Cache invalidation strategies in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What happens when you use stale-while-revalidate in Next.js caching?

In Next.js, when you use the stale-while-revalidate cache control header, what is the expected behavior when a cached page is requested after the stale time has passed?

A The cached page is discarded and a 404 error is returned until the fresh page is generated.

B The request waits until the fresh page is fetched before serving any content to the user.

C The cached page is served immediately while Next.js fetches a fresh version in the background to update the cache.

D The cache is cleared and the user is redirected to the homepage.

💡 Hint

Think about how stale-while-revalidate balances speed and freshness.

The stale-while-revalidate strategy serves the stale cached content immediately to the user, ensuring fast response times. Meanwhile, it triggers a background fetch to update the cache with fresh content for future requests.

❓ component_behavior

intermediate

How does Next.js ISR (Incremental Static Regeneration) handle cache invalidation?

Given a Next.js page using ISR with revalidate: 10, what happens when a user requests the page after 15 seconds since the last generation?

NextJS

export async function getStaticProps() {
  return {
    props: { time: Date.now() },
    revalidate: 10
  };
}

A Next.js serves the cached page immediately and triggers a background regeneration of the page.

B Next.js waits for the page to regenerate before serving it to the user.

C Next.js serves a 404 error because the cache expired.

D Next.js disables caching and regenerates the page on every request.

💡 Hint

ISR allows pages to update without rebuilding the whole site.

ISR serves the existing cached page immediately even if the revalidate time has passed. It then regenerates the page in the background so future requests get fresh content.

📝 Syntax

advanced

Which Next.js cache control header syntax correctly sets a 1-hour max-age with stale-while-revalidate?

Choose the correct way to set the Cache-Control header in Next.js API route to cache for 1 hour and allow stale content while revalidating.

NextJS

export default function handler(req, res) {
  // Set cache control header here
  res.status(200).json({ message: 'Hello' });
}

A res.setHeader('Cache-Control', 'public max-age=3600 stale-while-revalidate=3600');

B res.setHeader('Cache-Control', 'max-age=3600 stale-while-revalidate=3600');

C res.setHeader('Cache-Control', 'public, max-age=3600; stale-while-revalidate=3600');

D res.setHeader('Cache-Control', 'public, max-age=3600, stale-while-revalidate=3600');

💡 Hint

Headers use commas to separate directives.

The correct syntax separates directives with commas. Option D correctly uses commas between public, max-age, and stale-while-revalidate.

🔧 Debug

advanced

Why does this Next.js ISR page never update after deployment?

This Next.js page uses ISR with revalidate: 60. After deployment, the page content never changes even after 5 minutes. What is the most likely cause?

NextJS

export async function getStaticProps() {
  const data = await fetch('https://api.example.com/data').then(res => res.json());
  return {
    props: { data },
    revalidate: 60
  };
}

A The <code>revalidate</code> value must be a string, not a number.

B The API response is cached and does not change, so the page content stays the same.

C ISR only works in development mode, not in production.

D The <code>getStaticProps</code> function must return <code>revalidate</code> inside <code>props</code>.

💡 Hint

Consider what controls the data freshness besides ISR timing.

If the API data does not change or is cached upstream, ISR will regenerate the page but the content remains the same. The issue is likely with the API data source, not ISR itself.

❓ state_output

expert

What is the output of this Next.js API route with cache headers and multiple requests?

Consider this Next.js API route that sets cache headers and returns a timestamp. What will be the output of two requests made 5 seconds apart?

NextJS

let lastGenerated = 0;

export default function handler(req, res) {
  const now = Date.now();
  if (now - lastGenerated > 10000) {
    lastGenerated = now;
  }
  res.setHeader('Cache-Control', 'public, max-age=10');
  res.status(200).json({ timestamp: lastGenerated });
}

A Both requests return the same timestamp because the cache max-age is 10 seconds.

B The second request returns 0 because the cache expired and lastGenerated reset.

C Both requests return different timestamps because the handler updates on every call.

D The first request returns 0, the second returns a new timestamp 5 seconds later.

💡 Hint

Think about how max-age controls browser and CDN caching behavior.

The max-age=10 header tells caches to keep the response for 10 seconds. Since the second request is within 5 seconds, it receives the cached response with the same timestamp.