Response headers (Cache-Control, ETag) in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HTTP response header?

Consider a REST API response with the following headers:

Cache-Control: max-age=3600, must-revalidate
ETag: "abc123"

If a client sends a request with header If-None-Match: "abc123", what will the server most likely respond with?

A HTTP 304 Not Modified with no body

B HTTP 200 OK with full response body

C HTTP 500 Internal Server Error

D HTTP 403 Forbidden

💡 Hint

ETag helps the server decide if the resource has changed.

If the If-None-Match header matches the current ETag, the server returns 304 Not Modified without the body to save bandwidth.

🧠 Conceptual

intermediate

Which Cache-Control directive forces revalidation every time?

Which Cache-Control directive ensures that the client must check with the server before using a cached response?

A public

B max-age=3600

C no-cache

D private

💡 Hint

Think about which directive means 'always check before use'.

no-cache means the client must revalidate with the server before using the cached response, even if it is fresh.

❓ Predict Output

advanced

What is the effect of this Cache-Control header?

Given this response header:

Cache-Control: private, max-age=0, must-revalidate

What does this mean for caching behavior?

A The response is not cached anywhere

B The response is cached by any cache and never revalidated

C The response is publicly cached and served without revalidation for 1 hour

D The response is cached only by the browser and must be revalidated before each use

💡 Hint

Consider what private and max-age=0 imply together.

private means only the browser caches it, max-age=0 means it is immediately stale, and must-revalidate forces revalidation before reuse.

❓ Predict Output

advanced

What error occurs with this ETag usage?

Consider this server code snippet that sets ETag header incorrectly:

response.headers['ETag'] = 12345

What error or behavior will this cause?

A TypeError because header value must be a string

B SyntaxError due to invalid header format

C The ETag header is ignored silently

D The server sends ETag as a string "12345" without error

💡 Hint

HTTP headers must be strings.

Setting a non-string value for a header usually raises a TypeError in most server frameworks.

🧠 Conceptual

expert

How does ETag improve REST API efficiency?

Which statement best explains how ETag headers improve REST API efficiency?

A ETag forces servers to always send full responses to clients

B ETag enables clients to send conditional requests to avoid downloading unchanged resources

C ETag allows clients to cache responses indefinitely without revalidation

D ETag disables caching and forces fresh data every time

💡 Hint

Think about how clients can save bandwidth using ETag.

ETag lets clients send If-None-Match headers so servers can respond with 304 Not Modified if the resource is unchanged, saving bandwidth.

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