4.2. Network Configurations
You can select from three basic types of network configurations when designing an Oracle infrastructure:
Single tier
Two tier
n-tier
Single tier is the simplest type. It has been around for years and is characterized by the use of terminals for serial connections to the Oracle server. The two-tier configuration is also referred to as the client/server architecture, and more recently the n -tier architecture has been introduced. Let's take a look at each of these configuration alternatives.
4.2.1. Single-Tier Architecture
Single-tier architecture was the standard for many years before the birth of the PC. Applications using single-tier architecture are sometimes referred to as green-screen applications because most of the terminals that used them, such as the IBM 3270, had green screens. Single-tier architecture is commonly associated with mainframe-type applications.
This architecture is still in use today for many mission-critical applications, such as order processing and fulfillment and inventory control, because it is the simplest architecture to configure and administer. Because the terminals are directly connected to the host computer, the complexities of network protocols and multiple operating systems don't exist.
When a single-tier architecture is used, users interact with the database using terminals, which are nongraphical, character-based devices. Figure 4.1 shows an example of the single-tier architecture. In this type of architecture, client terminals are directly connected to larger server systems such as mainframes. All the intelligence exists on the mainframe, and all processing takes place there. Simple serial connections also exist on the mainframe. Although no complex network architecture is necessary, a single-tier architecture is somewhat limiting in terms of scalability and flexibility. Because all the processing must take place on the server, the server can become the bottleneck to increasing performance.
Figure 4.1. Single-tier architecture
4.2.2. Two-Tier Architecture
Two-tier architecture gained popularity with the introduction of the PC and is commonly referred to as client/server computing. In a two-tier environment, clients connect to servers over a network using a network protocol, which is the agreed-upon method for the client to communicate with the server. TCP/IP (Transmission Control Protocol/Internet Protocol) is a popular network protocol and has become the de facto standard of network computing. Whether you choose TCP/IP or some other network protocol, both the client and the server must be able to understand it. Figure 4.2 shows an example of a two-tier architecture.
This architecture has definite benefits over single-tier architecture. First, client/server computing introduces the graphical user interface (GUI). This interface is easier to understand and learn, and it offers more flexibility than the traditional character-based interfaces of the singletier architecture. Also, two-tier architecture allows the client computer to share the application process load. To a certain degree, this reduces the processing requirements of the server.
The two-tier architecture does have some faults, even though at one time, this configuration was thought to be the panacea of all networking architectures. Unfortunately, the main problem—that being scalability—persists. Notice that the term client/server contains a slash (/). The slash represents the invisible component of the two-tier architecture and the one that is often overlooked: the network! The limitation of client/server computing is one of scalability.
When prototyping projects, many developers fail to consider the network component and soon find out that what worked well in a small environment does not scale effectively to larger, more complex systems. The two-tier architecture model was subject to a great deal of redundancy because application software was required on every desktop. As a result, many companies end up with bloated PCs and large servers that still do not perform adequately. What is needed is a more scalable model for network communications. That is what n -tier architecture provides.
4.2.3. N-Tier Architecture
N-tier architecture is the next logical step after two-tier architecture. Instead of dividing application processing work between a client and a server, you divide the work among three or more machines. The n-tier architecture introduces middleware components, such as application servers or web servers, situated between the client and the database server, which can be used for a variety of tasks, including the following:
Moving data between machines that work with different network protocols
Serving as firewalls that can control client access to the servers
Offloading processing of the business logic from the clients and servers to the middle tier
Executing transactions and monitoring activity between clients and servers to balance the load among multiple servers
Acting as a gateway to bridge existing systems to new systems
The Internet is an example of the ultimate n -tier architecture, with the user's browser providing a consistent presentation interface. This common interface means less training of staff and also increases the potential reuse of client-side application components.
Figure 4.2. Two-tier architecture
N-tier architecture is rapidly becoming the architecture of choice for enterprise networks. This model is scalable and divides the tasks of presentation, business logic and routing, and database processing among many machines, which means that this model accommodates large applications. Many factors are driving n -tier computing, such as the Internet and Oracle grid computing, which uses a large number of back-end processors to scale database services and connectivity.
By reducing the processing load on the database servers, those servers can do more work with the same number of resources. Also, the transaction servers can balance the flow of network transactions intelligently, and application servers can reduce the processing and memory requirements of the client (see Figure 4.3).