Response headers (Cache-Control, ETag) in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Cache-Control header in HTTP responses?

The Cache-Control header tells browsers and other clients how to store and reuse the response. It controls caching behavior like how long to keep the data or whether to cache it at all.

beginner

Explain what an ETag header does in HTTP responses.

An ETag is a unique identifier for a specific version of a resource. It helps clients check if the resource has changed since the last request, enabling efficient caching and conditional requests.

intermediate

What does the directive Cache-Control: no-cache mean?

It means the client must revalidate with the server before using a cached copy. The cached response can be stored but must be checked for freshness before reuse.

intermediate

How does the client use the If-None-Match header with ETag?

The client sends If-None-Match with the ETag value it has. The server compares it to the current ETag. If they match, the server replies with 304 Not Modified, saving bandwidth.

beginner

Why are Cache-Control and ETag headers important for REST APIs?

They improve performance by reducing unnecessary data transfer and server load. Cache-Control manages how responses are cached, and ETag helps detect changes to resources efficiently.

What does the Cache-Control: max-age=3600 header instruct the client to do?

A Immediately revalidate the response

B Never cache the response

C Cache the response forever

D Cache the response for 3600 seconds before revalidating

The max-age=3600 directive tells the client to consider the cached response fresh for 3600 seconds (1 hour) before checking with the server again.

Which header is used by the client to tell the server the version of the resource it has cached?

A Cache-Control

B If-None-Match

C ETag

D Last-Modified

If-None-Match is sent by the client with the ETag value to ask the server if the resource has changed.

What response status code does the server send if the resource has not changed when using ETag validation?

A 200 OK

B 404 Not Found

C 304 Not Modified

D 500 Internal Server Error

The server sends 304 Not Modified to tell the client the cached version is still valid.

If a response has Cache-Control: no-store, what does it mean?

A The response must not be cached anywhere

B The response can be cached indefinitely

C The response can be cached but must be revalidated

D The response is private

no-store means the response must not be stored in any cache.

Which header helps reduce bandwidth by avoiding sending unchanged resources?

A ETag

B Cache-Control

C Content-Type

D Authorization

ETag allows the server to tell the client if the resource has changed, avoiding resending unchanged data.

Describe how Cache-Control and ETag headers work together to improve API performance.

Think about caching rules and how the client checks if data changed.

Explain the difference between Cache-Control: no-cache and Cache-Control: no-store.

Focus on whether caching is allowed or not.

Practice

1. What is the main purpose of the Cache-Control header in an HTTP response?

easy

A. To indicate the response content type

B. To encrypt the response data for security

C. To specify the server's IP address

D. To tell browsers how long to keep the response data

Solution

1. Step 1: Understand the role of Cache-Control

   The Cache-Control header instructs browsers and caches how long they can store the response data before requesting it again.

2. Step 2: Compare options with Cache-Control purpose

   Only To tell browsers how long to keep the response data correctly describes this behavior. Other options describe unrelated functions.

3. Final Answer:

   To tell browsers how long to keep the response data -> Option D

4. Quick Check:

   Cache-Control = Duration for caching [OK]

Hint: Cache-Control controls data storage time in browsers [OK]

Common Mistakes:

• Confusing Cache-Control with security headers
• Thinking Cache-Control sets content type
• Assuming Cache-Control reveals server info

2. Which of the following is the correct syntax to set a Cache-Control header to cache content for 3600 seconds?

easy

A. "Cache-Control: max-age=3600"

B. "Cache-Control: timeout=3600"

C. "Cache-Control: expires=3600"

D. "Cache-Control: duration=3600"

Solution

1. Step 1: Recall Cache-Control max-age syntax

   The correct way to specify caching time in seconds is using max-age=seconds.

2. Step 2: Check each option's correctness

   Only "Cache-Control: max-age=3600" uses max-age=3600, which is valid. Others use invalid directives.

3. Final Answer:

   "Cache-Control: max-age=3600" -> Option A

4. Quick Check:

   max-age sets cache time [OK]

Hint: Use max-age to set cache time in seconds [OK]

Common Mistakes:

• Using 'timeout' instead of 'max-age'
• Confusing Cache-Control with Expires header syntax
• Using unsupported directives like 'duration'

3. Given this HTTP response header snippet:

Cache-Control: max-age=0, must-revalidate
ETag: "abc123"

What happens when the browser sends a request with header If-None-Match: "abc123"?

medium

A. Server returns 304 Not Modified if content unchanged

B. Server returns full content ignoring ETag

C. Server returns 500 Internal Server Error

D. Server always returns 200 OK with new ETag

Solution

1. Step 1: Understand ETag and If-None-Match interaction

   The ETag header is a unique identifier for the content version. The browser sends If-None-Match with the ETag value to check if content changed.

2. Step 2: Interpret Cache-Control and server response

   With max-age=0 and must-revalidate, the browser asks server to validate content. If ETag matches, server responds with 304 Not Modified, saving bandwidth.

3. Final Answer:

   Server returns 304 Not Modified if content unchanged -> Option A

4. Quick Check:

   ETag + If-None-Match = 304 if unchanged [OK]

Hint: ETag + If-None-Match = 304 Not Modified if same [OK]

Common Mistakes:

• Assuming server always sends full content
• Confusing 304 with error codes
• Ignoring Cache-Control directives

4. A developer sets the response header as Cache-Control: max-age=abc. What is the likely result?

medium

A. The browser caches the response for 'abc' seconds

B. The server returns a syntax error

C. The browser ignores the Cache-Control header and does not cache

D. The browser caches the response indefinitely

Solution

1. Step 1: Analyze max-age value validity

   The max-age directive requires a numeric value in seconds. 'abc' is invalid.

2. Step 2: Understand browser behavior on invalid Cache-Control

   Browsers typically ignore invalid Cache-Control headers and treat the response as non-cacheable.

3. Final Answer:

   The browser ignores the Cache-Control header and does not cache -> Option C

4. Quick Check:

   Invalid max-age causes no caching [OK]

Hint: max-age must be a number, else ignored [OK]

Common Mistakes:

• Thinking server returns error for header syntax
• Assuming browser caches with invalid value
• Confusing max-age with other headers

5. You want to optimize a REST API to reduce bandwidth by sending full data only when it changes. Which combination of headers should you use in the response?

hard

A. Cache-Control: max-age=0 and no ETag

B. Cache-Control: no-cache and ETag with unique value

C. Cache-Control: no-store and no ETag

D. Cache-Control: max-age=3600 and no ETag

Solution

1. Step 1: Identify headers that enable conditional requests

   ETag provides a unique identifier for content versions, allowing clients to ask if content changed.

2. Step 2: Understand Cache-Control no-cache directive

   no-cache forces the browser to validate with the server before using cached data, ensuring fresh content without unnecessary full data transfer.

3. Step 3: Evaluate other options

   Options without ETag cannot validate changes efficiently; no-store disables caching; max-age=0 without ETag disables validation.

4. Final Answer:

   Cache-Control: no-cache and ETag with unique value -> Option B

5. Quick Check:

   no-cache + ETag = bandwidth saving [OK]

Hint: Use no-cache with ETag for efficient validation [OK]

Common Mistakes:

• Omitting ETag disables change detection
• Using no-store disables caching entirely
• Assuming max-age alone reduces bandwidth