Multi-client XML web applications

Modern XML technologies provide a full stack of modeling and implementation languages as well as tools for web applications that are stable, platform independent, and based on open standards. A particular strong point of what we call the X stack is that data are encoded with XML end-to-end and that XML technologies can be used wherever XML data need to be processed. In this paper, we demonstrate two solutions how to incorporate server push into the X stack. Server push is needed in modern web applications such as Google Docs, when a client request results in an update in server state, an update of which a second client needs to be notified without having issued a preceding request to which the server can respond. Both solutions are based on the WebSocket protocol and browser-side implementations of WebSocket. The first solution provides a middleware router component that uses an existing server-side WebSocket implementation, at the cost of introducing a new kind of component into our architectures. The second solution provides a native server-side implementation of WebSocket in an XML database system, in our case BaseX, at the cost of extending a standard component in our architecture. Both solutions adhere to the principle of end-to-end encoding of data in XML and processing of data with XML technologies. Finally, we propose and support a new HTML element xml-stream that represents the receiving end of a WebSocket connection in an HTML document. The new element allows developers to configure in a declarative fashion how incoming date should be handled. This element can be used in the X stack but can also declaratively support the technologies.