The New Distribution of The 3-Tiered Architecture Changes Everything

As the majority of an application’s presentation layer logic moves to the client it induces changes that impact the entire application delivery ecosystem

The increase in mobile clients, in demand for rich, interactive web applications, and the introduction of the API as one of the primary means by which information and content is shared across applications on the web is slowly but surely forcing a change back toward a traditional three-tiered architecture, if not in practice then in theory. This change will have a profound impact on the security, delivery, and scalability of the application but it also forces changes in the underlying network and application network infrastructure to support what is essentially a very different delivery model.

What began with Web 2.0 – AJAX, primarily – is continuing to push in what seems a backward direction in architecture as a means to move web applications forward. In the old days the architecture was three-tiered, yes, but those tiers were maintained almost exclusive on the server-side of the architecture, with the browser acting only as the interpreter of the presentation layer data that was assembled on the  server. Early AJAX applications continued using this model, leveraging the out-of-band (asynchronous) access provided by the XMLHTTPRequest object in major  browsers as a means to dynamically assemble smaller pieces of the presentation layer. The browser was still relegated primarily to providing little more than rendering support.

Enter Web 2.0 and RESTful APIs and a subtle change occurred. These APIs returned not presentation layer fragments, but data. The presentation layer logic required to display that data in a meaningful way based on the application became the responsibility of the browser. This was actually a necessary evolution in web application architecture to support the increasingly diverse set of end-user devices being used to access web applications. Very few people would vote for maintaining the separation of presentation layer logic used to support mobile devices and richer, desktop clients like browsers. By forcing the client to assemble and maintain the presentation layer that complexity on the server side is removed and a single, unified set of application logic resources can be delivered to every device without concern for cross-browser, cross-device support being “built in” to the presentation layer logic. 

This has a significant impact on the ability to rapidly support emerging clients – mobile and otherwise – that may not support the same robust set of capabilities available on a traditional browser. By reducing the presentation layer assembly on the server side to little more than layout – if that – the responsibility for assembling all the components and their display and routing data to the proper component is laid on the client. This means one server-side application truly can support both mobile and desktop clients with very little modification. It means an API provided by a web application can not only be used by the provider of that API to build its own presentation layer (client) but third-party developers can also leverage that API and the data it provides in whatever way it needs/chooses/desires.

This is essentially the point to which we are almost at today.