Last-Modified and If-Modified-Since in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Last-Modified HTTP header?

The Last-Modified header tells the client the date and time when the requested resource was last changed. It helps the client know if the resource has been updated since the last time it was fetched.

beginner

How does the If-Modified-Since header work in an HTTP request?

The client sends the If-Modified-Since header with a date to ask the server: "Send me the resource only if it has changed since this date." If the resource is unchanged, the server replies with a 304 Not Modified status, saving bandwidth.

beginner

What HTTP status code does the server return if the resource has NOT been modified since the date in If-Modified-Since?

The server returns 304 Not Modified. This tells the client it can use its cached copy because the resource has not changed.

intermediate

Why are Last-Modified and If-Modified-Since headers useful in REST APIs?

They reduce unnecessary data transfer by letting clients check if data changed before downloading it again. This improves performance and saves bandwidth.

intermediate

Can If-Modified-Since be used with POST requests?

No, If-Modified-Since is typically used with GET requests to check resource freshness. POST requests usually change data and do not use this header.

What does the server send if the resource has changed since the date in If-Modified-Since?

A 200 OK with the updated resource

B 500 Internal Server Error

C 404 Not Found

D 304 Not Modified

If the resource has changed, the server sends a 200 OK status with the updated resource data.

Which header does the server include to tell the client when the resource was last changed?

A If-Modified-Since

B Content-Type

C Last-Modified

D Cache-Control

The server includes the Last-Modified header with the date/time the resource was last updated.

What is the main benefit of using If-Modified-Since in client requests?

A To reduce data transfer if resource is unchanged

B To authenticate the client

C To speed up server processing

D To encrypt the data

It helps avoid downloading unchanged data, saving bandwidth and improving speed.

If the client does NOT send If-Modified-Since, what will the server do?

A Always return 304 Not Modified

B Return the full resource with 200 OK

C Return 403 Forbidden

D Return 400 Bad Request

Without If-Modified-Since, the server returns the full resource with 200 OK.

Which HTTP method is If-Modified-Since most commonly used with?

A PUT

B POST

C DELETE

D GET

If-Modified-Since is mainly used with GET requests to check if the resource has changed.

Explain how Last-Modified and If-Modified-Since headers work together to optimize data transfer in REST APIs.

Think about how the client and server communicate about resource freshness.

Describe a scenario where using If-Modified-Since would improve user experience in a web application.

Imagine a news app or social media feed updating often.

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