A common model for a three tier system is:
- Data Management Tier
- Business Logic Tier
- UI Presentation Tier
The Web services technologies allow you to create rapidly reusable program-to-program communication interfaces. Those interfaces can allow access to any of the components (at any of the tiers) depending on your design choice. For example, RDBMSs are now shipping with native Web service interfaces. However, the most common case is that you put a Web service interface into the middle business logic tier.
Today enforcement of transactional integrity is typically done by the infrastructure that implements the business logic (for example a Java Application Server) or the data management (for example a RDBMS). We are not currently seeing transactional integrity being enforced in the Web services infrastructure itself. The Web services infrastructure is focused on communication related issues: security, monitoring, standards interoperability, service virtualization, etc.