Expiration-based caching in Rest API - Mini Project: Build & Apply

Expiration-based caching in a REST API

📖 Scenario: You are building a simple REST API that returns data about products. To make your API faster and reduce repeated work, you want to add a cache that stores results temporarily. The cache should keep data only for a short time, then remove it automatically.

🎯 Goal: Build a basic expiration-based cache for your REST API. You will create a cache dictionary, set an expiration time, store data with timestamps, and return cached data only if it is still fresh.

📋 What You'll Learn

Create a cache dictionary to store product data and timestamps

Add a variable for cache expiration time in seconds

Write logic to check if cached data is still valid before returning it

Print the cached data or a message if cache expired

💡 Why This Matters

🌍 Real World

APIs often use caching to speed up responses and reduce repeated work. Expiration-based caching helps keep data fresh while saving resources.

💼 Career

Understanding caching is important for backend developers and API designers to build efficient and scalable web services.

1

Create the cache dictionary

Create a dictionary called cache that will store cached product data and their timestamps. Initialize it as an empty dictionary.

Rest API

# Create an empty dictionary called cache
# Your code here

Hint

Use {} to create an empty dictionary in Python.

2

Set the cache expiration time

Create a variable called cache_expiration and set it to 10. This number means the cache will keep data for 10 seconds before it expires.

Rest API

cache = {}
# Set cache_expiration to 10 seconds
# Your code here

Hint

Just assign the number 10 to the variable cache_expiration.

3

Add caching logic with expiration check

Write code to store product data in cache with the current time, and check if cached data is still valid before returning it. Use variables product_id set to 101 and product_data set to {'name': 'Book', 'price': 12.99}. Use time.time() to get the current time. If cached data exists and is not expired, use it. Otherwise, update the cache with new data and timestamp.

Rest API

cache = {}
cache_expiration = 10
import time

product_id = 101
product_data = {'name': 'Book', 'price': 12.99}

# Your code here

Hint

Use time.time() to get current time and store it with the data in the cache dictionary.

4

Print the cached result

Write a print statement to display the cached_result variable.

Rest API

cache = {}
cache_expiration = 10
import time

product_id = 101
product_data = {'name': 'Book', 'price': 12.99}

current_time = time.time()
if product_id in cache and current_time - cache[product_id]['timestamp'] < cache_expiration:
    cached_result = cache[product_id]['data']
else:
    cache[product_id] = {'data': product_data, 'timestamp': current_time}
    cached_result = product_data

# Print the cached_result
# Your code here

Hint

Use print(cached_result) to show the cached data.

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