Retry-After header in Rest API

Introduction

The Retry-After header tells a client when to try a request again after it was told to wait.

When a server is busy and wants the client to wait before retrying.

When a resource is temporarily unavailable and will be ready later.

When rate limits are reached and the client must pause before sending more requests.

When maintenance is happening and the server asks clients to retry later.

Syntax

Rest API

Retry-After: <HTTP-date>
Retry-After: <delta-seconds>

You can use either a date/time or a number of seconds.

The date/time is in HTTP-date format, like 'Wed, 21 Oct 2015 07:28:00 GMT'.

Examples

This tells the client to retry after 120 seconds (2 minutes).

Rest API

Retry-After: 120

This tells the client to retry after the given date and time.

Rest API

Retry-After: Wed, 21 Oct 2015 07:28:00 GMT

Sample Program

This is a simple HTTP response showing the Retry-After header telling the client to wait 60 seconds before retrying.

Rest API

HTTP/1.1 503 Service Unavailable
Retry-After: 60
Content-Type: text/plain

Service is busy. Please try again after 60 seconds.

Important Notes

Clients should respect the Retry-After header to avoid overloading the server.

If Retry-After is missing, clients decide when to retry on their own.

Using Retry-After helps improve user experience by giving clear wait times.

Summary

The Retry-After header tells clients when to try again.

It can use seconds or a date/time format.

It helps servers manage load and clients avoid repeated failures.

Practice

1.

What is the main purpose of the Retry-After header in REST APIs?

easy

A. To provide the client's IP address to the server

B. To specify the content type of the response

C. To tell the client how long to wait before making another request

D. To indicate the server's current time

Solution

1. Step 1: Understand the role of Retry-After header

   The Retry-After header is used by servers to tell clients when they can retry a request after being told to wait.

2. Step 2: Match purpose with options

   To tell the client how long to wait before making another request correctly states that it tells the client how long to wait before retrying, which matches the header's purpose.

3. Final Answer:

   To tell the client how long to wait before making another request -> Option C

4. Quick Check:

   Retry-After = wait time before retry [OK]

Hint: Retry-After tells when to retry, not server info [OK]

Common Mistakes:

• Confusing Retry-After with content type headers
• Thinking Retry-After provides server time
• Assuming Retry-After sends client info

2.

Which of the following is a correct example of the Retry-After header syntax?

Retry-After: ?

easy

A. Wed, 21 Oct 2015 07:28:00 GMT

B. "120 seconds"

C. 120 seconds

D. After 2 minutes

Solution

1. Step 1: Review Retry-After header formats

   Retry-After accepts either a number of seconds (integer) or a HTTP-date string.

2. Step 2: Check each option

   120 seconds includes invalid text; "120 seconds" incorrectly quotes the number with text; Wed, 21 Oct 2015 07:28:00 GMT is a valid HTTP-date format; After 2 minutes is not a valid format.

3. Final Answer:

   Wed, 21 Oct 2015 07:28:00 GMT -> Option A

4. Quick Check:

   Retry-After = seconds or HTTP-date [OK]

Hint: Retry-After uses seconds or HTTP-date format only [OK]

Common Mistakes:

• Adding quotes around seconds value
• Using natural language like 'After 2 minutes'
• Using invalid date formats

3.

Given the following HTTP response header:

HTTP/1.1 503 Service Unavailable
Retry-After: 60

What should the client do?

medium

A. Wait 60 seconds before retrying the request

B. Retry the request immediately

C. Ignore the Retry-After header and retry after 5 seconds

D. Abort the request permanently

Solution

1. Step 1: Interpret the Retry-After header value

   The header value '60' means the client should wait 60 seconds before retrying.

2. Step 2: Match client action to header instruction

   Wait 60 seconds before retrying the request correctly instructs to wait 60 seconds before retrying, which follows the server's guidance.

3. Final Answer:

   Wait 60 seconds before retrying the request -> Option A

4. Quick Check:

   Retry-After: 60 means wait 60 seconds [OK]

Hint: Retry-After number means wait that many seconds [OK]

Common Mistakes:

• Retrying immediately ignoring Retry-After
• Assuming Retry-After is in milliseconds
• Treating Retry-After as a permanent failure

4.

Identify the error in this HTTP response header snippet:

HTTP/1.1 429 Too Many Requests
Retry-After: "30 seconds"

medium

A. The header name should be lowercase

B. Retry-After value should not be quoted or include text

C. Retry-After header is missing

D. Status code 429 is invalid

Solution

1. Step 1: Check Retry-After header format

   The Retry-After header must be either an integer number of seconds or a valid HTTP-date string without quotes or extra text.

2. Step 2: Identify the error in the given header

   The value "30 seconds" is quoted and includes text, which is invalid syntax.

3. Final Answer:

   Retry-After value should not be quoted or include text -> Option B

4. Quick Check:

   Retry-After must be seconds or date, no quotes/text [OK]

Hint: Retry-After value is raw seconds or date, no quotes [OK]

Common Mistakes:

• Adding quotes around Retry-After value
• Including descriptive text in Retry-After
• Confusing header case sensitivity

5.

A server wants to tell clients to retry after 2 minutes using the Retry-After header. Which is the best way to set this header to ensure compatibility?

hard

A. Retry-After: Wed, 21 Oct 2030 07:28:00 GMT (current time + 2 minutes)

B. Retry-After: "120 seconds"

C. Retry-After: After 2 minutes

D. Retry-After: 120

Solution

1. Step 1: Understand Retry-After header formats

   Retry-After accepts either an integer number of seconds or a valid HTTP-date string.

2. Step 2: Evaluate options for compatibility

   Retry-After: 120 uses integer seconds (120) which is simple and widely supported. Retry-After: "120 seconds" incorrectly quotes and adds text. Retry-After: Wed, 21 Oct 2030 07:28:00 GMT (current time + 2 minutes) uses a date but must be exact and updated dynamically, which is complex. Retry-After: After 2 minutes is invalid text.

3. Final Answer:

   Retry-After: 120 -> Option D

4. Quick Check:

   Use seconds as integer for simple Retry-After [OK]

Hint: Use integer seconds for Retry-After to ensure compatibility [OK]

Common Mistakes:

• Adding quotes or text to seconds value
• Using a fixed date without updating
• Writing natural language instead of valid format