Last-Modified and If-Modified-Since in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the HTTP status code returned?

Consider a REST API endpoint that returns a resource with the header Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT. A client sends a GET request with the header If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT. What status code will the server most likely return?

A 200 OK - The resource is returned with full content.

B 304 Not Modified - The resource has not changed since the given date.

C 404 Not Found - The resource does not exist.

D 500 Internal Server Error - Server failed to process the request.

💡 Hint

Think about what the If-Modified-Since header asks the server to do.

The If-Modified-Since header tells the server to send the resource only if it has been modified after the specified date. Since the Last-Modified date matches the If-Modified-Since date, the server responds with 304 Not Modified to save bandwidth.

🧠 Conceptual

intermediate

Purpose of Last-Modified header

What is the main purpose of the Last-Modified HTTP header in REST APIs?

A To tell the client when the resource was last changed, enabling conditional requests.

B To specify the expiration time of the resource in the cache.

C To provide the server's current time.

D To indicate the size of the resource in bytes.

💡 Hint

Think about how clients can avoid downloading unchanged data.

The Last-Modified header tells clients the last time the resource was changed. Clients can use this information in If-Modified-Since requests to avoid downloading data that hasn't changed.

❓ Predict Output

advanced

What happens if If-Modified-Since is after Last-Modified?

A server resource has Last-Modified: Mon, 01 Jan 2024 10:00:00 GMT. A client sends a GET request with If-Modified-Since: Tue, 02 Jan 2024 10:00:00 GMT. What will the server respond?

A 400 Bad Request due to invalid date.

B 200 OK with the full resource content.

C 304 Not Modified with no body.

D 503 Service Unavailable.

💡 Hint

Consider if the resource has changed since the date the client provides.

The client asks if the resource has changed since a date later than the resource's last modification. Since the resource was last modified before that date, the server responds with 304 Not Modified, indicating no new changes.

🔧 Debug

advanced

Why does the server always return 200 OK?

A developer implements a REST API that sets Last-Modified header correctly. However, clients always receive 200 OK responses even when sending If-Modified-Since headers. What is the most likely cause?

A The client is sending the wrong HTTP method instead of GET.

B The <code>Last-Modified</code> header is set to a future date.

C The server is missing the <code>Content-Type</code> header.

D The server does not check the <code>If-Modified-Since</code> header and always returns the full resource.

💡 Hint

Think about what the server must do to support conditional GET requests.

Even if the Last-Modified header is set, the server must explicitly check the If-Modified-Since header in the request and decide whether to return 304 Not Modified or the full content. If this logic is missing, the server always returns 200 OK.

🚀 Application

expert

Implementing conditional GET with Last-Modified in pseudocode

Given a REST API endpoint, which pseudocode snippet correctly implements conditional GET using Last-Modified and If-Modified-Since headers?

A

if request.headers['If-Modified-Since'] >= resource.last_modified:
    return 304 Not Modified
else:
    return 200 OK with resource and Last-Modified header

B

if request.headers['If-Modified-Since'] < resource.last_modified:
    return 304 Not Modified
else:
    return 200 OK with resource and Last-Modified header

C return 200 OK with resource and ignore If-Modified-Since header

D

if resource.last_modified == None:
    return 304 Not Modified
else:
    return 200 OK with resource

💡 Hint

Remember the server returns 304 only if the resource was not modified after the date the client provides.

The server compares the If-Modified-Since date with the resource's Last-Modified date. If the resource was not modified after the date, it returns 304 Not Modified. Otherwise, it returns the full resource with Last-Modified header.

Practice

1. What is the main purpose of the Last-Modified header in a REST API response?

easy

A. To tell the client when the resource was last changed

B. To tell the client the size of the resource

C. To tell the client the type of the resource

D. To tell the client the server's current time

Solution

1. Step 1: Understand the role of Last-Modified header

   The Last-Modified header indicates the date and time when the resource was last changed on the server.

2. Step 2: Compare options with the header's purpose

   Only To tell the client when the resource was last changed correctly describes this purpose. Other options describe unrelated information.

