Edge latency atlas

Every CDN status page tells you where the network is. None of them tell you what it actually costs you from where you are sitting. I wanted to see the shape of my own latency, so I measured it.

How it works

The browser opens a WebSocket to the nearest Worker, which upgrades and hands the connection to a Durable Object. That object pings a fixed list of 30 colo endpoints and streams results back as they arrive.

export class Probe extends DurableObject {
	async measure(targets: string[]) {
		const samples = await Promise.all(
			targets.map(async (target) => {
				const start = performance.now();
				await this.env.TARGET.fetch(target, { method: "HEAD" });
				return { target, rtt: performance.now() - start };
			}),
		);
 
		return samples.sort((a, b) => a.rtt - b.rtt);
	}
}

Results land in a plain array of { target, rtt } pairs. The canvas layer converts that into a contour field with a marching squares pass, which is the part I underestimated.

What I got wrong

I assumed the interesting variation would be geographic. It is not. The dominant term is peering, and two locations 400km apart can differ by a factor of four depending on which transit provider each one prefers that week.

The map is therefore somewhat honest and somewhat useless: it shows real numbers, but those numbers move enough that a single screenshot is misleading.

What I would change

Store a rolling seven-day median per colo instead of showing the instantaneous value. Right now the tool tells you something true and unactionable.