Examples of HTTP API Rate Limiting HTTP Response headers

One of the Additional HTTP Status Codes (RFC6585) is

• 429 Too Many Requests

Where can I find examples of HTTP / REST API Rate-Limiting HTTP response headers useful with this HTTP response status?

Here are some examples of HTTP API Rate Limiting HTTP Response headers. Taken from four common REST APIs: Github, Vimeo, Twitter and Imgur:

Github Rate Limiting <http://developer.github.com/v3/#rate-limiting>

#=============================#=============================================#
# HTTP Header                 # Description                                 #
#=============================#=============================================#
| X-RateLimit-Limit           | Request limit per hour                      |
+-----------------------------+---------------------------------------------+
| X-RateLimit-Remaining       | The number of requests left for the time    |
|                             | window                                      |
+-----------------------------+---------------------------------------------+

Vimeo Rate Limiting <http://developer.vimeo.com/guidelines/rate-limiting>

#=============================#=============================================#
# HTTP Header                 # Description                                 #
#=============================#=============================================#
| X-RateLimit-Limit           | Request limit per day / per 5 minutes       |
+-----------------------------+---------------------------------------------+
| X-RateLimit-Remaining       | The number of requests left for the time    |
|                             | window                                      |
+-----------------------------+---------------------------------------------+
| X-RateLimit-Reset           | The remaining window before the rate limit  |
|                             | resets in UTC epoch seconds                 |
+-----------------------------+---------------------------------------------+

Twitter REST API Rate Limiting <https://dev.twitter.com/docs/rate-limiting/1.1>

> Note: Twitter uses headers with similar names like Vimeo, but has another dash in each name.

#=============================#=============================================#
# HTTP Header                 # Description                                 #
#=============================#=============================================#
| X-Rate-Limit-Limit          | The rate limit ceiling for that given       |
|                             | request                                     |
+-----------------------------+---------------------------------------------+
| X-Rate-Limit-Remaining      | The number of requests left for the         |
|                             | 15 minute window                            |
+-----------------------------+---------------------------------------------+
| X-Rate-Limit-Reset          | The remaining window before the rate limit  |
|                             | resets in UTC epoch seconds                 |
+-----------------------------+---------------------------------------------+

Imgur API Rate Limits <http://api.imgur.com/>

#=============================#=============================================#
# HTTP Header                 # Description                                 #
#=============================#=============================================#
| X-RateLimit-UserLimit       | Total credits that can be allocated         |
+-----------------------------+---------------------------------------------+
| X-RateLimit-UserRemaining   | Total credits available                     |
+-----------------------------+---------------------------------------------+
| X-RateLimit-UserReset       | Timestamp (unix epoch) for when the credits |
|                             | will be reset                               |
+-----------------------------+---------------------------------------------+
| X-RateLimit-ClientLimit     | Total credits that can be allocated for the |
|                             | application in a day                        |
+-----------------------------+---------------------------------------------+
| X-RateLimit-ClientRemaining | Total credits remaining for the application |
|                             | in a day                                    |
+-----------------------------+---------------------------------------------+

In addition to API specific headers, don't forget the humble, standard Retry-After header

> Servers send the "Retry-After" header field to indicate how long the user agent ought to wait before making a follow-up request.... > The value of this field can be either an HTTP-date or a number of seconds to delay after the response is received.

The standard makes specific additional recommendations when using it with a 503 or 3xx status code:

> When sent with a 503 (Service Unavailable) response, Retry-After indicates how long the service is expected to be unavailable to the client. When sent with any 3xx (Redirection) response, Retry-After indicates the minimum time that the user agent is asked to wait before issuing the redirected request.