Last-Modified and If-Modified-Since in Rest API - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the Last-Modified header in the HTTP response.

Rest API

response.headers['Last-Modified'] = [1]

Drag options to blanks, or click blank then click option'

A'If-Modified-Since'

B'Wed, 21 Oct 2015 07:28:00 GMT'

C'200 OK'

D'Content-Type'

Attempts:

3 left

💡 Hint

Common Mistakes

Using header names instead of date strings.

Setting unrelated header values.

The Last-Modified header must be set to a valid HTTP date string indicating the last modification time of the resource.

2fill in blank

medium

Complete the code to read the If-Modified-Since header from the HTTP request.

Rest API

last_seen = request.headers.get([1])

Drag options to blanks, or click blank then click option'

A'Authorization'

B'Last-Modified'

C'Content-Length'

D'If-Modified-Since'

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing with Last-Modified header.

Using unrelated headers.

The If-Modified-Since header is sent by the client to tell the server the last time it saw the resource.

3fill in blank

hard

Fix the error in the code that compares If-Modified-Since with Last-Modified to decide if the resource is modified.

Rest API

if last_modified <= [1]:
    return 304

Drag options to blanks, or click blank then click option'

Alast_seen

Brequest.headers['If-Modified-Since']

CNone

Dresponse.headers['Last-Modified']

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing header strings directly without parsing.

Using wrong variables.

You must compare the stored last_modified date with the parsed last_seen date from the If-Modified-Since header.

4fill in blank

hard

Fill both blanks to parse the If-Modified-Since header and compare it with the last modified time.

Rest API

from datetime import datetime

client_time = datetime.strptime([1], '%a, %d %b %Y %H:%M:%S GMT')
if last_modified <= [2]:
    return 304

Drag options to blanks, or click blank then click option'

Alast_seen

Brequest.headers.get('If-Modified-Since')

Cclient_time

Dresponse.headers['Last-Modified']

Attempts:

3 left

💡 Hint

Common Mistakes

Parsing the wrong header.

Comparing string with datetime.

First, get the If-Modified-Since header string, then parse it into a datetime object, and finally compare with last_modified.

5fill in blank

hard

Fill all three blanks to set Last-Modified header, parse If-Modified-Since, and return 304 if not modified.

Rest API

from datetime import datetime

last_modified = datetime(2023, 6, 1, 12, 0, 0)
response.headers['Last-Modified'] = [1]
client_header = request.headers.get('If-Modified-Since')
client_time = datetime.strptime([2], '%a, %d %b %Y %H:%M:%S GMT') if client_header else None
if client_time and last_modified <= [3]:
    return 304

Drag options to blanks, or click blank then click option'

A'Thu, 01 Jun 2023 12:00:00 GMT'

Bclient_header

Cclient_time

D'Wed, 21 Oct 2015 07:28:00 GMT'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong date strings.

Not checking if client_header exists.

Comparing string with datetime.

Set the Last-Modified header to the correct date string, parse the If-Modified-Since header if present, and compare the datetime objects to decide if 304 should be returned.

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