The Last-Modified response header specifies the last time a change was made in the returned content, in the form of a time stamp. ETag values are unique identifiers generated by the server and changed every time the object is modified. Either can be used to determine if cached content is up to date.
The portlet code on the remote server uses the header value to determine if the content being requested has changed since the last request, and responds with either fresh content or a 304 Not Modified Response. If the Portal Server receives the latter, it displays the cached content.
JSP portlets can access the value in the If-Modified-Since request header using the getLastModified(HttpServletRequest req) method provided by the Java class HttpServlet.
Response.Cache.SetLastModified(DateTime.Now);To use ETag in .NET, use the Response.Cache.SetETag method to pass in the string to be used as the ETag. The SetETagFromFileDependencies method creates an ETag by combining the file names and last modified timestamps for all files on which the handler is dependent.