Cache backends (memory, Redis, Memcached) in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Django cache code using memory backend?

Consider this Django view code snippet using the default memory cache backend:

from django.core.cache import cache

def get_data():
    cache.set('key', 'value', timeout=5)
    return cache.get('key')

What will get_data() return immediately after setting the cache?

Django

from django.core.cache import cache

def get_data():
    cache.set('key', 'value', timeout=5)
    return cache.get('key')

A None

B Raises KeyError

C 'value'

D Empty string ''

💡 Hint

Think about what happens when you set and get a key immediately in the same cache.

The memory cache stores the key-value pair immediately. So getting the key right after setting it returns the stored value.

📝 Syntax

intermediate

Which cache backend setting is syntactically correct for Redis in Django?

Which of the following CACHES settings correctly configures Redis as the cache backend in Django?

A """CACHES = { 'default': { 'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache', 'LOCATION': 'redis://127.0.0.1:6379', } }"""

B """CACHES = { 'default': { 'BACKEND': 'django.core.cache.backends.redis.RedisCache', 'LOCATION': 'redis://localhost:6379', } }"""

C """CACHES = { 'default': { 'BACKEND': 'django_redis.cache.RedisCache', 'LOCATION': 'memcached://127.0.0.1:11211', } }"""

D """CACHES = { 'default': { 'BACKEND': 'django_redis.cache.RedisCache', 'LOCATION': 'redis://127.0.0.1:6379/1', } }"""

💡 Hint

Redis backend in Django usually uses django_redis.cache.RedisCache and a Redis URL.

Option D correctly uses the django_redis backend and a Redis URL with database 1. Other options mix backends and locations incorrectly.

🔧 Debug

advanced

Why does this Memcached cache code raise a connection error?

Given this Django cache setting and code snippet:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': '127.0.0.1:11211',
    }
}

from django.core.cache import cache
cache.set('foo', 'bar')
cache.get('foo')

Running this code raises a connection error. What is the most likely cause?

Django

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': '127.0.0.1:11211',
    }
}

from django.core.cache import cache
cache.set('foo', 'bar')
cache.get('foo')

A Memcached server is not running on 127.0.0.1:11211

B Django cache API does not support Memcached

C The backend string is misspelled

D The cache key 'foo' is too long for Memcached

💡 Hint

Check if the Memcached service is active and reachable at the specified address.

The connection error usually means the Memcached server is not running or not reachable at the given IP and port.

❓ state_output

advanced

What is the value of the cache key after expiration with Redis backend?

Assume Redis cache backend is configured with a 1-second timeout. This code runs:

from django.core.cache import cache
cache.set('temp', 'data', timeout=1)
import time
time.sleep(2)
value = cache.get('temp')

What is the value of value after this code runs?

Django

from django.core.cache import cache
cache.set('temp', 'data', timeout=1)
import time
time.sleep(2)
value = cache.get('temp')

A None

B 'data'

C Raises TimeoutError

D Empty string ''

💡 Hint

Think about what happens when you get a cache key after its timeout expires.

After the 1-second timeout, the key expires and is removed from Redis, so getting it returns None.

🧠 Conceptual

expert

Which cache backend is best suited for sharing cache across multiple Django app instances?

You have multiple Django app servers running behind a load balancer. You want a cache backend that all instances share to keep data consistent. Which cache backend is best suited?

A File-based cache backend (django.core.cache.backends.filebased.FileBasedCache)

B Redis cache backend (django_redis.cache.RedisCache)

C Local memory cache backend (django.core.cache.backends.locmem.LocMemCache)

D Dummy cache backend (django.core.cache.backends.dummy.DummyCache)

💡 Hint

Consider which cache backend supports sharing data across multiple machines.

Redis is a networked cache server accessible by all app instances, making it ideal for shared caching. Local memory and file caches are local to each instance. Dummy cache does not store data.