The data access layer is used by the business logic layer. The business logic layer abstracts business transactions by hiding the logic and implementation details of the transaction from the upper layers. The workflow, or business fazade, layer is the next level. It provides simple interfaces, also known as fazades, to the presentation layer. Internally, it manages state and uses the atomic operations exposed by the business logic layer to complete complicated workflow. Lastly, there is the presentation layer, which transforms the results returned by the workflow layer for the user. .
Several physical architectures derive from this logical architecture.
Physical Architecture.
Distributing the Load.
SQL Server XML allows the database to read and write data but it also does much more. XML capabilities allow stored procedures to deal with large amounts of highly structured data. The information can be passed to the stored procedure as XML, allowing for the possibility of implementing business logic or workflow as stored procedures, instead of as COM+ or script. This means that more of the application's processing can be moved down to the database tier.
The decision on how to distribute the processing that the application does affests the hardware and software the application will need, the kind of expertise needed to develop it, and the process by which it is deployed, updated, and maintained. One can either choose a server configuration where the Web servers perform most of the work ("top-heavy") or a configuration where the database server does most of the work ("bottom-heavy").
Two factors make the "top-heavy" server farm the model of choice for most applications: .
1. Cost. The software and hardware for database servers tends to be more expensive than for Web servers. .
2. Scalability. Database scalability in SQL Server 2000 is improved over SQL Server 7.0, but to take full advantage of the power of new hardware, careful planning and an effective maintenance plan are necessary.
