This is a demo app written on Cloudflare Workers utilizing Durable Objects to implement real-time chat with stored history. This app runs 100% on Cloudflare's edge.
Try it here: https://edge-chat-demo.cloudflareworkers.com
The reason this demo is remarkable is because it deals with state. Before Durable Objects, Workers were stateless, and state had to be stored elsewhere. State can mean storage, but it also means the ability to coordinate. In a chat room, when one user sends a message, the app must somehow route that message to other users, via connections that those other users already had open. These connections are state, and coordinating them in a stateless framework is hard if not impossible.
This chat app uses a Durable Object to control each chat room. Users connect to the object using WebSockets. Messages from one user are broadcast to all the other users. The chat history is also stored in durable storage, but this is only for history. Real-time messages are relayed directly from one user to others without going through the storage layer.
Additionally, this demo uses Durable Objects for a second purpose: Applying a rate limit to messages from any particular IP. Each IP is assigned a Durable Object that tracks recent request frequency, so that users who send too many messages can be temporarily blocked -- even across multiple chat rooms. Interestingly, these objects don't actually store any durable state at all, because they only care about very recent history, and it's not a big deal if a rate limiter randomly resets on occasion. So, these rate limiter objects are an example of a pure coordination object with no storage.
This chat app is only a few hundred lines of code. The deployment configuration is only a few lines. Yet, it will scale seamlessly to any number of chat rooms, limited only by Cloudflare's available resources. Of course, any individual chat room's scalability has a limit, since each object is single-threaded. But, that limit is far beyond what a human participant could keep up with anyway.
For more details, take a look at the code! It is well-commented.
If you are in the Durable Objects beta, you can deploy this demo using Wrangler, the official Workers CLI. At the time of writing, Durable Object support in Wrangler is not yet available in a full release build, so you need to install a release candidate instead. See the release notes for installation instructions and more information.
After installing it, run wrangler login to connect it to your Cloudflare account.
Once you have Wrangler installed and authenticated, you can deploy the app for the first time by adding your Cloudflare account ID (which can be viewed by running wrangler whoami) to the wrangler.toml file and then running:
wrangler publish --new-class ChatRoom --new-class RateLimiter
If you get an error about the --new-class flag not being recognized, you need to update your version of Wrangler.
This command will deploy the app to your account under the name edge-chat-demo. The --new-class flags tell Cloudflare that you want the ChatRoom and RateLimiter classes to be callable as Durable Objects. The flags should be omitted on subsequent uploads of the same Worker because at that point the classes are already configured as Durable Objects.
This demo code does not have any dependencies, aside from Cloudflare Workers (for the server side, chat.mjs) and a modern web browser (for the client side, chat.html). Deploying the code requires Wrangler.