Runtime differences and limits

• Cloudflare Workers (V8 isolate): typical max wall time 30 s (paid); response body limit 100 MB; subrequest guidance: keep fan-out low (<10) where possible. V8 runtime receives periodic updates; use compatibility dates/flags to opt into behavior changes (see Compatibility dates below).
• Storage: KV, R2, D1, Durable Objects (SQLite-backed Durable Objects are GA and provide larger persistent options — monitor SQLite storage usage to avoid unexpected billing).

Notes and hard signals:

• Gen 13 servers + FL2 stack (Cloudflare, Mar 23, 2026): Cloudflare shipped Gen 13 servers and a new Rust-based FL2 request stack to optimize for core density and increase edge compute throughput (https://blog.cloudflare.com/gen13-launch/). Test your Workers under higher concurrency assumptions and measure tail latency; workloads sensitive to per-core cache locality should be validated against Gen 13.
• SQLite-backed Durable Objects billing (Dec 12, 2025 announcement; billing target in Jan 2026): Storage billing for SQLite-backed Durable Objects was enabled with a target date in January 2026. Monitor Durable Objects’ SQLite storage usage in the dashboard and follow pricing/limits in the Durable Objects docs (https://developers.cloudflare.com/changelog/post/2025-12-12-durable-objects-sqlite-storage-billing/).
• Compatibility dates & flags: Cloudflare uses a compatibility date to gate backwards-incompatible runtime fixes. Set and occasionally update the compatibility_date in Wrangler or the dashboard and test thoroughly before deploying (https://developers.cloudflare.com/workers/configuration/compatibility-dates/).

Request lifecycle

1. DNS resolves to nearest edge PoP
2. TLS terminates at the edge
3. Edge function executes in-region isolate
4. Function can respond directly or fetch from origin
5. Response cached at edge if cache headers allow

Quickstart workflow

1. Scaffold

npm create cloudflare@latest my-worker -- --template=javascript
cd my-worker

2. wrangler.toml (set compatibility_date)

name = "my-worker"
main = "src/index.ts"
compatibility_date = "2026-04-01" # set to the current date for access to latest runtime features; test before upgrading

3. Minimal worker handler

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const url = new URL(request.url);
    const country = request.cf?.country ?? "US";

    // Example KV read
    const cached = await env.CACHE_KV.get(url.pathname);
    if (cached) return new Response(cached, { headers: { "X-Cache": "HIT" } });

    const origin = await fetch("https://api.example.com" + url.pathname);
    const body = await origin.text();
    await env.CACHE_KV.put(url.pathname, body, { expirationTtl: 300 });

    return new Response(body, { headers: { "X-Cache": "MISS" } });
  }
};

4. Deploy

npx wrangler deploy

Durable Objects & SQLite-backed DOs (operational guidance)

• SQLite-backed Durable Objects are GA and provide a transactional local DB per object. They are useful for coordination, leader election, materialized views, and small datasets with complex queries.
• Billing: SQLite storage billing was announced (Dec 12, 2025) and enabled in January 2026. Free-tier accounts are not charged; paid plans should monitor storage usage and optimize or delete unused data ahead of billing windows (https://developers.cloudflare.com/changelog/post/2025-12-12-durable-objects-sqlite-storage-billing/).
• Size & limits: refer to the Durable Objects pricing and limits page. For large blobs, prefer R2; for SQL workloads that need global availability, consider D1 or external managed DBs.
• Migration tips: to move from in-memory patterns to SQLite-backed DOs, add a migration path that exports current state to the new DO and validate under load; measure request latency and compute billing (DO compute and storage charges are separate).

Testing & performance guidance (Gen 13/FL2 specifically)

• Assume higher per-PoP concurrency — run load tests that increase concurrent requests per PoP and measure tail latency (p95/p99).
• If your workload previously relied on L3 cache locality, benchmark on Gen 13 hardware assumptions: FL2 was designed to remove cache dependence; still validate hot-path latency.
• Watch CPU-bound tasks: Gen 13 increases available cores, which may make previously rejected concurrency patterns feasible — but billing and per-invocation limits still apply.
• Subrequests and fan-out: keep subrequests modest; more aggressive fan-out under higher concurrency increases the chance of running into subrequest or connection limits.

Migration and compatibility notes

• Compatibility dates: update compatibility_date in your wrangler.toml to opt into safe runtime fixes, and use compatibility flags for controlled testing (https://developers.cloudflare.com/workers/configuration/compatibility-dates/).
• V8 upgrades: Cloudflare updates V8 periodically. If you depend on specific JS engine behavior, lock compatibility_date and run an automated test suite when updating.
• Durable Objects storage billing: if you adopted SQLite-backed DOs during beta, review usage and cost under the new billing model; archive or compact old data where possible.

Examples

JWT validation, A/B testing, and other code examples remain unchanged — see the existing handler and examples in the Worker scaffold above.

Decision tree (short)

Need to run code on every request?

• Yes → Is the logic lightweight (<50ms CPU)? If yes, Edge function. If no, serverless/origin.
• No → Static asset or CDN cache.

Which edge platform?

• Need KV, R2, Durable Objects, D1 → Cloudflare Workers
• Next.js on Vercel → Vercel Edge Functions (Fluid Compute for concurrency)
• Deno-native codebase → Deno Deploy

Gotchas & operational checklist

• Set compatibility_date and test on a staging environment before updating in production.
• Monitor Durable Objects SQLite storage usage in the dashboard; optimize/evict data before billing windows.
• Re-run load tests with Gen 13 assumptions: higher core counts, different cache characteristics.
• Use workers' changelog and compatibility flags to stay informed about runtime changes.

References

• Gen 13 servers & FL2 (Cloudflare Blog, Mar 23, 2026): https://blog.cloudflare.com/gen13-launch/
• Durable Objects SQLite storage billing (Changelog, Dec 12, 2025): https://developers.cloudflare.com/changelog/post/2025-12-12-durable-objects-sqlite-storage-billing/
• Compatibility dates (Cloudflare Workers docs): https://developers.cloudflare.com/workers/configuration/compatibility-dates/