Datasheet

SQL Server 2000 – Particulars and History
21
Three-Tier
This model, and the closely related one that follows (n-tier), are the much-hyped architectures of today.
If you hear someone talking about how everything needs to be done one way regardless of what it is –
they are almost certainly talking three-tier or n-tier computing.
Three-Tier System
Client
User
Services
Server(s)
Data Services
Database
Business
Services
This model takes the approach of breaking up all three service levels into completely separate logical
models. Clients are responsible for UI issues only – just as they were under server-centric client-server.
The difference is that the business and data services are logically separated from each other. In addition,
this approach moves the logical model into a distinctly separate realm from the physical model. This
means that business and data services can run on the same server, but do not have to. This adds a
significant level of stability and scalability since you can split the workload onto two (and, depending on
how it's done, more) servers. In addition, this model has a tendency to be more extensible, since changes
and additions affect smaller pieces of code (instead of one huge build of everything, you can just rebuild
the affected components).
Since everything is component based, you can, if you use DCOM, take an approach where you
distribute the components over many servers. If you use Microsoft Transaction Server (MTS), or
COM+, which comes with Windows 2000, you can even keep copies of the same component on
multiple servers for load balancing.
Advantages Disadvantages
Some upgrades can be done entirely at the
server level.
Other upgrades still require a "touch" on every
client computer – upgrades and new installs are
both very tedious and difficult logistically.
An increasing number of homogeneous
products are available off the shelf – pre-
made software is cheap.
Typically there is still considerably more
downtime than with a host system.
Since only the information to be displayed is
sent on the network, there is little network
bandwidth used compared to the client-centric
model. The load may, however, be higher
between the business-logic and data-services
systems if they are on different servers.
Performance can be degraded due to COM
and marshalling, especially across servers or
even networks. This includes any access over
the Internet.
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