How it works

The Embedded Web server supports the HTTP local caching by the browser. For static resources, what are basically all except the scripts, the server sends the http header with the last modified date tag in the response.

Resource not cached

Here is an example, recorded from HTTP_demo. The browser opens a web page, which is not yet cached locally.

• The browser opens a default page:

  http://mcb2300
  

• This generates the following http request for the web server:

  GET / HTTP/1.1
  Accept: image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-
  shockwave-flash, application/vnd.ms-excel, application/vnd.ms-powerpoint,
  application/msword, application/xaml+xml, application/vnd.ms-xpsdocument,
  application/x-ms-xbap, application/x-ms-application, */*
  Accept-Language: en-us
  User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1;
  Trident/4.0; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727; .NET CLR
  3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.0.4506.2152; .NET CLR
  3.5.30729)
  Accept-Encoding: gzip, deflate
  Host: mcb2300
  Connection: Keep-Alive
  

• The web server opens a default page index.htm and sends it to the browser with the following http header:

  HTTP/1.1 200 OK
  Server: Keil-EWEB/2.1
  Content-Type: text/html
  Last-Modified: Thu, 19 Nov 2009 07:46:25 GMT
  Connection: close
  // and the content of 'index.htm'
  

• The browser, when receiving the Last-Modifed http header, stores this file to local cache together with the url and date tag.

Resource cached

When the browser tries to open the same page again, it first checks the local cache. The file index.htm is already there, so it sends a different request to the Web server.

• The browser sends a different http header in the request:

  GET / HTTP/1.1
  Accept: */*
  Accept-Language: en-us
  User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1;
  Trident/4.0; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727; .NET CLR
  3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.0.4506.2152; .NET CLR
  3.5.30729)
  Accept-Encoding: gzip, deflate
  If-Modified-Since: Thu, 19 Nov 2009 07:46:25 GMT
  Host: mcb2300
  Connection: Keep-Alive
  

• Now, the Web server is also informed that the requested file is cached. The Web server checks the date, if the browser caches the same file version. If the date tags are equal, the Web server sends only a short response:

  HTTP/1.1 304 Not Modified
  Server: Keil-EWEB/2.1
  Connection: close
  

• The browser now uses the locally cached index.htm

Outdated resource cached

If the date tags are not equal, the file cached by browser is outdated. The Web server sends the updated index.htm. The browser updates the local cache and uses an updated file.

• HTTP/1.1 200 OK
  Server: Keil-EWEB/2.1
  Content-Type: text/html
  Last-Modified: Thu, 19 Nov 2009 09:38:54 GMT
  Connection: close
  // and the new content of 'index.htm'