In multi-tier computing, application logic is held in an application server, such as Sybase EAServer, WebLogic, or WebSphere, which sits between the database server and the client applications. In many situations, a single application server can access multiple databases in addition to non-relational data stores. In the Internet case, client applications are browser-based, and the application server is generally a web server extension. Many modern multi-tier applications use a service-oriented architecture (SOA) based on web services.
Sybase EAServer stores application logic in the form of components, and makes these components available to client applications. The components can be PowerBuilder components, Java beans, or COM components.
Application servers can also provide transaction logic to their client applications—guaranteeing that sets of operations are executed atomically across multiple databases. SQL Anywhere is well suited to multi-tier computing, and can participate in distributed transactions coordinated by Microsoft Distributed Transaction Coordinator. Both Sybase Enterprise Application Server and Microsoft Transaction Server use DTC to provide transaction services to their client applications.
The built-in support for web services makes SQL Anywhere a good choice for many multi-tier or SOA applications.
See Three-Tier Computing and Distributed Transactions.