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Setting HTTP Caching Headers - Last-Modified and ETag

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.

When an object stored in the cache includes a Last-Modified or ETag header, the Portal Server can use this value to ask the remote server if the object has changed since the last time it was seen.
  • The Portal Server sends the value from the Last-Modified header to the remote server in the If-Modified-Since Request header.
  • The remote server sends the ETag header to the Portal Server with portlet content. When another request is made for the same content, the Portal Server sends the value in the ETag header back to the remote server in the If-None-Match header.

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.

In .NET, the Response.Cache.SetLastModified method allows you to set the Last-Modified header to the date of your choice. Alternately, the SetLastModifiedFromFileDependencies method sets the header based on the time stamps of the handler’s file dependencies.
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.
Parent topic: About Pagelet Caching

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