Validation-based caching in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HTTP cache validation example?

Consider a REST API that uses ETag headers for validation-based caching. The server responds with an ETag value "abc123" for a resource. A client sends a GET request with header If-None-Match: "abc123". What will the server respond?

Rest API

HTTP/1.1 304 Not Modified
ETag: "abc123"

A HTTP 404 Not Found

B HTTP 500 Internal Server Error

C HTTP 304 Not Modified with no body

D HTTP 200 OK with full resource body

💡 Hint

Think about what the server does when the client's ETag matches the current resource's ETag.

When the client's If-None-Match header matches the server's current ETag, the server responds with 304 Not Modified and no body, telling the client to use its cached copy.

🧠 Conceptual

intermediate

Which HTTP header is used by clients to validate cached resources?

In validation-based caching, clients send a header to check if their cached resource is still valid. Which header is it?

A If-Modified-Since

B Cache-Control

C Authorization

D Content-Type

💡 Hint

It is a header that asks the server if the resource has changed since a certain date.

The If-Modified-Since header is sent by clients to ask the server if the resource has changed since the given date. If not, the server can respond with 304 Not Modified.

🔧 Debug

advanced

Why does this validation-based caching code always return full data instead of 304?

Given this simplified server code snippet in Python Flask, why does the server never return 304 Not Modified?

Rest API

from flask import Flask, request, jsonify, make_response
app = Flask(__name__)

@app.route('/data')
def data():
    etag = '"xyz789"'
    if request.headers.get('If-None-Match') == etag:
        return make_response('', 304)
    response = make_response(jsonify({'value': 42}))
    response.headers['ETag'] = etag
    return response

A The response body is always sent regardless of ETag

B The server does not check the If-Modified-Since header

C The ETag header is not set in the response

D The If-None-Match header is compared incorrectly (missing quotes)

💡 Hint

Check how the ETag value is formatted and compared with the header.

The ETag header value is usually quoted (e.g., "xyz789"), but the code compares the raw string without quotes to the header value which includes quotes. This causes the comparison to fail and the server always sends full data.

📝 Syntax

advanced

Which option correctly sets the ETag header in this Node.js Express code?

Choose the correct way to set the ETag header in an Express.js response.

Rest API

app.get('/resource', (req, res) => {
  const etagValue = 'abc123';
  // Set ETag header here
  res.send('Hello World');
});

A res.set('ETag', etagValue);

B res.setHeader('etag', etagValue);

C res.header('ETag', etagValue);

D res.etag = etagValue;

💡 Hint

Express response objects have a method to set headers with case-insensitive names.

The res.set() method is the correct way to set headers in Express. It is case-insensitive and sets the header properly. res.header() is an alias but less recommended. res.setHeader() is not a method on Express response. res.etag is not a valid property.

🚀 Application

expert

How many items are cached when using validation-based caching with ETag and If-None-Match?

A REST API serves 3 different resources, each with unique ETags. A client caches all 3 resources and sends requests with If-None-Match headers for each. How many resources will the client avoid downloading if all ETags match?

A 0

B 3

C 2

D 1

💡 Hint

If the client's ETag matches the server's, the server responds with 304 Not Modified.

If all 3 ETags match, the server responds with 304 Not Modified for all 3 resources, so the client avoids downloading all 3 again.

Practice

1. What is the main purpose of validation-based caching in REST APIs?

easy

A. To encrypt data during transmission

B. To speed up the server response by skipping validation

C. To store data permanently on the client side

D. To check if the data has changed before downloading it again

Solution

1. Step 1: Understand validation-based caching concept

   It is used to avoid downloading data again if it has not changed.

2. Step 2: Identify the main purpose

   The main goal is to check if data changed before downloading again to save bandwidth and time.

