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How do you migrate a serverless app between edge platforms without downtime? Deploy and validate the destination while the current service still handles production, then move traffic in measured stages with monitoring and a tested rollback. That process can keep an application available, but no platform or procedure can guarantee uninterrupted service for every architecture.
The exact runbook depends on your framework and runtime, stateful services, DNS provider, domain setup, and the routing controls offered by both platforms. Treat the migration as a period of parallel operation rather than a single DNS switch.
What “without downtime” means in an edge migration
In this context, “without downtime” is an operational objective: users should continue receiving a correct response while the new deployment is introduced. The objective is supported by keeping the old service reachable, pre-validating the new one, shifting traffic gradually, watching service health, and retaining a reversible path.
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- Parallel operation: the existing and destination deployments remain available at the same time.
- Prevalidation: the destination is tested on its provider URL or a preview domain before production traffic is sent there.
- Staged routing: paths, users, regions, or percentages move in controlled increments.
- Observability: application and platform signals are compared with the old service’s baseline.
- Rollback: traffic can return to the old version without assuming that data or infrastructure changes will reverse themselves.
Cloudflare Workers documentation describes the risk directly: “Because gradual deployments mean multiple versions of your Worker serve traffic simultaneously, clients and services can end up interacting with more than one version in ways that produce errors or inconsistent behavior.” The statement appears in its Gradual deployments documentation, last updated July 3, 2026.
Decide whether the migration is feasible before scheduling it
Do not start with a DNS record. First establish whether the destination can reproduce the application’s behavior and whether the state can be shared safely during the transition.
Questions that determine the runbook
- Which framework, adapter, language runtime, and package versions are in use?
- Does the destination support the APIs, filesystem assumptions, streaming behavior, background execution, and maximum request duration your code requires?
- Which requests are stateless, and which depend on sessions, caches, queues, databases, object storage, or third-party identity systems?
- Can both deployments read and write the same data model during the overlap period?
- Who controls authoritative DNS, and can you add records without changing the registrar?
- Do you need the apex domain,
www, or both? Are redirects and certificates already defined for each? - What routing mechanism is available: path rewrites, a reverse proxy, percentage splits, version pinning, or only an all-at-once DNS change?
If an essential dependency cannot be shared or made backward-compatible, plan a maintenance window or an application-level transition instead of promising a no-downtime cutover.
1. Map the existing system
Create an inventory that another engineer can use to recreate the service. Record the current value, its owner, its destination equivalent, and a test that proves it works.
Application and build inventory
- Every public route, HTTP method, rewrite, redirect, response header, cookie, and cache policy.
- Framework version, runtime version, package manager lockfile, build command, output directory, and required Node, Python, Go, or other runtime APIs.
- Function entry points, request and response signatures, streaming or multipart behavior, maximum execution time, concurrency assumptions, and regional placement.
- Static assets, image processing, middleware, edge configuration, and client-side code that embeds an API origin.
- Scheduled jobs, webhooks, queues, retries, cron expressions, and any process that runs outside an incoming request.
Configuration and state inventory
- Environment variables, secrets, certificates, signing keys, and which values differ between preview and production.
- Databases, key-value stores, object storage buckets, search indexes, caches, rate-limit counters, and analytics sinks.
- Authentication providers, callback URLs, cookie domains, token issuers, session stores, and trusted origins.
- DNS records, custom domains, TLS certificates, apex and
wwwbehavior, email-related records, and third-party verification records. - Observability: logs, traces, metrics, alert rules, dashboards, synthetic checks, and the current baseline for latency and errors.
Mark each item as stateless, shared during migration, copied with a synchronization plan, or not portable. This classification exposes the work that source-code conversion alone cannot solve.
2. Prove the app on the destination
Deploy the destination before moving production traffic. Use the provider’s generated URL or an isolated preview domain so the old service remains untouched.
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- Reproduce the build. Translate the build command, output directory, install command, runtime version, and framework adapter. Save the exact build logs and artifact identifiers.
