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.