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.