Conditional requests are a part of the HTTP specification. They define a condition, determining when the server should send a response. They can define the condition that the resource must have been modified since a given timestamp (if-modified-since) or that the resource should not match with a given checksum (if-none-match). If the client has the latest, actual data, the if-none-match or if-modified-since validation will fail, and a 304 response is sent, telling the client there is no new data is available. If new data is available, a 200 response is sent, including the new data.
The If-None-Match HTTP request header makes the request conditional. For GET and HEAD methods, the server will send back the requested resource, with a 200 status, only if it doesn't have an ETag matching the given ones. For other methods, the request will be processed only if the eventually existing resource's ETag doesn't match any of the values listed.
When the condition fails (when the file has not been changed) for GET and HEAD methods, then the server must return HTTP status code 304 (Not Modified).
The comparison with the stored ETag uses the weak comparison algorithm, meaning two files can be considered identical if the content is equivalent — they don't have to be identical byte for byte. For example, two files that differ by the date of generation could still be considered as identical.
When used in combination with If-Modified-Since, If-None-Match has precedence.
Source: Mozilla Developer Network
The If-Modified-Since request HTTP header makes the request conditional: the server will send back the requested resource, with a 200 status, only if it has been last modified after the given date. If the request has not been modified since, the response will be a 304 without any body; the Last-Modified response header of a previous request will contain the date of last modification.
When used in combination with If-None-Match, it is ignored, unless the server doesn't support If-None-Match.
The most common use case is to update a cached entity that has no associated ETag.
Source: Mozilla Developer Network
- GTFS Regional
- GTFS Regional Realtime
- NeTEx Regional
Conditional requests on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/Conditional_requests
ETag on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
If-None-Match on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match
If-Modified-Since on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since