Smart Placement Probe

Demonstrate Workers Smart Placement by fetching an origin URL and reporting latency alongside the request colo. wrangler.json sets "placement": { "mode": "smart" } so Cloudflare can place the Worker closer to back-end origins based on traffic patterns.

API ReferenceCopy Anchor Link

GET /Copy Anchor Link

Explains Smart Placement and returns app metadata.

GET /probeCopy Anchor Link

Fetch a URL and report status, latency, and colo.

url string (required)

http:// or https:// target.

Example RequestCopy Anchor Link

curl "https://your-worker.workers.dev/probe?url=https://example.com"

Success ResponseCopy Anchor Link

{
  "url": "https://example.com/",
  "status": 200,
  "latencyMs": 42,
  "cf": { "colo": "SJC" },
  "workerPlacement": "Smart Placement may move this Worker closer to back-end origins based on request patterns."
}

cf.colo comes from the incoming request.cf when available (empty object in local dev).

Error CodesCopy Anchor Link

• 400 - Missing or non-http(s) url (INVALID_URL)

Use CasesCopy Anchor Link

• Learn Smart Placement configuration and behavior
• Measure origin latency from the serving colo
• Compare placement effects after repeated traffic to distant origins
• Debug whether a Worker is running near your back-end

LimitationsCopy Anchor Link

• Smart Placement effects are most visible after deploy with repeated traffic
• Single PoP / colo per request - not a multi-region map
• cf.colo may be empty in local wrangler dev
• No control plane API to force a specific colo

Use in your projectCopy Anchor Link

import { FETCH_TIMEOUT_MS, SMART_PLACEMENT_HINT } from "../constants/defaults";import type { ProbeResult } from "../types/probe";export async function probeUrl(url: string, colo?: string): Promise<ProbeResult> {  const start = Date.now();  const controller = new AbortController();  const timeoutId = setTimeout(() => controller.abort(), FETCH_TIMEOUT_MS);  try {    const res = await fetch(url, {      method: "GET",      signal: controller.signal,      headers: { "User-Agent": "Cloudflare-Experiments-SmartPlacementProbe/1.0" },    });    clearTimeout(timeoutId);    return {      url,      status: res.status,      latencyMs: Date.now() - start,      cf: colo ? { colo } : {},      workerPlacement: SMART_PLACEMENT_HINT,    };  } catch (e) {    clearTimeout(timeoutId);    const message = e instanceof Error ? e.message : "Fetch failed";    return {      url,      status: 0,      latencyMs: Date.now() - start,      cf: colo ? { colo } : {},      workerPlacement: SMART_PLACEMENT_HINT,      error: message,    };  }}export function getRequestColo(request: Request): string | undefined {  const cf = (request as Request & { cf?: { colo?: string } }).cf;  return typeof cf?.colo === "string" ? cf.colo : undefined;}

DeploymentCopy Anchor Link

curl "https://your-worker.workers.dev/probe?url=https://example.com"

Local DevelopmentCopy Anchor Link

cd apps/experiments/smart-placement-probe
npm install
npm run dev

curl "http://localhost:8787/probe?url=https://example.com"

Cloudflare Features UsedCopy Anchor Link

• Workers - Edge compute runtime
• Smart Placement - Place Workers closer to back-end origins