com.smartgwt.client.docs

## Interface Caching

-     public interface Caching

  ### Caching

  Standard web browsers can cache server responses, associating the locally-cached files with the URLs (including query parameters) that were used to fetch the files from the server. Each file may be assigned an explicit expiration time. Requests for the associated URL will always be served from the local cache, without accessing the server, until the file expires.

  The recommended approach is to move as much content as possible into cacheable assets (these can be images, html, css, and js) and tell the browser to cache those for as long as possible (ideally indefinitely). Clearly, most things can't simply be cached permanently - new versions of the application will often require changes to these assets. To allow for this, the pages that direct the loading of the cached assets should be dynamic and should create version-specific URLs to these cacheable assets. This can be done by tacking the version number as a query parameter or as a path component. Here's an example of loading a javascript file versioned with a query parameter:

        <script src='/foo/bar.js?version=13'></script>

  Generally, the version number wouldn't be hard-coded into the dynamic page, but would instead pick up the value of a variable, such that you can simply bump up the value in one configuration file and have all versioned URLs change dynamically.

  Because Smart GWT performs the assembly of image URLs dynamically, versioning of Smart GWT skins must be done with a path component. You can do this by changing the base skinDir using `Page.setSkinDir()` - e.g:

        Page.setSkinDir('/version/5.6/isomorphic/skins/SmartGWT/');

  You can then either deploy the new skins under the versioned directory above or use a URL rewriting engine such as mod_rewrite for Apache to map all such versions into a single deploy directory.

  To actually tell the browser to cache images for a longer length of time than the browser session, you need to set the HTTP 'Expires' header. If you're not using the Smart GWT Java back-end there are several caching solutions available, depending on your server of choice. Microsoft's IIS has built-in caching capability, please check the reference manual for details. If you're using Apache, you can use mod_expires. Some servlet containers also natively support the setting of caching headers.

  The Smart GWT Java back-end supports setting caching headers via the FileDownload service on a per-mimetype basis. To use it, first register the FileDownload servlet in your web.xml as follows:

            <servlet>
              <servlet-name>FileDownload</servlet-name>
              <init-param>
                  <param-name>expires</param-name>
                  <param-value>text/javascript:3600,image/gif:86400</param-value>
              </init-param>
              <servlet-class>com.isomorphic.servlet.FileDownload</servlet-class>
            </servlet>

  The expires parameter controls the expiration time in seconds. In the block above, javascript files are set to expire in 1 hour and gif images are set to expire in 1 day from the time they are served to the browser. If you don't set explicit expires mappings, all images and css files will be set to expire in 1 day and javascript files will expire in 1 hour, by default.

  Next, map any resource that you want to serve with caching headers to the FileDownload servlet in your web.xml. Typically, you'll want to serve the Smart GWT modules and all skin images with caching headers. You can do so by adding the following servlet-mapping directives to your web.xml:

            <servlet-mapping>
              <servlet-name>FileDownload</servlet-name>
              <url-pattern>/isomorphic/system/modules/*</url-pattern>
            </servlet-mapping>

            <servlet-mapping>
              <servlet-name>FileDownload</servlet-name>
              <url-pattern>/isomorphic/skins/*</url-pattern>
            </servlet-mapping>