Onionoo serves invalid cache headers
Onionoo serves invalid cache headers the first time you receive a 304. All 304 responses after that are ok until the cache max-age is reached. Then the next 304 response will have invalid headers and after that it's ok again until the new max-age is reached etc.
They are invalid because the Date header isn't reset once the max-age is reached and the Age header carries on incrementing. This results in an age that is higher than the max-age, which means the headers are saying the content has already expired before it's been sent.
Make a request at 09:10:32
Response code: 200 Date: Wed, 21 Dec 2016 09:10:32 GMT Age: 0 Cache-Control: public, max-age=300 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
AKA This content is from 09:10:32 (now) and you can cache it until 09:15:32 (5 minutes)
Now lets make a request at 09:15:41, it's been more than 5 minutes so we'll set the If-Modified-Since header to see if our cached version is still up to date
Response code: 304 Date: Wed, 21 Dec 2016 09:10:32 GMT Age: 309 Cache-Control: public, max-age=300 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
AKA This content is from 09:15:41 (now) and you can cache it until 09:15:32 (9 seconds ago).
So the headers are saying it was out of date 9 seconds before the response was sent. It wasn't, it's still fresh, it's just the Date header wasn't updated so the max-age was calculated from an old date.
If we make another request with If-Modified-Since set at 09:15:54 (13 seconds later) everything catches up:
Response code: 304 Date: Wed, 21 Dec 2016 09:15:41 GMT Age: 13 Cache-Control: public, max-age=300 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
AKA This content is from 09:15:54 (now) and you can cache it until 09:20:41 (4:47 minutes)
Notice how the Date header is now correctly set as the date of our previous request with the age header calculated from that.
This is technically not abiding by the HTTP protocol:
The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain.
In the second request in the example above the max-age in the Cache-Control headers is 300 and the age is 309 which means that Onionoos caching mechanism isn't obeying the Cache-Control header.
The end result of this is that the first time a 304 response is sent it will never be cached by a client even though the content is still fresh. In the above example 3 HTTP requests needed to be made, if the headers were set correctly only 2 requests would have been required. Fixing this should reduce the amount of traffic that hits the Onionoo server.