Oracle Application Server 10g Architecture and Administration - Client Tier, Web Tier, OHS, and Web Cache (
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Client Tier
The client level consists of either a Java client application or a web browser. Using a web browser as the client allows the entire application to be located on the server. The client always gets the latest version when the application starts. Also, the client can use any web browser from any location, provided that the client can connect to the application server. The client tier can also consist of an application running on the client’s desktop (usually presenting a rich user interface) and connecting directly or through HTTP to the application server. This requires that the client have the application installed on the desktop.
The web server layer contains two important components, the Oracle HTTP Server (OHS) and Web Cache component (Figure 1-6). This tier is responsible for managing incoming HTTP requests, caching web messages, and sending XML and HTML back to the client.
Let’s take a closer look at the components inside the web tier.
Oracle HTTP Server (OHS)
All Oracle web systems must have enough listener processes so that a single port is not overwhelmed with incoming requests. The Oracle HTTP Server is a component of Application Server 10g that listens on a specific port and forwards J2EE incoming requests through mod_oc4j to the least-loaded OC4J container. It is imperative that the web servers have load-balancing intelligence so that a single OC4J container is not overloaded with work. Oracle has addressed this issue by incorporating the open source Apache product into the Oracle HTTP Server and providing the load-balancing capability to the mod_oc4j module. This makes customization quite easy.
Another huge benefit of the web listener load balancing is that you can customize the Web Cache to load-balance multiple Oracle HTTP Servers, thereby improving scalability. When the existing app servers become overwhelmed, more app servers can be easily added to the architecture.
It is the job of the web servers to manage the flow of the HTML and XML. On the incoming end, the web server validates and parses incoming XML strings. For outbound transactions, the web server takes data from the application server and creates the outbound HTML pages or XML strings. When an incoming transaction requests services, OHS either serves the HTML page or forwards the transaction to an OC4J container where the appropriate component (JSP engine, servlet, or Enterprise JavaBean) services the request.
Web Cache
The Application Server 10g Web Cache significantly enhances performance by reducing the need to regenerate dynamic or static information. The Web Cache is positioned in front of the HTTP server and stores both static and dynamic web content. It has a number of unique features, including partial-page caching, content-aware web server load balancing, the ability to cluster Web Caches so that multiple caches operate as a single logical cache, and the ability to cache content from third-party servers such as Sun, IBM, BEA, and others. Internal Oracle tests reveal that adding the Application Server 10g Web Cache to a three-tiered application (client, application server, and database tiers) can reduce the load on the database back end by 95 percent. The Web Cache feature has a dramatic impact on the ability of the application to scale to meet growing e-commerce demands.
Now, let’s drill-down and examine the central tier, the app server tier.
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 This chapter is from Oracle Application Server 10g Administration Handbook, by Garmany and Burleson. (McGraw-Hill/Osborne, 2004, ISBN: 0072229586). Check it out at your favorite bookstore today. Buy this book now.
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