The Active Cache Protocol

  In Active Cache, the Web server specifies the association between a cache applet and a URL-named document by sending a new entity header, ``CacheApplet,'' with the document:

• CacheApplet: code = ``code.class'', archive = ``code.jar'', codebase = ``codebase_url''

The header follows the convention for applet specification in HTML documents. It specifies that the applet ``code.class'' at code base "codebase_url" is the cache applet for the document, and associated classes are grouped in an archive (4, 4). Codebase and archive directives are optional. Since HTTP/1.1 allows the introduction of new entity headers, and requires that if a proxy does not recognize an entity header, it should forward the header, the server can be assured that Active-Cache enabled proxies will receive the header even if they have parent proxies.

For security concerns, we require that the codebase of the applet, if present, has the same server URL as the document. That is, only the Web server who provides the document can supply the applet for it.

An Active-Cache enabled proxy agrees to fulfill the following obligations:

• if the document is cached and a user request hits on the document, it will either invoke the cache applet, or send the request directly to the server.
• if the applet's execution fails due to any reason, the request is sent to the server;
• if the applet's execution succeeds, the proxy will take the appropriate action based on the return value of the FromCache method (see below);
• each applet can deposit information in a special log object (whose name is the name of the applet appended by ``.log''), and the proxy will send the log object back to the server periodically. If the proxy evicts the log object from the cache, it will first send the object to the server.

In other words, the proxy will not return a cached reply to the user unless the cache applet has been executed successfully, and the applet can deposit information in the log object which will eventually be reflected back to the server.

However, the proxy decides whether it wants to cache the document, when it fetches the applet and the archive, and whether it wants to invoke the applet. Furthermore, the proxy can evict any document or any applet from its cache at any time. The only constraint on the proxy is the above agreement.

The proxy's freedom means that the cache applet may not process every request to the document, and some requests might instead go to the original server. The applet does not capture every request to the document, but captures every ``cache hit'' to the document.