In the SQL-NS application model, both events and subscriptions are just data. SQL-NS uses SQL joins to match the events and subscriptions, and because SQL Server can usually execute these efficiently, notification applications built to this model tend to scale well.
Building an application on SQL-NS involves writing an ADF: an XML document that describes the schemas, logic, and component configuration of the application. This chapter described the ADF for a sample application: one that sends stock price updates to subscribers. The information in this chapter should give you a feel for SQL-NS applications and what's involved in building them. Later chapters provide the details and teach you how to build your own applications.