, you can also use CRDT techniques even when you do have a central server. For example, a collaborative text editor could use a CRDT to manage the text, but a server DB to manage permissions.

Outline of this Survey

In this blog series, I will survey CRDT techniques for collaborative apps. My goal is to demystify and summarize the techniques I know about, so that you can learn them too without a PhD’s worth of effort.

The techniques are divided into two “topics”, corresponding to the sections above:

Part 2 (the next post) covers semantic techniques; I’ll cover algorithmic techniques in Part 3.

Since I’m the lead developer for the Collabs CRDT library, I’ll also mention where various techniques appear in Collabs. You can find a summary in the Collabs docs.

This survey is opinionated: I omit or advise against techniques that I believe don’t translate well to collaborative apps. (CRDTs are also used in distributed data stores, which have different requirements.) I also can’t hope to mention every CRDT paper or topic area; for that, see crdt.tech. If you believe that I’ve treated a technique unfairly - including your own! - please feel free to contact me: mweidner037@gmail.com

I thank Martin Kleppmann, Jake Teton-Landis, and Florian Jacob for feedback on portions of this survey. Any mistakes or bad ideas are my own.

Sources

I’ll cite specific references in-line, but here are some sources of general inspiration.

Next Post - Part 2: Semantics

This blog post is Part 1 of a series.


Home • Matthew Weidner • PhD student at CMU CSD • mweidner037@gmail.com • @MatthewWeidner3LinkedInGitHub