- Translate function conventions. A platform may expect a different directory, handler signature, event object, response format, or middleware entry point. Cloudflare’s migration documentation shows that function conventions can differ between platforms.
- Load configuration deliberately. Create every required environment variable and secret in the destination’s production context. Do not assume that a variable exported by the source platform exists automatically at the destination.
- Replace platform bindings. Connect databases, KV stores, object storage, queues, scheduled tasks, authentication callbacks, and logging integrations. Verify permissions with least-privilege credentials.
- Exercise representative traffic. Test anonymous and authenticated requests, cache misses and hits, uploads, redirects, error responses, webhooks, scheduled work, and the largest normal payloads.
- Compare behavior with the source. Capture status codes, headers, cookies, JSON shapes, HTML, cache directives, and latency for the same test cases. Differences that look cosmetic can break clients or search indexing.
Keep a written list of deviations. A destination deployment is not ready for traffic merely because it builds successfully.
3. Prepare the domain and TLS before changing DNS
Add the production domain to the destination and complete ownership validation and certificate provisioning while the old service remains authoritative.
- Enter the exact domain in the destination project, including separate entries when both the apex and
wwwhost are required. - Publish the ownership-verification record requested for your domain. For the Cloudflare DNS example in Vercel’s migration guidance, this is a TXT record that must be present before changing the target record.
- Wait for the destination to report successful verification and an issued certificate. Check that the certificate covers every hostname you will serve.
- Record the destination’s exact A or CNAME value. Do not copy a sample record from a tutorial; use the value shown for your actual project and domain.
- Confirm redirect rules, canonical host behavior, cookies, and callback URLs for apex and
wwwindependently.
DNS changes are not instantaneous. Vercel’s 2026 Knowledge Base guidance says Cloudflare domains generally use a five-minute TTL, recommends checking the actual record, and notes that global propagation can take up to 48 hours. Those figures describe that provider workflow, not a universal DNS SLA.
During propagation, some resolvers can still send users to the old service. Keep it healthy and compatible until authoritative DNS, major recursive resolvers, and your own synthetic checks show that the destination receives the intended traffic.
4. Choose a traffic-transition pattern
Select the narrowest transition that your architecture can operate safely. The following patterns are examples, not interchangeable features available on every pair of platforms.
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| Route-by-route rewrite | The existing front door remains in place and sends selected paths to the destination. | Different teams can validate independent routes or APIs. | Cross-route cookies, shared APIs, redirects, and asset URLs can cross versions unexpectedly. |
| Legacy reverse proxy | The old service proxies selected requests to the new deployment while retaining a fallback. | The source platform can proxy reliably and you need one public domain during testing. | Proxy headers, streaming, timeouts, client IP handling, and cache keys may differ. |
| Percentage split | A routing layer assigns a fraction of requests to each version. | The destination platform supports weighted or gradual deployments and the app tolerates mixed versions. | Consecutive requests from one user may land on different versions, causing session, asset, or API skew. |
| DNS cutover | The domain’s target record is changed from the old service to the destination. | Both platforms lack an application-layer traffic control, or the final switch is small and well tested. | Resolver caching makes timing unpredictable and does not provide per-request rollback. |
Route-by-route migration
Vercel documents an incremental approach using destination-side fallbacks and a legacy-server or reverse-proxy arrangement. Keep the old service in front, select a path whose dependencies are understood, and rewrite only that path after its destination tests pass. Leave a fast fallback for an origin failure. Increase the set of migrated paths only after checking shared authentication, redirects, client bundles, and API contracts.
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Percentage-based migration
Cloudflare Workers supports gradual deployments that split traffic between Worker versions. Start with a small share that can exercise real production conditions, then increase it according to your service objectives. Confirm whether the platform offers version affinity or overrides when a user must remain on one version.
Neither pattern removes the need to verify that the source and destination can share cookies, sessions, caches, queues, and data. If your routing layer cannot preserve those relationships, a split can create failures even when each version works alone.
