Validation-based caching in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is validation-based caching in REST APIs?

Validation-based caching is a technique where the server tells the client if the cached data is still valid using validators like ETag or Last-Modified headers, avoiding sending the full data again.

beginner

What does the ETag header do in validation-based caching?

ETag is a unique identifier for a specific version of a resource. The client sends it back in If-None-Match header to ask if the resource changed. If not, the server replies with 304 Not Modified.

beginner

How does the Last-Modified header help with validation-based caching?

Last-Modified tells when the resource was last changed. The client sends this date in If-Modified-Since header. If the resource is unchanged, the server responds with 304 Not Modified.

beginner

What HTTP status code indicates that cached data is still valid?

The HTTP status code 304 Not Modified means the cached data is still valid and the server does not resend the resource body.

beginner

Why is validation-based caching better than no caching?

It saves bandwidth and speeds up responses by letting clients reuse cached data when it hasn't changed, while still ensuring data freshness by validating with the server.

Which HTTP header is used by the client to ask if the cached resource has changed using ETag?

A If-None-Match

B If-Modified-Since

C Cache-Control

D ETag

The client sends the ETag value in the If-None-Match header to check if the resource has changed.

What response status code does the server send if the cached resource is still valid?

A 500 Internal Server Error

B 404 Not Found

C 200 OK

D 304 Not Modified

304 Not Modified tells the client the cached resource is still valid and no new data is sent.

Which header tells the client when the resource was last changed?

A Last-Modified

B Content-Type

C ETag

D Expires

Last-Modified header contains the date/time when the resource was last updated.

What does the client send in the If-Modified-Since header?

A The ETag value

B The last modified date it has cached

C The full resource data

D The cache expiration time

The client sends the date of the cached resource in If-Modified-Since to check if it is still fresh.

Why use validation-based caching instead of no caching?

A To disable caching completely

B To always get fresh data without delay

C To reduce bandwidth and improve speed by reusing unchanged data

D To force the server to send full data every time

Validation-based caching saves bandwidth and speeds up responses by reusing cached data when valid.

Explain how validation-based caching works in REST APIs using ETag and Last-Modified headers.

Think about how the client asks the server if cached data is still fresh.

Why is the 304 Not Modified status code important in validation-based caching?

Consider what happens when the server confirms data hasn't 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