Expiration-based caching in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this cache expiration check?

Consider a REST API cache that stores data with an expiration timestamp. Given the following code snippet, what will be the output?

Rest API

import time

cache = {'data': 'value', 'expires_at': time.time() + 2}

# Wait 3 seconds to simulate delay
time.sleep(3)

if time.time() > cache['expires_at']:
    print('Cache expired')
else:
    print('Cache valid')

A TypeError

B Cache expired

C KeyError

D Cache valid

💡 Hint

Think about how the expiration time compares to the current time after the delay.

The cache expiration time is set 2 seconds ahead. After sleeping 3 seconds, the current time is greater than the expiration time, so the cache is expired.

🧠 Conceptual

intermediate

Which HTTP header is commonly used to control cache expiration?

In REST APIs, which HTTP header tells clients how long to keep a cached response before requesting it again?

A Cache-Control

B Authorization

C Content-Type

D User-Agent

💡 Hint

It controls caching policies like max-age and no-cache.

The Cache-Control header specifies caching directives including expiration times for responses.

🔧 Debug

advanced

Why does this cache never expire?

Look at this cache expiration code snippet. Why does the cache never expire?

Rest API

import time

cache = {'data': 'value', 'expires_at': time.time() + 5}

# Check expiration
if time.time() < cache['expires_at']:
    print('Cache expired')
else:
    print('Cache valid')

A Because the comparison operator is reversed, so it prints 'Cache expired' when cache is still valid

B Because the expiration time is set in the past

C Because time.time() returns a string, causing a TypeError

D Because the cache dictionary is missing the 'data' key

💡 Hint

Check the logic of the if condition comparing current time and expiration time.

The condition uses < instead of >, so it treats valid cache as expired and vice versa, causing wrong output.

📝 Syntax

advanced

Which option correctly sets a cache expiration time 10 seconds from now?

Choose the correct Python code to set a cache expiration timestamp 10 seconds in the future.

A cache['expires_at'] = time.now() + 10

B cache['expires_at'] = time.time() - 10

C cache['expires_at'] = time.time() + 10

D cache['expires_at'] = time.time + 10

💡 Hint

Remember the correct function to get current time in seconds is time.time()

Option C correctly calls time.time() and adds 10 seconds. Option C sets expiration in the past. Option C uses a non-existent function time.now(). Option C uses time.time without parentheses, which is a function object, not a number.

🚀 Application

expert

How many items remain in cache after expiration cleanup?

Given this cache dictionary with expiration timestamps, how many items remain after removing expired entries?

Rest API

import time

cache = {
    'a': {'value': 1, 'expires_at': time.time() + 5},
    'b': {'value': 2, 'expires_at': time.time() - 1},
    'c': {'value': 3, 'expires_at': time.time() + 10},
    'd': {'value': 4, 'expires_at': time.time() - 10}
}

# Remove expired
now = time.time()
cache = {k: v for k, v in cache.items() if v['expires_at'] > now}

print(len(cache))

A 1

B 3

C 4

D 2

💡 Hint

Check which items have expiration times greater than current time.

Only keys 'a' and 'c' have expiration times in the future, so 2 items remain after cleanup.

Practice

1. What does the Cache-Control: max-age header do in expiration-based caching?

easy

A. It deletes the cache immediately after the response.

B. It disables caching for the resource.

C. It sets how long the cached data is considered fresh.

D. It forces the client to always fetch new data.

Solution

1. Step 1: Understand the role of Cache-Control header

   The Cache-Control header controls caching behavior for HTTP responses.

2. Step 2: Identify the meaning of max-age directive

   The max-age directive specifies the time in seconds that the response is considered fresh and can be served from cache.