5. Define monitoring and a ramp plan
Write the promotion and rollback rules before sending the first user to the destination. Use the source deployment as the comparison baseline rather than relying on a generic percentage or latency target.
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Signals to watch
- HTTP failure rates by route, status code, region, runtime, and version.
- Unhandled exceptions, rejected promises, worker exceptions, timeouts, and resource-limit events.
- Latency percentiles, queue age, database errors, cache hit ratio, and origin load.
- Authentication failures, session churn, missing assets, incorrect redirects, and client-side JavaScript errors.
- Business checks such as completed sign-ins, successful payments, message delivery, or document generation.
Promotion procedure
- Record a baseline window on the existing service that includes normal peaks and scheduled work.
- Send the smallest practical traffic share or one low-risk route to the destination.
- Observe long enough to cover cache behavior, asynchronous jobs, token expiry, and your normal request pattern.
- Compare destination signals with the baseline and inspect sampled requests, not only aggregate dashboards.
- Promote to the next share only when your predeclared error, latency, and correctness conditions remain within your service objectives.
- Pause or roll back on a material regression; do not continue a ramp to “see if it settles” without an explanation.
Cloudflare specifically advises monitoring errors and exceptions during gradual deployments. Its documentation does not prescribe a universal threshold or ramp schedule; derive yours from the application’s normal variance and accepted error budget.
6. Prevent version skew during parallel service
Mixed-version traffic is a compatibility problem, not just a routing problem. A single browser can make requests to different versions during one page view.
Sessions and authentication
- Use a shared session store or stateless tokens that both versions can validate.
- Keep signing keys, cookie names, domains, paths, and expiration rules compatible.
- Ensure login callbacks and logout requests reach a version that understands the issued session.
- Use platform affinity or a version override when a workflow requires a user to remain on one version.
Client assets
- Serve immutable, content-addressed bundles so an HTML response from one version does not request files removed by another.
- Retain old asset paths for the entire browser-cache lifetime and DNS transition.
- Check service-worker scope and cache invalidation; a service worker can keep directing requests to old code after routing changes.
APIs and asynchronous work
- Use additive API changes first. Keep old fields and response shapes until every client has moved.
- Make queue consumers tolerate messages produced by both versions, and prevent duplicate processing with idempotency keys.
- Version webhooks and verify that retries can be handled by either deployment.
7. Make data and rollback safe
A code rollback does not rewind a database migration, object upload, queue message, secret rotation, or infrastructure binding. Design the data path so the old version can continue operating if traffic returns to it.
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Use an expand-and-contract data plan
- Expand: add new columns, indexes, tables, or fields without removing or changing the old representation.
- Dual-read or dual-write: deploy code that can read both forms and, where needed, write both during the overlap.
- Migrate: backfill or transform data with a resumable, observable job.
- Switch: move traffic only after both versions have passed compatibility tests.
- Contract: remove the old representation only after rollback is no longer required.
Keep the old platform’s credentials, bindings, queues, buckets, and database permissions until the rollback window closes. Test a real recovery: route traffic back, process a request, consume a queued message, and verify that sessions and writes remain correct.
Cloudflare’s rollback documentation states that 100 recent versions are available for rollback. That version-history limit applies to the documented Workers feature; it does not restore external data or guarantee that an older version can use a changed resource.
8. Complete the cutover and retire the old service
Retire components only after the destination has handled the intended workload and your checks show correct behavior across every required hostname.
- Move the remaining routes or increase the traffic share to the destination according to the approved plan.
- Verify the apex and
wwwhosts separately when both are part of the public site. - Test redirects, canonical URLs, cookies, authentication callbacks, webhooks, scheduled jobs, and static assets from external networks.
- Keep dashboards and alerts split by version so a late regression is attributable.
- Freeze destructive changes on the old platform until DNS caches have aged and the rollback decision point has passed.
