If-None-Match and 304 responses in Rest API - Mini Project: Build & Apply

Using If-None-Match Header and 304 Responses in REST API

📖 Scenario: You are building a simple REST API that returns user profile data. To save bandwidth and improve speed, you want to use the If-None-Match header and 304 Not Modified responses.This means the client sends a version tag (ETag) it has, and the server checks if the data has changed. If not, the server replies with 304 and no data, telling the client to use its cached copy.

🎯 Goal: Build a REST API endpoint that returns user data with an ETag. It should check the If-None-Match header from the client and respond with 304 Not Modified if the data has not changed, or 200 OK with the data if it has changed.

📋 What You'll Learn

Create a dictionary called user_profile with keys name, age, and email and exact values 'Alice', 30, and '[e-mail]' respectively.

Create a string variable called etag with the exact value "v1" representing the version of the data.

Write a function called handle_request that takes a parameter if_none_match (string or None). Inside, check if if_none_match equals etag. If yes, return a tuple (304, None). Otherwise, return (200, user_profile).

Call handle_request twice: once with if_none_match set to "v1" and once with None. Print the results exactly as shown.

💡 Why This Matters

🌍 Real World

Web servers use ETags and 304 responses to reduce bandwidth and speed up page loads by telling browsers when cached data is still valid.

💼 Career

Understanding HTTP caching headers like If-None-Match and 304 responses is important for backend developers building efficient REST APIs.

1

Create the user profile data

Create a dictionary called user_profile with these exact entries: 'name': 'Alice', 'age': 30, and 'email': '[e-mail]'.

Rest API

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

Hint

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

2

Create the ETag version string

Create a string variable called etag and set it to the exact value "v1".

Rest API

user_profile = {'name': 'Alice', 'age': 30, 'email': '[e-mail]'}
# Create the etag string with value "v1"
# Your code here

Hint

Use double quotes "v1" exactly for the etag string.

3

Write the request handler function

Write a function called handle_request that takes one parameter if_none_match. Inside, check if if_none_match == etag. If yes, return (304, None). Otherwise, return (200, user_profile).

Rest API

user_profile = {'name': 'Alice', 'age': 30, 'email': '[e-mail]'}
etag = "v1"
# Define handle_request(if_none_match) function
# Your code here

Hint

Use an if-else statement to compare if_none_match with etag and return the correct tuple.

4

Call the function and print results

Call handle_request with if_none_match set to "v1" and print the result. Then call it with None and print the result.

Rest API

user_profile = {'name': 'Alice', 'age': 30, 'email': '[e-mail]'}
etag = "v1"

def handle_request(if_none_match):
    if if_none_match == etag:
        return (304, None)
    else:
        return (200, user_profile)

# Call handle_request with "v1" and print
# Call handle_request with None and print
# Your code here

Hint

Use print() to show the results of calling handle_request with "v1" and None.

Practice

1. What does the If-None-Match header do in a REST API request?

easy

A. It asks the server to send data only if it has changed since the last request.

B. It forces the server to ignore caching and send fresh data every time.

C. It tells the server to delete the cached data on the client side.

D. It requests the server to send the full data regardless of changes.

Solution

1. Step 1: Understand the purpose of If-None-Match

   This header sends the ETag value from the client to check if the resource has changed.

2. Step 2: Determine the server's response based on the ETag

   If the ETag matches, the server returns 304 Not Modified, meaning no new data is sent.

3. Final Answer:

   It asks the server to send data only if it has changed since the last request. -> Option A

4. Quick Check:

   If-None-Match checks changes [OK]

Hint: If-None-Match checks if data changed before sending [OK]

Common Mistakes:

• Thinking it forces fresh data every time
• Confusing it with cache deletion
• Assuming it always sends full data

2. Which of the following is the correct syntax for sending an If-None-Match header with ETag value "abc123" in an HTTP request?

easy

A. If-None-Match: abc123

B. If-None-Match = "abc123"

C. If-None-Match: "abc123"

D. If-None-Match => abc123

Solution

1. Step 1: Recall HTTP header syntax

   Headers use colon : to separate name and value, and ETag values are quoted strings.

