SDN Controller Programming Guide

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Figure 1 Controller Tiers
The Administration tier of the controller appliance will host a web-layer through which software
modules installed on the appliance can expose REST API [1] [2] (or RESTful web services) to other
external entities. Similarly, modules can extend the available web-based GUI to allow network
administrators and other personae to directly interact with the features of the software running on
the SDN Controller Appliance.
A web application is an application that is accessed by users over a network such as the Internet
or an intranet. The HP VAN SDN Controller runs on a web server as illustrated in Figure 2.
Figure 2 Web Application Architecture
Servlets [3] [4] is the technology used for extending the functionality of the web server and for
accessing business systems. Servlets provide a component-based, platform-independent method for
building Web-based applications.
SDN applications don’t implement Servlets directly but instead they implement RESTful web
services [1] [2] which are based on Servlets; however RESTful web services also act as controllers
as described in the pattern from Figure 3.