3. Final Answer:

   To tell the client when the resource was last changed -> Option A

4. Quick Check:

   Last-Modified = resource last change time [OK]

Hint: Last-Modified shows last change date of resource [OK]

Common Mistakes:

• Confusing Last-Modified with Content-Length
• Thinking it shows server current time
• Mixing it up with Content-Type header

2. Which of the following is the correct syntax to send an If-Modified-Since header in an HTTP request?

easy

A. If-Modified-Since: 2023-01-01T12:00:00Z

B. If-Modified-Since = "2023-01-01 12:00:00"

C. If-Modified-Since -> 01/01/2023 12:00

D. If-Modified-Since: Sun, 01 Jan 2023 12:00:00 GMT

Solution

1. Step 1: Recall HTTP header date format

   HTTP headers use the RFC 1123 date format like Sun, 01 Jan 2023 12:00:00 GMT for date/time values.

2. Step 2: Match options to correct format

   If-Modified-Since: Sun, 01 Jan 2023 12:00:00 GMT matches the correct header syntax and date format. Other options use invalid syntax or date formats.

3. Final Answer:

   If-Modified-Since: Sun, 01 Jan 2023 12:00:00 GMT -> Option D

4. Quick Check:

   If-Modified-Since uses RFC 1123 date format [OK]

Hint: Use RFC 1123 date format for If-Modified-Since header [OK]

Common Mistakes:

• Using ISO 8601 instead of RFC 1123 format
• Using '=' instead of ':' in header
• Using arrow '->' instead of colon

3. Given the server responds with header Last-Modified: Wed, 10 May 2023 10:00:00 GMT, what will the server respond if the client sends If-Modified-Since: Wed, 10 May 2023 09:00:00 GMT?

medium

A. HTTP 400 Bad Request

B. HTTP 304 Not Modified

C. HTTP 200 OK with resource data

D. HTTP 500 Internal Server Error

Solution

1. Step 1: Compare If-Modified-Since with Last-Modified

   The client's If-Modified-Since date is earlier (09:00) than the server's Last-Modified date (10:00).

2. Step 2: Determine server response based on dates

   Since the resource was modified after the client's date, the server sends full data with HTTP 200 OK.

3. Final Answer:

   HTTP 200 OK with resource data -> Option C

4. Quick Check:

   If-Modified-Since < Last-Modified -> 200 OK [OK]

Hint: If client date before last change, server sends full data [OK]

Common Mistakes:

• Thinking server sends 304 when resource changed after client date
• Confusing 400 Bad Request with conditional headers
• Assuming server error on date mismatch

4. A client sends If-Modified-Since: Tue, 09 May 2023 10:00:00 GMT but the server always responds with HTTP 200 OK and full data, ignoring the header. What is the likely error in server handling?

medium

A. Server does not check If-Modified-Since header before sending data

B. Server sends wrong Last-Modified date format

C. Client sent If-Modified-Since header incorrectly

D. Server is missing Content-Type header

Solution

1. Step 1: Understand expected server behavior

   The server should compare If-Modified-Since with resource's Last-Modified and respond 304 if not changed.

2. Step 2: Identify why server always sends 200 OK

   If server ignores the header and always sends full data, it likely does not check the If-Modified-Since header properly.

3. Final Answer:

   Server does not check If-Modified-Since header before sending data -> Option A

4. Quick Check:

   Ignoring If-Modified-Since means always 200 OK [OK]

Hint: Server must check If-Modified-Since to send 304 [OK]

Common Mistakes:

• Blaming client for server ignoring header
• Thinking wrong Last-Modified format causes 200 OK
• Confusing Content-Type header with caching headers

5. You want to optimize your REST API to reduce bandwidth by using Last-Modified and If-Modified-Since. Which approach correctly implements this optimization?

hard

A. Ignore If-Modified-Since and always send full resource with Last-Modified

B. Send Last-Modified header with resource date; on request, check If-Modified-Since and respond 304 if unchanged

C. Send If-Modified-Since header from server to client to indicate last change

D. Use Last-Modified header only on POST requests

Solution

1. Step 1: Understand how Last-Modified and If-Modified-Since work together

   The server sends Last-Modified header with the resource's last change date. The client sends If-Modified-Since header on next request to ask if resource changed since that date.

2. Step 2: Implement correct server logic for optimization

   The server compares If-Modified-Since with resource's last modified date and returns HTTP 304 Not Modified if unchanged, saving bandwidth.

3. Final Answer:

   Send Last-Modified header with resource date; on request, check If-Modified-Since and respond 304 if unchanged -> Option B

4. Quick Check:

   Send Last-Modified + check If-Modified-Since -> 304 optimization [OK]

Hint: Send Last-Modified; respond 304 if If-Modified-Since matches [OK]

Common Mistakes:

• Sending If-Modified-Since from server instead of client
• Ignoring If-Modified-Since header in server logic
• Using Last-Modified only on POST requests