Static files (example: JS\CSS\HTML.) are cached by client browsers and if they don't carry cache-control directive in the HTTP-header, it is up to the browsers to determine, how often to check to check for updated value from server. Some times browsers could cache stale content for days.
This could cause the static file to be obsolete and pick from local browser cache and out of sync and could potentially result in broken pages and corrupt files.
The solution is to add cache-control directive to the web server configuration and this will force the browser to check for the updated version from the server when the time specified expires.
The functionality would essentially result in HTTP_STATUS code 200 when a file is modified and other wise would return HTTP_STATUS code 304
Testing the HTTP-Header changes:
You can using tamper data as Firefox plugin or fiddler for IE to look at the HTTP-header
Make changes to httpd.conf
1. Add the following at the end of LoadModule section in web server configuration file (httpd.conf).
LoadModule headers_module modules/mod_headers.so
2. Add the following at the end of web server configuration file (httpd.conf).
Scenario1: [Browser cache not expired and file changed - File should be fetched with 200 response]
Scenario2: [Browser cache expired and file changed - should be fetched from server with 200 response]
Scenario3: [File not changed but browser cache expired - should get 304 response]
Scenario4:[File not changed and browser cache not expired - should be serverd from browser cache]