Service-oriented architecture (SOA) is an evolution in enterprise architecture, not a revolution. It captures and uses the best practices of the architectures-monolithic, two-tier and N-tier -- that came before it. Yet arriving at a common definition of SOA is not easy.

Applicability of SOA
Enterprise IT departments typically run applications that can be broadly classified into backbones and front-ends. The backbone is comprised of all the back-end systems. This includes the applications developed on J2EE, Microsoft .NET, CICS mainframes and various other technologies that contain the business logic. It also includes the stored procedures, data in the databases and data stored in various other formats. The backbone can be considered the combination of all the business tiers and data tiers of the enterprise.