When we press Enter, F5 or Ctrl+F5 to refresh a webpage, is there any difference among them? Actually it's yes, we can find the difference from the request header and response header information.
There are two cases about pressing Enter in the address bar. First if the page requested is cached and not expired in the browser, we can find the header information sent by the browser is :
Host 192.168.3.174:8080 User-Agent Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0 Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language zh-cn,zh;q=0.5 Accept-Encoding gzip, deflate Accept-Charset GB2312,utf-8;q=0.7,*;q=0.7 Connection keep-alive
The HTTP response status is 200 OK, however, there is no record for this request in the access.log of Nginx server, it means the request is not sent to the HTTP server since there is a cached file. The request header and response header in firebug are faked by the browser. This kind of refresh costs minimum bandwidth and takes little time.
The second case is the page is expired, now the HTTP request will be:
Host 192.168.3.174:8080 User-Agent Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0 Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language zh-cn,zh;q=0.5 Accept-Encoding gzip, deflate Accept-Charset GB2312,utf-8;q=0.7,*;q=0.7 Connection keep-alive If-Modified-Since Mon, 04 Jul 2011 10:12:40 GMT
There is one more line appended to the request header : If-Modified-Since, there is also one record in access.log of Nginx server. What happens is it will first check whether the file is modified after If-Modified-Since, and this time is determined by the last HTTP response's Last-Modified attribute, if it's not modified, then server will respond with 304 Not Modified status and browser then will read content from the cache. If it's modified, server will respond 200 OK and send the new content.
Regarding F5, the HTTP request header will be :
Host 192.168.3.174:8080 User-Agent Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0 Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language zh-cn,zh;q=0.5 Accept-Encoding gzip, deflate Accept-Charset GB2312,utf-8;q=0.7,*;q=0.7 Connection keep-alive If-Modified-Since Mon, 04 Jul 2011 10:12:40 GMT Cache-Control max-age=0
There is one more line Cache-Control : max-age=0 appended,it means it doesn't matter whether the file in the browser cache is expired or not, it will check on the server about the file status.
Finally regarding Ctrl+F5, the HTTP request header is :
Host 192.168.3.174:8080 User-Agent Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0 Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language zh-cn,zh;q=0.5 Accept-Encoding gzip, deflate Accept-Charset GB2312,utf-8;q=0.7,*;q=0.7 Connection keep-alive Pragma no-cache Cache-Control no-cache
If-Modified-Since is gone, instead Cache-Control is changed to no-cache, and also Pragma line is for compabtible with HTTP 1.0. This means we don't care abot the cache and we need a forced refresh and download the file from server directly. This kind of refresh cost the most bandwidth and takes most time.
