Introduction

This chapter discusses the issues you will need to take into account when planning a multiserver installation.

Although WebLogic server can be used in a standalone configuration, in most environments you will want to take advantage of the load-balancing and high-availability features available when multiple servers are installed and configured to act together as a cluster. The exact nature of the cluster you create will depend on the needs of the application(s) you run at your site.

Considerations include:

availability

application architecture

security

Application Structure

In the most basic configuration, all the components of your application may run on a single physical machine. It is, of course, possible and frequently desirable to spread the application across multiple pieces of physical hardware. However, regardless of the number of physical components, a properly designed application will generally be divided into two or more logical components, called tiers.

Web	The Web tier is responsible for serving the static components of your application (HTML pages) to clients .
Presentation	The presentation tier generates dynamic content. Historically, the presentation tier included CGI programs run as separate executables and perhaps scripted output executed by mod_perl or some other server module. In the case of WebLogic this tier is more likely to be composed of JSPs or servlets. However, it is still possible to incorporate legacy scripts through the use of Apache, IIS, and Netscape server proxy plug-ins.
Object	The object tier is optional. [1] If your application provides Enterprise JavaBeans or Remote Method Invocation classes to its clients, you have it.
Database	For security and performance reasons, the database tier is likely to be located on a separate machine, perhaps behind a second firewall.

^[1] Well, strictly speaking, all the tiers are optional, but the other three are always recommended.

BEA Systems recommends that you install your servers in one of two possible configurations, basic or multiple tier. In a basic installation most or all of the tiers are run on a single server or a single cluster of identically configured servers. This architecture is the easiest to administer and has the capability for load balancing and high availability.

In a multiple-tier architecture you sacrifice some ease of administration for better load balancing and failover capabilities. In particular, with multiple-tier architectures you have the capability for balancing the load to method calls of clustered EJBs.