Cache invalidation strategies in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this cache invalidation simulation?

Consider a REST API cache that stores user data. The cache invalidation is triggered when a user updates their profile. What will be the output of the following Python simulation code?

Rest API

cache = {'user_1': 'John Doe', 'user_2': 'Jane Smith'}

# User 1 updates profile
cache['user_1'] = 'John A. Doe'

# Invalidate cache for user_2
cache.pop('user_2', None)

print(cache)

A {'user_1': 'John A. Doe', 'user_2': 'Jane Smith'}

B {'user_1': 'John Doe', 'user_2': 'Jane Smith'}

C {'user_1': 'John A. Doe'}

D {'user_2': 'Jane Smith'}

💡 Hint

Think about what happens when you update a cache entry and then remove another.

The cache updates the value for 'user_1' to 'John A. Doe'. Then it removes 'user_2' from the cache. So only 'user_1' remains with the updated value.

🧠 Conceptual

intermediate

Which cache invalidation strategy is demonstrated here?

A REST API cache is set to expire entries automatically after 10 minutes. Which cache invalidation strategy does this represent?

A Time-to-live (TTL) expiration

B Write-through caching

C Manual invalidation

D Cache aside

💡 Hint

Think about automatic expiration based on time.

TTL expiration means cache entries are invalidated automatically after a set time period.

🔧 Debug

advanced

Why does this cache invalidation code fail to remove the stale entry?

Look at the code below that tries to invalidate a cache entry after updating the database. Why does the stale cache entry remain?

Rest API

cache = {'item_1': 'old_value'}

def update_database():
    # Simulate database update
    return 'new_value'

new_val = update_database()

# Later code tries to remove stale cache but fails
if 'item_1' in cache:
    pass  # supposed to remove stale cache but does nothing

print(cache)

A The cache invalidation code does not remove the entry; it only checks existence.

B The cache dictionary is immutable, so assignment fails.

C The cache entry is overwritten, so no stale entry remains.

D The update_database function returns null, so cache is not updated.

💡 Hint

Look carefully at the 'if' block that is supposed to remove the cache entry.

The 'if' block only checks if 'item_1' is in cache but does not remove it. So the stale entry remains.

📝 Syntax

advanced

Which option causes a runtime error in cache invalidation code?

Identify the option that will cause a runtime error when trying to invalidate a cache entry in Python.

Rest API

cache = {'key1': 'value1', 'key2': 'value2'}

A cache.pop('key1')

B del cache['key2']

C cache['key1'] = None

D cache.remove('key1')

💡 Hint

Check which method is not valid for Python dictionaries.

Dictionaries do not have a 'remove' method. Using cache.remove('key1') causes an AttributeError.

🚀 Application

expert

How many cache entries remain after this sequence of invalidations?

Given the following cache invalidation sequence, how many entries remain in the cache?

Rest API

cache = {'a': 1, 'b': 2, 'c': 3, 'd': 4}

# Invalidate entries where value is even
cache = {k: v for k, v in cache.items() if v % 2 != 0}

# Add new entry
cache['e'] = 5

# Remove entry 'a'
cache.pop('a', None)

print(len(cache))

A 1

B 2

C 4

D 3

💡 Hint

Count entries after filtering, adding, and removing.

After removing even values (b:2, d:4), cache has 'a':1, 'c':3 (2 entries). Then 'e':5 added (3 entries). Removing 'a' leaves 'c':3, 'e':5 (2 entries).