Three Tier Architecture on a LAN
As the complexities and security requirements of a company increase the IT department will move to a
three (or more) tier architecture. The front end will still be the user's desktop PCs. But now the server
function has been divided into two (or more) parts.
- A data server that runs the DBMS or other data store.
- A departmental server that usually routes traffic, controls security and does some business processing.
One of those business processes is the generation of SQL statements to communicate with the data
server. Now, when the request is received by the departmental server it is sent on to the data server
through a physical link between the machines, most probably a 100 Mb or 1 Gb Ethernet LAN.
As we will see in an example on the Web below, the three-tier model can expand to many more levels,
called n-tier. Each additional machine is specialized to perform one part of the business process. Adding
to the base of a departmental server and a data server, common additional servers include a security
server and a server to handle the processing of the business logic.
Note that not all requests will be routed through all the servers. For example, a security server may
perform some initial authentication and then the actual SQL requests may then be permitted to flow
directly to the data server.
Two-Tier Architecture on the Web
Here we have the user's PC running a browser, which is connected via the web to a server. This is a
lightweight website because the web server, the script processor and the database software are all held
on one machine. Thus we have a two-tier system: PC/Browser and Web Server. In this case, the
connection is between the script interpreter and the database.
Note that the way we use the word "connection" in this scenario is not the connection from the
browser to the web site. In a two-tier system both the SQL requester and the datastore ends of the
connection are on the same machine. The connection here is only a logical channel through the
server's memory; there is no need for an external physical connection as in some other scenarios.
Three-Tier Architecture on the Web
As with a web server, the datastore may be on the same server or a separate machine(s). This
architecture is the logical next move when the two-tier system (above) becomes overloaded. Just as with
the 3-tier LAN system, we have moved the DBMS to another physical machine and have thus been able
to optimize the hardware and OS to support reliability, security and scaling of a data store. The web
server can also then be re-optimized for serving pages and executing scripts without having to
accommodate a data service.
The front end shifts further from the SQL requester in a three-tier system. The front end (point of user
interface) becomes the PC running a Browser. The SQL requester is the business logic software on the
web server, not the user's PC. The web server will then pass information back and forth to the end
user's PC.
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