3. Final Answer:

   It sets how long the cached data is considered fresh. -> Option C

4. Quick Check:

   max-age = cache freshness duration [OK]

Hint: Remember max-age sets cache freshness time in seconds [OK]

Common Mistakes:

• Confusing max-age with disabling cache
• Thinking max-age deletes cache immediately
• Assuming max-age forces fresh fetch every time

2. Which of the following is the correct syntax to set a cache expiration of 300 seconds in an HTTP response header?

easy

A. Cache-Control: maxage: 300

B. Cache-Control: max-age=300

C. Cache-Control: expire=300

D. Cache-Control: timeout=300

Solution

1. Step 1: Recall correct Cache-Control syntax

   The Cache-Control header uses directives separated by commas, with max-age specified as max-age=seconds.

2. Step 2: Compare options to correct syntax

   Only Cache-Control: max-age=300 uses the correct format: max-age=300.

3. Final Answer:

   Cache-Control: max-age=300 -> Option B

4. Quick Check:

   Correct header syntax = max-age=seconds [OK]

Hint: Use equal sign (=) with max-age, no spaces [OK]

Common Mistakes:

• Using colon instead of equal sign
• Using incorrect directive names like expire or timeout
• Adding spaces inside the directive

3. Given this HTTP response header:
Cache-Control: max-age=10
What will happen if a client requests the same resource 5 seconds later?

medium

A. The client will use the cached response without contacting the server.

B. The client will ignore the cache and fetch new data from the server.

C. The client will receive a 404 error.

D. The client will refresh the cache but still use old data.

Solution

1. Step 1: Understand max-age meaning

   max-age=10 means the cached data is fresh for 10 seconds after the first fetch.

2. Step 2: Check time elapsed since caching

   5 seconds is less than 10 seconds, so cache is still fresh and valid.

3. Final Answer:

   The client will use the cached response without contacting the server. -> Option A

4. Quick Check:

   Elapsed time < max-age = use cache [OK]

Hint: If request time < max-age, cache is used [OK]

Common Mistakes:

• Assuming cache expires immediately
• Thinking client always fetches new data
• Confusing max-age with cache invalidation

4. You set Cache-Control: max-age=60 but clients still fetch new data every time. What is the likely mistake?

medium

A. The client does not support caching headers.

B. The max-age value should be in milliseconds, not seconds.

C. The resource URL is missing a file extension.

D. The server is sending no-cache header overriding max-age.

Solution

1. Step 1: Check for conflicting headers

   If the server sends a no-cache directive, it forces clients to revalidate or fetch fresh data despite max-age.

2. Step 2: Understand max-age units and client support

   max-age is in seconds, and most clients support caching headers, so these are unlikely causes.

3. Final Answer:

   The server is sending no-cache header overriding max-age. -> Option D

4. Quick Check:

   no-cache overrides max-age [OK]

Hint: Check for no-cache header overriding max-age [OK]

Common Mistakes:

• Confusing seconds with milliseconds for max-age
• Blaming client support without checking headers
• Assuming URL format affects caching

5. You want to cache API responses for 5 minutes but ensure clients always get fresh data after expiration. Which header setup is best?

hard

A. Cache-Control: max-age=300, must-revalidate

B. Cache-Control: max-age=300, no-store

C. Cache-Control: no-cache, max-age=300

D. Cache-Control: max-age=0, public

Solution

1. Step 1: Understand caching for 5 minutes

   max-age=300 sets cache freshness for 300 seconds (5 minutes).

2. Step 2: Ensure fresh data after expiration

   must-revalidate forces clients to check with server after max-age expires before using cached data.

3. Step 3: Evaluate other options

   no-store disables caching, no-cache forces revalidation every time, max-age=0 means no caching.

4. Final Answer:

   Cache-Control: max-age=300, must-revalidate -> Option A

5. Quick Check:

   max-age + must-revalidate = cache then fresh after expiry [OK]

Hint: Use must-revalidate with max-age for fresh data after expiry [OK]

Common Mistakes:

• Using no-store disables caching completely
• Using no-cache forces revalidation every request
• Setting max-age=0 disables caching