Retry-After header in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HTTP response header?

Consider a server response with the following HTTP header:

Retry-After: 120

What does the value 120 represent?

A The client should retry the request after 120 seconds.

B The client should retry the request after 120 milliseconds.

C The client should retry the request after 120 minutes.

D The client should retry the request immediately.

💡 Hint

Think about the unit of time used in the Retry-After header when a number is given.

The Retry-After header with a numeric value indicates the number of seconds the client should wait before retrying.

🧠 Conceptual

intermediate

What type of value can the Retry-After header contain?

The Retry-After header can have two types of values. Which of the following correctly describes them?

A Only a HTTP-date timestamp.

B Only a number of seconds.

C A number of seconds or a HTTP-date timestamp.

D A boolean value true or false.

💡 Hint

Think about how servers can tell clients when to retry: either a delay or a specific time.

The Retry-After header accepts either a delay in seconds or a HTTP-date indicating when to retry.

🔧 Debug

advanced

Identify the error in this Retry-After header usage

A server sends this header:

Retry-After: 2024-13-01T12:00:00Z

What is wrong with this header value?

A The Retry-After header cannot contain a date, only seconds.

B The month value 13 is invalid in the date format.

C The time zone Z is not allowed in HTTP-date format.

D The date format is correct and valid.

💡 Hint

Check the month part of the date carefully.

Months in dates must be between 01 and 12. 13 is invalid.

🚀 Application

advanced

How should a client handle a Retry-After header with a date value?

A client receives this header:

Retry-After: Wed, 21 Oct 2025 07:28:00 GMT

What should the client do?

A Calculate the difference between the current time and the given date, then wait that long before retrying.

B Ignore the header and retry immediately.

C Retry after 21 seconds as indicated by the day number.

D Retry after 7 minutes and 28 seconds.

💡 Hint

Think about how to interpret a date in the Retry-After header.

The client must wait until the specified date/time before retrying, so it calculates the wait time.

❓ Predict Output

expert

What is the output of this Python code parsing Retry-After header?

Given this Python code snippet:

from datetime import datetime, timezone
import time

def parse_retry_after(value):
    try:
        delay = int(value)
        return delay
    except ValueError:
        retry_time = datetime.strptime(value, '%a, %d %b %Y %H:%M:%S %Z')
        now = datetime.now(timezone.utc)
        delta = (retry_time - now).total_seconds()
        return max(0, int(delta))

# Example usage:
print(parse_retry_after('120'))
print(parse_retry_after('Wed, 21 Oct 2030 07:28:00 GMT'))

Assuming the current UTC time is Wed, 21 Oct 2025 07:28:00 GMT, what will be the output?

Rest API

from datetime import datetime, timezone
import time

def parse_retry_after(value):
    try:
        delay = int(value)
        return delay
    except ValueError:
        retry_time = datetime.strptime(value, '%a, %d %b %Y %H:%M:%S %Z')
        now = datetime.now(timezone.utc)
        delta = (retry_time - now).total_seconds()
        return max(0, int(delta))

# Example usage:
print(parse_retry_after('120'))
print(parse_retry_after('Wed, 21 Oct 2030 07:28:00 GMT'))

A ValueError exception

B

120
0

C

120
-157766400

D

120
157766400

💡 Hint

Calculate the seconds between 2025 and 2030 in years, then convert to seconds.

The first call returns 120 as integer. The second calculates seconds between 2025 and 2030, about 5 years, which is roughly 157766400 seconds.