Braid Polyfills for CDNs

These polyfills give the power of Braid-HTTP subscriptions to CDNs:

• Cloudflare
• Fastly planned
• Bunny.net planned

This provides two major performance improvements:

1. Regular pages get updates pushed to the cache in 0.5 RTT, rather than the existing 1.5 RTT that it takes to:
   • Tell the CDN to purge a cache entry (in 0.5 RTT)
   • Have a client try to GET it again
   • And then (in 1 RTT) have the CDN go to the origin to fetch the new value
2. Dynamic pages that use a WebSocket or SSE today (bypassing the CDN) can instead fan out through the CDN, and cache updates there
   • First load is way faster, because the page doesn't have to wait on a WebSocket to open to the origin and fetch the data after the page has loaded.
   • The subscriptions can fan out through the CDN to N clients, requiring only 1 subscription to the origin server

Presented at Braid Meeting 144.

How to use

• Add the polyfill to your CDN (see below)
• Now you can put it in front of any origin server that speaks Braid-HTTP, and it will become a distributed synchronized cache that fans out subscriptions to clients, keeping everyone up to date with minimal load on your server.

Implementation Status

Thus far, we've implemented a polyfill for Cloudflare. It adds a Cloudflare edge Worker that reads the Braid headers and routes requests to an internal Durable Object, which maintains a persistent subscription upstream to the Braid-HTTP origin.

Cloudflare Instructions

You need a Cloudflare account on the Workers Paid plan (the Free plan's CPU limit ends a streaming subscription within a minute) and an origin that speaks Braid-HTTP, such as a node server using braidify. Then:

1. cd cloudflare && npm install && npx wrangler login
2. In wrangler.toml, set ORIGIN to your origin's URL.
3. npx wrangler deploy

Your site is now served through the worker's workers.dev address, or through a custom domain you add to the worker in the dashboard. Every answer carries a Cache-Status header saying whether it came from a live copy at the edge, from the regional object, or from the origin. The settings, the cost, and npm test are described in cloudflare/readme.md.

Demo app: Fake News

The ./fake-news/ directory holds a demo Braid-HTTP app: a fictional news site whose front page is pushed to every open reader as soon as an editor changes it.

To install and run:

cd fake-news
npm install
node server.js 8080

Open http://localhost:8080/ to see the front page. Go to /admin to publish and unpublish stories, and watch the front page update live.

To try it through the polyfill, run Fake News where Cloudflare can reach it and set ORIGIN in cloudflare/wrangler.toml to its URL. The resources it serves, the same page over a websocket for comparison, and the settings that make it purge a plain CDN on every edit are described in fake-news/readme.md.