Cache invalidation strategies in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a cache expiration time in Express middleware.

Express

app.use((req, res, next) => {
  res.set('Cache-Control', 'public, max-age=[1]');
  next();
});

Drag options to blanks, or click blank then click option'

Aexpires

Bcache

C3600

Dno-cache

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cache' or 'expires' instead of a numeric value.

Setting 'no-cache' which disables caching instead of setting expiration.

The max-age directive sets the cache expiration time in seconds. Here, 3600 means the cache expires after one hour.

2fill in blank

medium

Complete the code to clear a specific cache key in an Express route.

Express

app.get('/clear-cache', (req, res) => {
  cache.[1]('user-data');
  res.send('Cache cleared');
});

Drag options to blanks, or click blank then click option'

Adelete

Bget

Cset

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' which only retrieves data.

Using 'clear' which removes all cache entries.

The delete method removes a specific key from the cache.

3fill in blank

hard

Fix the error in the cache middleware to properly invalidate cache after response.

Express

app.use((req, res, next) => {
  res.on('finish', () => {
    cache.[1](req.originalUrl);
  });
  next();
});

Drag options to blanks, or click blank then click option'

Adelete

Binvalidate

Cset

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' which adds or updates cache instead of removing.

Using 'invalidate' which is not a standard method.

The delete method is used to remove the cache entry for the requested URL after the response finishes.

4fill in blank

hard

Fill both blanks to create a cache middleware that skips caching for authenticated users.

Express

app.use((req, res, next) => {
  if (req.user && req.user.[1]) {
    res.set('Cache-Control', '[2]');
  }
  next();
});

Drag options to blanks, or click blank then click option'

AisAuthenticated

Bno-store

Cpublic

DisAdmin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'isAdmin' instead of 'isAuthenticated'.

Setting 'public' which allows caching instead of disabling it.

The middleware checks if the user is authenticated using isAuthenticated. If true, it sets Cache-Control to no-store to prevent caching sensitive data.

5fill in blank

hard

Fill all three blanks to implement a cache invalidation strategy that clears cache on data update.

Express

app.post('/update-data', (req, res) => {
  updateDatabase(req.body);
  cache.[1](req.body.[2]);
  res.status(200).send({ message: '[3]' });
});

Drag options to blanks, or click blank then click option'

Adelete

Bid

CData updated and cache cleared

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'delete' to remove cache.

Using wrong key like 'key' instead of 'id'.

Sending a generic message instead of a clear confirmation.

After updating the database, the cache entry identified by id is deleted using delete. Then a success message is sent.

Practice

1. What is the main purpose of cache invalidation in an Express app?

easy

A. To permanently store data without expiration

B. To increase server CPU usage

C. To keep cached data fresh and up-to-date

D. To disable caching completely

Solution

1. Step 1: Understand cache invalidation

   Cache invalidation means removing or updating cached data when it becomes outdated.

2. Step 2: Purpose in Express apps

   Express uses cache invalidation to ensure users get fresh data, not stale cached content.

3. Final Answer:

   To keep cached data fresh and up-to-date -> Option C

4. Quick Check:

   Cache invalidation = fresh data [OK]

Hint: Cache invalidation means keeping data fresh [OK]

Common Mistakes:

• Thinking cache invalidation disables caching
• Confusing cache invalidation with permanent storage
• Believing it increases CPU usage intentionally

2. Which of the following is the correct way to set a Cache-Control header in Express to prevent caching?

easy

A. res.set('Cache-Control', 'max-age=3600')

B. res.set('Cache-Control', 'no-store')

C. res.set('Cache-Control', 'public, max-age=86400')

D. res.set('Cache-Control', 'private, max-age=600')

Solution

1. Step 1: Understand Cache-Control values

   'no-store' tells browsers not to cache the response at all.

2. Step 2: Identify the option that prevents caching

   res.set('Cache-Control', 'no-store') uses 'no-store', which disables caching completely.

3. Final Answer:

   res.set('Cache-Control', 'no-store') -> Option B

4. Quick Check:

   Prevent caching = 'no-store' [OK]

Hint: Use 'no-store' to disable caching in Cache-Control [OK]

Common Mistakes:

• Using 'max-age' which allows caching
• Confusing 'private' with disabling cache
• Missing quotes around header value

3. Consider this Express middleware code snippet:

app.use((req, res, next) => {
  res.set('Cache-Control', 'public, max-age=10');
  next();
});

app.get('/data', (req, res) => {
  res.json({ time: Date.now() });
});

What will happen if a user requests /data twice within 10 seconds?

medium

A. The second request will return cached data from the first request

B. The second request will always fetch fresh data from the server

C. The server will throw an error on the second request

D. The cache will be cleared after the first request

Solution

1. Step 1: Analyze Cache-Control header

   'public, max-age=10' means cache is valid for 10 seconds.

2. Step 2: Effect on repeated requests

   Within 10 seconds, the browser or proxy can serve cached response without contacting server.

3. Final Answer:

   The second request will return cached data from the first request -> Option A

4. Quick Check:

   max-age=10 caches for 10 seconds [OK]

Hint: max-age sets cache duration in seconds [OK]

Common Mistakes:

• Assuming server always sends fresh data
• Confusing 'public' with disabling cache
• Thinking cache clears immediately after first request

4. You wrote this Express route to disable caching:

app.get('/info', (req, res) => {
  res.set('Cache-Control', 'no-cache');
  res.json({ message: 'Hello' });
});

But clients still see cached responses. What is the likely problem?

medium

A. Express does not support Cache-Control headers

B. The header name "Cache-Control" is misspelled

C. res.json() automatically overrides Cache-Control headers

D. Using "no-cache" allows caching but requires revalidation

Solution

1. Step 1: Understand "no-cache" meaning

   'no-cache' means caches can store response but must revalidate before reuse.

2. Step 2: Why clients see cached data

   If revalidation is skipped or proxy ignores it, cached data may be served.

3. Final Answer:

   Using "no-cache" allows caching but requires revalidation -> Option D

4. Quick Check:

   'no-cache' means revalidate before reuse [OK]

Hint: "no-cache" means revalidate, not disable caching [OK]

Common Mistakes:

• Thinking 'no-cache' disables caching completely
• Assuming header name is wrong without checking
• Believing res.json() removes headers

5. You want to cache user profile data in Express but ensure it updates immediately after a user changes their info. Which cache invalidation strategy is best?

hard

A. Set a long max-age and manually clear cache on user update

B. Use 'no-store' to never cache user profiles

C. Set a short max-age and rely on automatic expiration

D. Cache indefinitely and ignore updates

Solution

1. Step 1: Understand caching user profiles

   User profiles benefit from caching for speed but must update after changes.

2. Step 2: Choose invalidation strategy

   Long max-age improves performance; manual cache clearing on update ensures fresh data.

3. Final Answer:

   Set a long max-age and manually clear cache on user update -> Option A

4. Quick Check:

   Manual invalidation + long cache = fresh and fast [OK]

Hint: Use manual cache clear after updates for fresh user data [OK]

Common Mistakes:

• Using 'no-store' loses caching benefits
• Relying only on short max-age causes frequent reloads
• Ignoring cache invalidation leads to stale data