As we mentioned, application servers are usually built around Java applications. This is for several reasons: Java technologies--particularly Enterprise JavaBeans--allow you to extend the basic capabilities inherent within Java to areas like multitasking, authentication and security. However, the reality is that almost any programming language or scripting language can be implemented on any of the leading application servers, and it's not uncommon to find C/C++ programs or Perl or Tcl scripts running on the application server in addition to Java. The move toward Java Server Pages, or JSP, among application-server vendors shows that Java is still the primary language used in application servers. (JSP technology is similar to Microsoft's Active Server Pages, or ASP technology, in creating Web pages with dynamic content.)

Within the world of application servers, data sources can mean many things. In most situations, the main data sources will be databases from the likes of Oracle, Informix, Sybase, or Microsoft. This access can come in the form of native database drivers--which is preferable, as a native driver will outperform a generic ODBC or JDBC driver--or other back-end data sources like ERP systems (such as BEA Tuxedo or SAP R/3) and transaction managers. It can also mean connecting with authentication tools like a RADIUS server.

How do you shop for an application server to run on Linux? First, you'll need to look at your traffic levels and decide whether a Linux-based server makes sense. Generally speaking, the leading application servers are designed for high-traffic areas, and adding a Linux server to a cluster--especially with the improved symmetrical-processing support found in kernel versions 2.2 or better--makes a lot of sense in a distributed-computing environment. You'll need to make sure that your particular Linux distribution is supported, first of all; Red Hat Linux is uniformly supported, but a mainstream application server should run on any mainstream Linux distribution.

Of more importance is the Java support, and it's here that application servers become finicky. IBM WebSphere, for instance, works with the Java Development Kit (JDK) from Blackdown.org (a volunteer project porting the Sun JDK to Linux) and the IBM AlphaWorks JDK, as well as the Apache HTTP Server and IBM's DB2 database. (This reliance, obviously, is of great concern to anyone not working in an IBM shop.) However, most application servers ship with both native drivers for popular databases and ODBC drivers as well.

Application servers are becoming a staple of the enterprise data-processing world. And with the advent of Linux support among the leading application-server vendors, Linux is well on its way to become an even bigger player in the enterprise-computing world.

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