Before moving into understanding the 3-Tier architecture, let us understand difference between Node, Tier and server. This is a basic understanding that all dbas should be knowing.
Node: A node is a physical entity. Example is laptop, computer, PC etc.
Tier: Tier is a logical grouping of services. Example, middle tier consist of Web, Forms, Reports etc services. Other tiers are desktop and database tiers.
Server: Server is a process or a group of processes that provides a specific functionality. Example, Web server listens and processes HTTP requests.
Now, Very Important point - one node can have more than one tier on it. For example, let us take your PC as the node. Now if you install Oracle E-Business suite to your PC including the Oracle database, then your PC will have all the three tiers available in it i.e., desktop tier (which is your PC), application and database tiers (that you will install).
The same case is true vice-versa as well i.e., one tier can be on more than one node. Example, Application tier can be distributed on 2 machines.
Now, as we have understood the difference between node, tier and server and also the relation between each other, let us understand the 3-Tier architecture with the help of the below diagram.
As the name suggests, 3-Tier architecture is made of 3 tiers (I hope you are clear with the terminology tier now). These tiers are:
Desktop Tier
This tier is used by the users to access the application or the database. These can be client machines that are accessing the applications or the database. Like in the diagram, our desktop tier is made of PCs that are accessing the applications. All these clients can be located anywhere across the world. They connect to the internet through the network. These requests then hit a firewall on a specific port that is open. This firewall can be a load balancer as well that distributes the requests among the application servers based on the load.
Middle Tier
This tier is also knwon as Application Tier because most of all application softwares are installed in this tier. As can be seen in the diagram, there are 3 nodes on which application tier is installed. Components that come under application tier in Oracle E-Business Suite are Web server, forms server, admin server, reports server etc. We will discuss all these components one by one in later posts.
Database Tier
This is the tier where your database resides and hence the data. Desktop tier uses application tier to access data in the database tier. I hope this is clear for you now.
I will submit other posts soon.
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