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Sumeet Shroff Freelancer
Sumeet Shroff Freelancer

Posted on • Originally published at prateeksha.com

Next.js 15 Upgrade Playbook: App Router, Caching Pitfalls, and Safe Migration Steps

#nextjs #tutorial #performance #react

Quick summary

  • What changed: App Router behavior, caching defaults, middleware/header propagation, and image handling.
  • Risks: stale content, broken auth, and hydration errors from server/client boundary changes.
  • Goals: audit, migrate incrementally, verify caching and headers, and canary deploy with observability.

Why upgrade now?

Next.js 15 brings tightened App Router semantics, updated caching defaults, and runtime adjustments that can improve performance and security. Those same changes can also alter behavior in subtle ways — which is why the upgrade should be planned and tested, not rushed.

What changed in Next.js 15 (at a glance)

  • App Router boundaries are stricter: server vs client responsibilities are enforced earlier.
  • Caching model refined: more aggressive edge caching in some configs and finer fetch-level controls.
  • Middleware and headers: propagation and rewrite behavior changed in ways that can affect auth and proxies.
  • Assets and images: loader integrations and cache headers got updates that affect delivery.

Core technical differences (quick reference)

  • Server Components: calling browser-only APIs in server components now breaks sooner.
  • getServerSideProps: migration toward App Router patterns continues; consider server components + fetch.
  • Edge runtime: environment differences (no process/fs) require verification for edge-deployed code.

Pages Router vs App Router — quick comparison

  • Data fetching
    • Pages: getStaticProps / getServerSideProps
    • App: server components, fetch with cache control
  • File layout
    • Pages: pages/
    • App: app/ with nested layouts and templates
  • Streaming
    • Pages: limited
    • App: built-in streaming and partial rendering
  • Middleware
    • Pages: older APIs
    • App: updated header and rewrite behavior
  • Caching
    • Pages: page-level declarations
    • App: fetch-level cache controls and edge-oriented defaults

Common breaking issues and symptoms

  • Missing or stale content: default cache modes or edge rewrites not forwarding headers.
  • Hydration errors: browser APIs used in server components or mismatched component types.
  • Auth failures: cookies or authorization headers dropped by middleware or rewrites.
  • Over-caching: immutable or aggressive caching preventing content updates.

Recommended upgrade flow (safe and staged)

  1. Inventory and audit
    • List routes using getServerSideProps, getInitialProps, direct cookie reads, or dynamic headers.
    • Identify custom middleware, rewrites, and any proxy logic.
  2. Read migration docs and test locally
    • Follow official changelogs and migration notes for Next.js 15.
    • Build a branch environment and fix compile-time errors first.
  3. Convert incrementally
    • Migrate a few non-critical routes to app/ rather than converting the entire site.
    • Replace server-side props patterns with server components + fetch where practical.
  4. Verify caching and runtime behavior
    • Exercise fetch cache modes (no-store, revalidate, default) and inspect Cache-Control headers.
    • Test middleware header propagation and rewrites in staging.
  5. Canary deploy and monitor
    • Deploy to a subset of traffic, run E2E and smoke tests, and watch metrics.
  6. Prepare rollback and runbooks
    • Keep a rollback path and dashboards for errors, SSR latencies, and cache hit/miss rates.

Tip: Use feature flags or route-level toggles to limit the blast radius while migrating.

Testing and verification checklist

  • Unit & integration tests focused on cookie/header flows and API interactions.
  • End-to-end tests for login, personalization, and session refresh flows.
  • Load tests for SSR and edge functions to measure CPU and latency.
  • Cache verification: ensure Cache-Control values match expectations for static and dynamic routes.
  • Observability: track error rates, SSR time, and cache metrics for the first 48 hours post-rollout.

Helpful tools and references:

  • Google Search Central for SEO and crawlability.
  • Lighthouse for performance and accessibility checks.
  • MDN for web API behavior.
  • OWASP for header and cookie security guidance.

Real-world scenarios and fixes

Scenario: stale landing page after enabling edge caching

  • Cause: immutable cache policy served promotional content for hours.
  • Fix: add revalidate keys or switch to revalidate-based caching for marketing routes.

Scenario: login breaks after middleware migration

  • Cause: rewrites stopped forwarding set-cookie/authorization headers.
  • Fix: explicitly forward required headers in middleware and validate token refresh flow in staging.

Scenario: hydration mismatch from client-only code used in server component

  • Cause: library importing window.localStorage inside what became a server component.
  • Fix: move the code to a client component and add unit tests to prevent regressions.

Troubleshooting quick tips

  • Hydration errors: scan stack traces for "Hydration" and check for window/document usage.
  • Missing cookies: log request headers at middleware entry to confirm forwarding.
  • Stale responses: inspect Cache-Control and CDN/edge caching behavior with curl or a network tab.

Tip: Add CI assertions that validate Cache-Control headers on representative endpoints.

How Prateeksha Web Design approaches production upgrades

Prateeksha uses a phased approach: audit, convert low-risk routes first, add CI checks for headers and cache, canary deploy, and monitor closely. Runbooks and rollback plans are prepared before any production rollout.

Key verification commands and snippets

  • Local build and checks: npm run build && npx next build
  • Inspect headers: curl -I https://staging.example.com/path
  • Simulate edge behavior: run a staging environment that mirrors CDN/edge settings

Warning: local dev server behavior can differ from edge runtimes and CDNs — always validate in staging.

Key takeaways

  • Audit routes and middleware before upgrading — App Router and caching defaults are stricter.
  • Test header and cookie propagation; middleware rewrites can break auth flows.
  • Migrate incrementally and canary deploy to reduce risk.
  • Verify Cache-Control in staging and monitor cache hit/miss after rollout.
  • Prepare a rollback plan and observe the first 48 hours closely.

Conclusion

Upgrading to Next.js 15 offers performance and security benefits but introduces behavioral changes that can affect caching, headers, and component boundaries. Follow a staged plan: inventory, incremental migration, thorough testing, canary deployment, and observability. If you need hands-on help, document a rollback plan and monitor key metrics immediately after rollout.

Ready to plan an upgrade or run an audit? Contact Prateeksha for migration guidance and runbook help.

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Canonical: https://prateeksha.com/blog/nextjs-15-upgrade-guide-app-router-caching-migration

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