- Remove obsolete rewrites, proxy rules, secrets, bindings, and resources in a documented sequence. Preserve logs and deployment artifacts for incident analysis.
Do not delete the old service merely because the destination’s health check is green. A health check proves only the paths it exercises; delayed jobs, infrequent webhooks, and cached clients can surface problems later.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Platform comparison: what to evaluate
No edge platform is universally best for migration. Compare the source and destination against the workload you actually operate.
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|---|---|---|
| Runtime and framework | Are the language runtime, framework adapter, package APIs, streaming model, limits, and build output compatible? | A successful production-equivalent build and route-level behavior tests. |
| State and service portability | Can databases, KV, object storage, queues, schedules, auth, and secrets be shared or migrated without breaking the old version? | Dependency map, access tests, data compatibility plan, and restore test. |
| Traffic controls | Can you rewrite paths, proxy to the other origin, split percentages, pin a user to a version, or fall back quickly? | Documented routing configuration and a tested failure switch. |
| Observability | Can you separate logs, exceptions, latency, and business outcomes by deployment version? | Dashboards, alerts, trace correlation, and synthetic checks. |
| Rollback | How quickly can traffic return, how many versions are retained, and what external changes remain after code rollback? | Rollback drill covering code, data, queues, secrets, and bindings. |
| Domain and DNS workflow | How are ownership, certificates, apex and www, redirects, A records, and CNAMEs handled? |
Verified certificate, exact project-specific records, and resolver checks. |
Vercel’s migration guidance emphasizes runtime, environment-variable, domain, DNS, and incremental-routing concerns. Cloudflare’s migration and deployment documentation emphasizes function-convention differences, gradual traffic, version behavior, and rollback. Treat both as platform-specific examples rather than a vendor-neutral ranking.
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Common failure modes and recovery actions
The destination returns 404s or wrong handlers
Compare the source route map with the destination’s function directories, rewrites, trailing-slash rules, and method handling. Test the generated provider URL directly before involving DNS. Keep the route on the old service until every method and redirect behaves correctly.
Users are logged out during the split
Inspect cookie domain and path, signing keys, session storage, clock tolerance, and callback URLs. If the versions cannot validate the same session, use version affinity or keep the authenticated route on one version until compatibility is deployed.
New JavaScript loads with old HTML, or the reverse
Check cache headers, asset retention, service-worker caches, and whether filenames are immutable. Restore old asset paths and invalidate only with a tested strategy; do not remove files while either version can still emit references to them.
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Rollback fixes code but writes fail
Compare the rollback version’s schema and resource permissions with the current state. Re-enable compatible bindings, use the backward-compatible schema, and stop destructive migrations. A traffic rollback cannot undo data already written.
DNS appears inconsistent
Query authoritative nameservers and several recursive resolvers with dig. Confirm that the record value matches the destination project and that the old service remains healthy while caches expire. Do not repeatedly change records to chase individual resolver results.
Gradual deployment shows intermittent errors
Correlate each failed request with its version, region, session, asset set, and downstream dependency. Intermittence often indicates mixed-version state, not random platform failure. Pause the ramp, preserve evidence, and route affected workflows to the known-good version while you correct compatibility.
Quick Recap
Operational checklist
- System map lists routes, runtimes, builds, variables, secrets, bindings, jobs, domains, and state.
- Destination build and route tests pass on a provider or preview URL.
- Environment variables, certificates, callbacks, and managed-service permissions are verified.
- Domain ownership and TLS are complete before the production target changes.
- Routing method, fallback, version affinity, and rollback owner are documented.
- Session, asset, API, queue, and schema compatibility are tested with both versions live.
- Dashboards, synthetic checks, promotion criteria, and stop conditions are ready.
- Rollback is exercised with real traffic and does not depend on reversing data changes.
- Apex and
wwwbehavior, redirects, webhooks, and scheduled tasks are checked after cutover. - Old resources remain available until DNS caches, delayed jobs, and the agreed rollback window have passed.
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