Level 0 SOAs are SOAs that simply send SOAP messages from system to system. There is little notion of true services, but instead, they leverage Web services as an information integration mechanism. Hardly SOA, but certainly a first step.

Level 1 SOAs are SOAs that also leverage everything in Level 0 but add the notion of a messaging/queuing system. Most ESBs are level 1 SOAs, leveraging a messaging environment that uses service interfaces, but really does not deal with true services (behavior), and instead moves information between entities as messages through queues.

Level 2 SOAs are SOAs that leverage everything in Level 1, and add the element of transformation and routing. This means that the SOA can move information from source and target systems, leveraging service interfaces, as well as transform the data/schemas to account for the differences in application semantics. Moreover, by adding the element of intelligent routing, you're able to route the information based on elements such as source, content, and logical operators in the SOA.

Level 3 SOAs are SOAs that leverage everything in Level 2, adding a common directory service. The directory provides a point of discovery of processes, services, schemas, and such, allowing all those who leverage the SOA to easily locate and leverage assets such as services. Without directories, the notion of service reuse--the real reason for building SOAs--won?t work. Directories are typically standards-based, including UDDI, LDAP, and sometimes more proprietary directories such as Active Directory.

Level 4 SOAs are SOAs that leverage everything in Level 3, adding the notion of brokering and managing true services. Here is where the brokering of application behavior comes into play. In other words, at this level we are not only about managing information movement, but the discovery and leveraging of true services.

Finally, Level 5 SOAs are SOAs that leverage everything in Level 4, adding the notion of orchestration. Orchestration is key, providing the architect with the ability to leverage exposed services and information flows, creating, in essence, a "meta-application" above the existing processes and services to solve business problems.