Cache invalidation strategies in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Cache Invalidation Types

Which cache invalidation strategy immediately removes cached data when the underlying data changes?

A Time-based expiration where cache clears after a set time

B Lazy invalidation where cache is cleared only on next access

C Manual invalidation triggered by data update events

D Cache warming that preloads data before expiration

💡 Hint

Think about which method reacts instantly to data changes.

Manual invalidation triggers cache removal right when data changes, ensuring fresh data immediately.

❓ component_behavior

intermediate

Cache Behavior in Express Middleware

Given an Express middleware that caches responses with a TTL of 10 seconds, what happens if a client requests the same resource twice within 5 seconds?

Express

app.use((req, res, next) => {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      cache[key] = body;
      setTimeout(() => { delete cache[key]; }, 10000);
      res.sendResponse(body);
    };
    next();
  }
});

A The cache never expires because setTimeout is not cleared

B The second request triggers next() and fetches fresh data

C The cache clears immediately after the first request

D The second request returns cached data instantly without calling next()

💡 Hint

Consider what happens when cache[key] exists.

If cache[key] exists, the middleware sends cached data directly and does not call next(), so no fresh data fetch occurs.

🔧 Debug

advanced

Identifying Cache Invalidation Bug

Why does this Express cache invalidation code fail to clear the cache after data update?

Express

app.post('/update', (req, res) => {
  updateData(req.body);
  delete cache[req.originalUrl];
  res.send('Updated');
});

A Cache key uses req.originalUrl which differs from cached GET request URL

B delete operator cannot remove properties from cache object

C updateData is asynchronous but cache deletion runs before update completes

D res.send should be called before deleting cache

💡 Hint

Check if the cache key matches the cached data key.

The POST request URL differs from the GET request URL used as cache key, so deleting cache[req.originalUrl] does not remove the cached GET data.

📝 Syntax

advanced

Correct Cache Invalidation Syntax

Which option correctly invalidates a cache entry for a given key in an Express app using a simple object cache?

Express

const cache = {};

function invalidateCache(key) {
  // Invalidate cache here
}

A cache.remove(key);

B delete cache[key];

C cache[key] = null;

D cache.pop(key);

💡 Hint

Think about how to remove a property from an object in JavaScript.

The delete operator removes a property from an object. Other options are invalid or only nullify the value.

❓ state_output

expert

Cache State After Multiple Requests

Consider this Express cache middleware with manual invalidation. After these requests, what keys remain in the cache object? 1. GET /data (cache miss, stores response) 2. POST /data (updates data, invalidates cache) 3. GET /data (cache miss, stores new response) 4. GET /info (cache miss, stores response)

Express

const cache = {};

app.use((req, res, next) => {
  if (req.method === 'GET') {
    if (cache[req.originalUrl]) {
      res.send(cache[req.originalUrl]);
    } else {
      res.sendResponse = res.send;
      res.send = (body) => {
        cache[req.originalUrl] = body;
        res.sendResponse(body);
      };
      next();
    }
  } else if (req.method === 'POST') {
    updateData(req.body);
    delete cache['/data'];
    res.send('Updated');
  } else {
    next();
  }
});

A ['/data', '/info']

B ['/data']

C ['/info']

D []

💡 Hint

Remember the POST request deletes '/data' cache key.

After POST /data, cache['/data'] is deleted. Then GET /data stores new response again. Then GET /info stores response. So keys after all requests are '/data' and '/info'.