If-Unmodified-Since
A requisição If-Unmodified-Since
do cabeçalho HTTP realiza uma requisição condicional: o servidor enviará de volta o recurso solicitado ou o aceitará no caso de um POST
ou non-safe método, somente se não tiver sido modificado pela última vez após a data especificada. Se a solicitação tiver sido modificada após a data já especificada, a resposta será um erro 412
(Precondition Failed).
A requisição
There are two common use cases:
- In conjunction with non-safe methods, like
POST
, it can be used to implement an optimistic concurrency control, like done by some wikis: editions are rejected if the stored document has been modified since the original has been retrieved. - In conjunction with a range request with a
If-Range
header, it can be used to ensure that the new fragment requested comes from an unmodified document.
Header type | Request header |
---|---|
Forbidden header name | no |
Sintaxe
If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
Diretivas
- <day-name>
- One of "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", or "Sun" (case-sensitive).
- <day>
- 2 digit day number, e.g. "04" or "23".
- <month>
- One of "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case sensitive).
- <year>
- 4 digit year number, e.g. "1990" or "2016".
- <hour>
- 2 digit hour number, e.g. "09" or "23".
- <minute>
- 2 digit minute number, e.g. "04" or "59".
- <second>
- 2 digit second number, e.g. "04" or "59".
GMT
-
Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.
Exemplos
If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT
Especificações
Especificação | Título |
---|---|
RFC 7232, sessão 3.4: If-Unmodified-Since | Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests |
Compatibilidade com o Browser
BCD tables only load in the browser
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Veja também
Last-Modified
If-Modified-Since
If-Match
If-None-Match
If-Range
412
Precondition Failed