ETag for conditional requests in Rest API - Mini Project: Build & Apply

ETag for conditional requests

📖 Scenario: You are building a simple REST API that serves user profile data. To save bandwidth and improve performance, you want to use ETags to let clients check if the data has changed before downloading it again.

🎯 Goal: Create a REST API endpoint that returns user profile data with an ETag header. The API should check the client's If-None-Match header and respond with 304 Not Modified if the data has not changed.

📋 What You'll Learn

Create a dictionary called user_profile with keys id, name, and email and exact values 1, 'Alice', and '[e-mail]'

Create a variable called etag that stores the ETag string value "abc123"

Write a function called get_user_profile that takes a parameter if_none_match and returns 304 if if_none_match equals etag, otherwise returns the user_profile dictionary and the etag

Print the result of calling get_user_profile with if_none_match set to "abc123"

💡 Why This Matters

🌍 Real World

ETags help web servers tell clients if data has changed, saving bandwidth and speeding up web apps.

💼 Career

Understanding ETags is important for backend developers working on APIs and web performance optimization.

1

Create the user profile data

Create a dictionary called user_profile with these exact entries: 'id': 1, 'name': 'Alice', and 'email': '[e-mail]'

Rest API

# Create the user_profile dictionary with id, name, and email
# Your code here

Hint

Use curly braces {} to create a dictionary with the exact keys and values.

2

Create the ETag variable

Create a variable called etag and set it to the string "abc123"

Rest API

user_profile = {'id': 1, 'name': 'Alice', 'email': '[e-mail]'}
# Create the etag variable with value "abc123"
# Your code here

Hint

Use double quotes to create the string "abc123".

3

Write the function to handle conditional requests

Write a function called get_user_profile that takes a parameter if_none_match. Inside the function, if if_none_match equals etag, return 304. Otherwise, return a tuple with user_profile and etag.

Rest API

user_profile = {'id': 1, 'name': 'Alice', 'email': '[e-mail]'}
etag = "abc123"

# Define the get_user_profile function
# Your code here

Hint

Use an if statement to compare if_none_match with etag. Return 304 if they match, else return the data and etag.

4

Test the function with a matching ETag

Print the result of calling get_user_profile with if_none_match set to "abc123"

Rest API

user_profile = {'id': 1, 'name': 'Alice', 'email': '[e-mail]'}
etag = "abc123"

def get_user_profile(if_none_match):
    if if_none_match == etag:
        return 304
    else:
        return user_profile, etag

# Print the result of get_user_profile with if_none_match="abc123"
# Your code here

Hint

Call get_user_profile with the string "abc123" and print the result.

Practice

1.

What is the main purpose of an ETag in REST APIs?

easy

A. To help clients check if the resource has changed since last fetch

B. To encrypt the data sent from server to client

C. To authenticate the client making the request

D. To specify the content type of the response

Solution

1. Step 1: Understand ETag role in REST

   ETags are unique identifiers for resource versions, used to detect changes.

2. Step 2: Match purpose with options

   Only To help clients check if the resource has changed since last fetch describes checking if resource changed, which is the ETag's purpose.

3. Final Answer:

   To help clients check if the resource has changed since last fetch -> Option A

4. Quick Check:

   ETag purpose = freshness check [OK]

Hint: ETags track changes, not encrypt or authenticate [OK]

Common Mistakes:

• Confusing ETag with authentication tokens
• Thinking ETag encrypts data
• Assuming ETag sets content type

2.

Which HTTP header should a client send to make a conditional GET request using an ETag?

easy

A. If-Modified-Since

B. If-None-Match

C. ETag

D. Cache-Control

Solution

1. Step 1: Identify client header for ETag conditional requests

   The client sends If-None-Match header with the ETag value to check freshness.

2. Step 2: Compare options

   If-Modified-Since uses dates, ETag is a response header, Cache-Control controls caching rules. Only If-None-Match fits.

3. Final Answer:

   If-None-Match -> Option B

4. Quick Check:

   Client uses If-None-Match for ETag check [OK]

Hint: Client sends If-None-Match with ETag to check freshness [OK]

Common Mistakes:

• Using ETag header in request instead of response
• Confusing If-Modified-Since with ETag usage
• Sending Cache-Control instead of conditional header

3.

Given this HTTP interaction:
GET /resource HTTP/1.1
Host: example.com
If-None-Match: "abc123"
Server responds:
HTTP/1.1 304 Not Modified
What does this response mean?

medium

A. The resource has changed and new data is sent

B. The server encountered an error

C. The resource has not changed; client should use cached data

D. The request is unauthorized

Solution

1. Step 1: Understand 304 Not Modified status

   304 means resource is unchanged since client's ETag, so no new data sent.

2. Step 2: Match meaning with options

   The resource has not changed; client should use cached data correctly states client should use cached data because resource is unchanged.

3. Final Answer:

   The resource has not changed; client should use cached data -> Option C

4. Quick Check:

   304 = Not Modified, use cache [OK]

Hint: 304 means no change; use cached copy [OK]

Common Mistakes:

• Thinking 304 means new data sent
• Confusing 304 with error or unauthorized status
• Ignoring ETag and caching role

4.

Consider this server code snippet handling ETags:

etag = generate_etag(resource)
if request.headers.get('If-None-Match') == etag:
    return Response(status=304)
else:
    return Response(data=resource, headers={'ETag': etag})

What is a potential bug in this code?

medium

A. It compares ETag strings without quotes, causing mismatch

B. It returns 304 even if ETag does not match

C. It does not handle If-Modified-Since header

D. It always returns data ignoring conditional requests

Solution

1. Step 1: Analyze ETag comparison

   ETags in headers often include quotes, so direct string equality may fail if quotes differ.

2. Step 2: Identify mismatch cause

   If client sends "abc123" but server etag is abc123 without quotes, comparison fails incorrectly.

3. Final Answer:

   It compares ETag strings without quotes, causing mismatch -> Option A

4. Quick Check:

   ETag quotes matter in comparison [OK]

Hint: ETag strings must match exactly including quotes [OK]

Common Mistakes:

• Ignoring quotes around ETag values
• Assuming If-Modified-Since is required for ETag logic
• Returning 304 without checking ETag

5.

You want to optimize your REST API to reduce bandwidth by using ETags. Which approach correctly implements conditional GET requests with ETags?

1. Server generates ETag for resource.
2. Server sends ETag in response header.
3. Client stores ETag.
4. Client sends GET with If-None-Match header.
5. Server compares ETag and returns 304 if unchanged.
6. Otherwise, server sends new data with new ETag.

Which step is missing or incorrect?

hard

A. Step 6: Server should always send 200 status even if unchanged

B. Step 4: Client should send If-Modified-Since instead of If-None-Match

C. Step 3: Client should not store ETag

D. Step 5: Server must compare ETag including quotes exactly

Solution

1. Step 1: Review each step for correctness

   Steps 1-4 are correct standard flow for ETag conditional requests.

2. Step 2: Identify subtle issue in step 5

   Server must compare ETag values exactly, including quotes, or else conditional check fails.

3. Step 3: Confirm other options are incorrect

   Client should store ETag (step 3), If-None-Match is correct header (step 4), and 304 status is correct for unchanged (step 6).

4. Final Answer:

   Step 5: Server must compare ETag including quotes exactly -> Option D

5. Quick Check:

   Exact ETag match including quotes is required [OK]

Hint: ETag comparison must be exact including quotes [OK]

Common Mistakes:

• Ignoring quotes in ETag comparison
• Using wrong conditional header
• Sending 200 instead of 304 for unchanged resource