2. Step 2: Identify correct quoting and separator

   If-None-Match: "abc123" uses colon and quotes correctly: If-None-Match: "abc123".

3. Final Answer:

   If-None-Match: "abc123" -> Option C

4. Quick Check:

   Headers use colon + quoted ETag [OK]

Hint: Headers use colon and quotes for ETag values [OK]

Common Mistakes:

• Omitting quotes around ETag
• Using equals or arrow instead of colon
• Not quoting the ETag string

3. Given this server logic:

if request.headers.get('If-None-Match') == current_etag:
    return 304
else:
    return 200 with data and ETag

What will the server respond if the client sends If-None-Match: "xyz789" but current_etag is "abc123"?

medium

A. 200 OK with data and ETag

B. 304 Not Modified

C. 400 Bad Request

D. 500 Internal Server Error

Solution

1. Step 1: Compare client's ETag with server's current ETag

   The client sends "xyz789" but the server's current ETag is "abc123", so they do not match.

2. Step 2: Determine server response based on mismatch

   Since ETags differ, the server sends full data with status 200 and the current ETag.

3. Final Answer:

   200 OK with data and ETag -> Option A

4. Quick Check:

   ETags differ -> send data [OK]

Hint: If ETags differ, server sends full data (200) [OK]

Common Mistakes:

• Assuming 304 when ETags differ
• Confusing 400 or 500 errors with ETag mismatch
• Ignoring quotes in ETag comparison

4. A developer wrote this code snippet to handle If-None-Match but it always returns 200 OK even when ETags match:

if request.headers['If-None-Match'] == current_etag:
    return 200, data
else:
    return 304

What is the main error?

medium

A. The server must always return 200 regardless of ETag.

B. The code should compare request.headers.get() instead of direct indexing.

C. The ETag values should be stripped of quotes before comparison.

D. The 200 and 304 status codes are swapped in the if-else blocks.

Solution

1. Step 1: Analyze the if condition and returned status codes

   The code returns 200 OK when ETags match, but it should return 304 Not Modified instead.

2. Step 2: Identify correct status code usage for ETag match

   When ETags match, server returns 304 to avoid sending data; else returns 200 with data.

3. Final Answer:

   The 200 and 304 status codes are swapped in the if-else blocks. -> Option D

4. Quick Check:

   ETag match -> 304, not 200 [OK]

Hint: 304 means no change; 200 means send data [OK]

Common Mistakes:

• Returning 200 on ETag match
• Confusing header access methods
• Ignoring quotes in ETag comparison

5. You want to optimize your REST API to reduce bandwidth by using If-None-Match. Which of the following sequences correctly describes the client-server interaction to achieve this?

hard

A. Client sends request with If-None-Match -> Server always returns 200 with data and ETag -> Client caches data

B. Client sends request without ETag -> Server sends data with ETag -> Client caches data and ETag -> Client sends request with If-None-Match header -> Server returns 304 if data unchanged

C. Client sends request with ETag in body -> Server returns 304 if data unchanged -> Client discards cached data

D. Client sends request without headers -> Server returns 304 if data unchanged -> Client updates cache

Solution

1. Step 1: Understand the correct use of If-None-Match and 304

   The client first gets data and ETag, caches them, then sends If-None-Match with ETag to check for changes.

2. Step 2: Identify the correct sequence

   The sequence that starts with an initial request without the ETag header, receives data and ETag from the server, caches both, and then uses If-None-Match on future requests, receiving 304 if unchanged, is correct.

3. Final Answer:

   Client sends request without ETag -> Server sends data with ETag -> Client caches data and ETag -> Client sends request with If-None-Match header -> Server returns 304 if data unchanged -> Option B

4. Quick Check:

   Proper If-None-Match flow [OK]

Hint: Initial fetch no ETag, then use If-None-Match with ETag [OK]

Common Mistakes:

• Sending ETag in request body instead of header
• Expecting 304 without sending If-None-Match
• Ignoring caching after 304 response