3. Final Answer:

   To check if the data has changed before downloading it again -> Option D

4. Quick Check:

   Validation-based caching = check data change before download [OK]

Hint: Remember: caching avoids re-downloading unchanged data [OK]

Common Mistakes:

• Thinking caching stores data permanently
• Confusing caching with encryption
• Assuming caching skips validation

2. Which HTTP header does the client send to ask the server if the cached data is still valid?

easy

A. If-Modified-Since

B. ETag

C. If-None-Match

D. Cache-Control

Solution

1. Step 1: Identify client headers for validation

   The client uses headers to tell the server what version it has.

2. Step 2: Match header for ETag validation

   The If-None-Match header is sent with the ETag value to check if data changed.

3. Final Answer:

   If-None-Match -> Option C

4. Quick Check:

   Client asks with If-None-Match header [OK]

Hint: Client sends If-None-Match with ETag to validate cache [OK]

Common Mistakes:

• Confusing If-Modified-Since with If-None-Match
• Using Cache-Control header for validation
• Thinking ETag is sent by client

3. Given this server response to a client request with If-None-Match: "abc123" header:

HTTP/1.1 304 Not Modified
ETag: "abc123"

What does this response mean?

medium

A. The data has changed and new data is sent

B. The data has not changed; client should use cached data

C. The server does not support caching

D. The client must resend the request without ETag

Solution

1. Step 1: Understand HTTP 304 status meaning

   304 means Not Modified, so data is unchanged since last fetch.

2. Step 2: Interpret ETag match

   ETag matches client's value, so server confirms data is still valid.

3. Final Answer:

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

4. Quick Check:

   304 Not Modified = use cached data [OK]

Hint: 304 status means no new data, use cache [OK]

Common Mistakes:

• Thinking 304 means new data sent
• Confusing 304 with 200 OK
• Assuming server rejects caching

4. A client sends a request with header If-None-Match: "xyz789". The server responds with:

HTTP/1.1 200 OK
ETag: "abc123"
{ "data": "new content" }

What is the likely problem in this interaction?

medium

A. The server sent a different ETag, indicating data changed, so 200 OK is correct

B. The server should respond with 304 Not Modified instead of 200 OK

C. The client sent an invalid ETag header

D. The server response is missing the If-None-Match header

Solution

1. Step 1: Compare client and server ETags

   Client sent "xyz789", server responded with "abc123" which is different.

2. Step 2: Understand server response code

   Since ETags differ, data changed, so server correctly sends 200 OK with new data.

3. Final Answer:

   The server sent a different ETag, indicating data changed, so 200 OK is correct -> Option A

4. Quick Check:

   Different ETag means data changed, send 200 OK [OK]

Hint: Different ETag means new data, so 200 OK is correct [OK]

Common Mistakes:

• Expecting 304 when ETags differ
• Thinking client ETag must match server's
• Confusing If-None-Match with If-Match

5. You want to implement validation-based caching in your REST API. Which sequence correctly describes the interaction between client and server?

hard

A. Client sends request with If-None-Match header; server replies 304 if data unchanged or 200 with new data and ETag if changed

B. Server sends ETag in request; client replies 304 if data unchanged or 200 with new data if changed

C. Client sends ETag in response; server sends If-None-Match header to validate cache

D. Server always sends 200 OK with data and never uses ETag

Solution

1. Step 1: Identify correct client-server header roles

   Client sends If-None-Match with ETag to ask if data changed.

2. Step 2: Understand server response logic

   Server replies 304 if data unchanged, or 200 with new data and new ETag if changed.

3. Final Answer:

   Client sends request with If-None-Match header; server replies 304 if data unchanged or 200 with new data and ETag if changed -> Option A

4. Quick Check:

   Client sends If-None-Match; server replies 304 or 200 [OK]

Hint: Client sends If-None-Match; server replies 304 or 200 with ETag [OK]

Common Mistakes:

• Mixing up who sends ETag and If-None-Match
• Thinking server sends If-None-Match header
• Assuming server never sends 304