Cache invalidation strategies in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding cache timeout in Django

What happens when you set a cache timeout to a very low value in Django's caching framework?

A The cache entries expire quickly, causing frequent cache misses and more database hits.

B The cache entries never expire, so the data stays forever in the cache.

C The cache automatically refreshes data from the database without any misses.

D The cache entries are deleted only when the server restarts.

💡 Hint

Think about what 'timeout' means for cached data.

Setting a low timeout means cached data expires quickly. This causes the cache to miss often and fetch fresh data from the database repeatedly.

❓ component_behavior

intermediate

Effect of cache key versioning in Django

Given a Django cache key with versioning, what is the effect of changing the version number when retrieving cached data?

A Changing the version number causes Django to look for a different cache entry, effectively invalidating the old cache.

B Changing the version number merges old and new cache entries.

C Changing the version number has no effect; Django ignores it.

D Changing the version number deletes all cache entries.

💡 Hint

Think about how versioning helps manage cache entries.

Cache key versioning lets you keep multiple versions of cached data. Changing the version makes Django look for a new key, so old cache is ignored.

❓ state_output

advanced

Cache invalidation with signals in Django

Consider a Django model with a post_save signal that clears a related cache key. What is the expected cache state after saving an instance?

Django

from django.db import models
from django.core.cache import cache
from django.db.models.signals import post_save
from django.dispatch import receiver

class Product(models.Model):
    name = models.CharField(max_length=100)

@receiver(post_save, sender=Product)
def clear_product_cache(sender, instance, **kwargs):
    cache_key = f'product_{instance.pk}'
    cache.delete(cache_key)

# Assume cache initially has {'product_1': 'Old Data'}
product = Product(pk=1, name='New Name')
product.save()

A A new cache entry 'product_1' is created with 'New Name'.

B The cache entry 'product_1' remains unchanged with 'Old Data'.

C The cache entry 'product_1' is deleted, so cache.get('product_1') returns None.

D The cache raises a KeyError because the key is missing.

💡 Hint

What does cache.delete do to the cache entry?

The post_save signal deletes the cache key for the saved product. So after saving, the cache entry is removed and returns None on get.

📝 Syntax

advanced

Correct syntax for cache invalidation in Django

Which of the following code snippets correctly invalidates a cache key named 'user_profile_5' in Django?

A cache.clear_key('user_profile_5')

B cache.remove('user_profile_5')

C cache.invalidate('user_profile_5')

D cache.delete('user_profile_5')

💡 Hint

Check Django cache API for deleting keys.

The correct method to remove a cache entry by key is cache.delete(key). Other methods do not exist in Django's cache API.

🔧 Debug

expert

Diagnosing stale cache after model update

A Django app caches user data under 'user_data_42'. After updating the user model instance with pk=42, the cache still returns old data. Which is the most likely cause?

A The cache backend automatically updates cache on model save, so this is impossible.

B The cache key was not deleted or updated after the model save, so stale data remains.

C The cache timeout is set to zero, so data never expires.

D The model instance was saved with update_fields, which clears the cache automatically.

💡 Hint

Think about what triggers cache invalidation after data changes.

Django cache does not auto-update on model changes. You must manually delete or update cache keys after saving models to avoid stale data.