RetryAfter

RetryAfter is an HTTP response header that communicates to a client when it should retry a request after a temporary failure, such as rate limiting or maintenance. It helps clients implement backoff strategies and reduces unnecessary traffic to the server.

The value of Retry-After can be either an HTTP-date or a delta-seconds value. If delta-seconds is used, it indicates the number of seconds to wait from the time the response is generated before retrying. If an HTTP-date is used, it specifies an absolute date and time after which the client may retry. The date is expected to be in the standard HTTP-date format.

Usage and scope: Retry-After is most commonly sent with 429 Too Many Requests and 503 Service Unavailable responses. It is defined in RFC 6585 and is optional; not all responses include the header. Servers use Retry-After to signal short outages, maintenance windows, or predictable rate-limit throttling.

Client considerations: Clients should respect the specified wait time and avoid hammering the server. If Retry-After is absent, clients may employ their own backoff strategy, such as exponential backoff. If the value is an HTTP-date in the past or otherwise invalid, behavior is not strictly defined by the standard, but many clients treat it as an immediate retry or ignore the header.

Examples: Retry-After: 120 and Retry-After: Fri, 12 Oct 2023 10:00:00 GMT illustrate the delta-seconds form and the absolute-date form, respectively.