Validation-based caching in Rest API

Introduction

Validation-based caching helps your app save time and data by checking if information has changed before downloading it again.

When you want to avoid downloading the same data again if it hasn't changed.

When your app talks to a server and you want to save internet data.

When you want your app to load faster by using saved data if possible.

When you want to reduce the load on the server by not asking for data too often.

Syntax

Rest API

GET /resource HTTP/1.1
Host: example.com
If-None-Match: "etag_value"

Response:
HTTP/1.1 304 Not Modified

or
HTTP/1.1 200 OK
Content-Type: application/json
ETag: "etag_value"

{...data...}

The client sends an If-None-Match header with the ETag value it has.

The server replies with 304 Not Modified if data is unchanged, or 200 OK with new data and a new ETag if changed.

Examples

The client asks for user 123 but only if the data changed since ETag "abc123".

Rest API

GET /users/123 HTTP/1.1
Host: api.example.com
If-None-Match: "abc123"

The server says the data is the same, so the client can use its saved copy.

Rest API

HTTP/1.1 304 Not Modified

The server sends new data with a new ETag "def456" because the data changed.

Rest API

HTTP/1.1 200 OK
Content-Type: application/json
ETag: "def456"

{"name": "Alice", "age": 30}

Sample Program

This program first gets data from the server and saves the ETag. Then it asks again with the ETag to see if data changed. If not, it uses cached data.

Rest API

import requests

url = 'https://api.example.com/data'

# First request, no ETag yet
response = requests.get(url)
print('Status:', response.status_code)
print('Data:', response.text)

# Save ETag from response
etag = response.headers.get('ETag')

# Second request with ETag to check if data changed
headers = {'If-None-Match': etag} if etag else {}
response2 = requests.get(url, headers=headers)
print('Status:', response2.status_code)
if response2.status_code == 304:
    print('Data not changed, use cached data')
else:
    print('New data:', response2.text)

Important Notes

ETag is a unique identifier for the version of the data.

304 means 'Not Modified' and tells the client to use its saved data.

Validation-based caching saves bandwidth and speeds up apps.

Summary

Validation-based caching checks if data changed before downloading again.

Clients send ETag with If-None-Match header to ask if data is new.

Servers reply with 304 if data is unchanged, or 200 with new data and ETag if changed.

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