In the past, the three-tier architecture
involved a transaction processing monitor, but now more frequently uses an
application server. Clients access a transaction processing (TP) monitor or application
server in the middle tier that, in turn, accesses a database server on the backend.
The notion of a TP monitor dates back to the original mainframe OLTP systems. Of
course, in the mainframe environment all logic ran on one machine. In an open system
environment, application servers typically run on a separate machine (or
machines), adding a middle tier between clients and the database server.
There are various classes of application servers:
??? Older, proprietary servers such as Tuxedo from BEA Systems on Unix and
Windows, or CICS from IBM on mainframes
??? Industry-standard application servers based on Java 2 Enterprise Edition (J2EE)
??? The Microsoft .NET application server environment as part of the Windows
operating systems for servers, for example, Windows 2000 or Windows 2003
Application servers provide an environment for running services that clients call.
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