Web Logic Server is an Application Server which executes Web Application(WAR) and Enterprise Application (EAR) developed in Java.
The Oracle Fusion Middleware allows the installation of Web Logic Admin Server as well as Managed Servers like SOA Server, OSB Server and ADF Server to provide the complete deployment environment for Oracle Fusion Middleware Applications.
It has become common to have multiple server to provide performance as well as increased scalability and reliability when large applications with critical components to be executed in the heterogeneous environment. Thus, IT companies are preferring the usage of Web Logic Clustering as the optimized solution for running such applications.
Web Logic Clustering is the mechanism in which multiple Web Logic Server Instances are running and thus provides client with the single web logic server instance. Web Logic Cluster could have server instances on the same machine or can connect to multiple machines for increasing clustering capacity. It is important to note that each server instance in the Web Logic Cluster must be running with the same version of Web Logic Server.
To run the Web Logic Server through Admin Server, we need to CREATE the Web Logic DOMAIN. A Web Logic Domain is a combination of Web Logic server resources (Data Sources, Adapters, JMS Queues and Connections, etc.), application components, application services and server instances. Which means that a DOMAIN is a combination of multiple server instances which can be clustered, non-clustered, or a combination of clustered and non-clustered instances.
Each domain consists of Admin Server and other managed servers like SOA Server, OSB Server, ADF Server. The Admin Server configures, manages and monitors all other server instances. Thus, it clarifies that all server instances must resides in the same DOMAIN.
Also read to know how Web Logic Clustered environment is different from Non-clustered environment from the below given link
How Web Logic Clustered environment is different from Non-clustered environment
Which Objects are considered as part of Web Logic Clustered Environment?
In the Web Logic Clustered Environment , each task executed in terms of Object. An application object could be different type, having different behavior including object invocation process, features and functions within an application when gets consumed. Below given are the main types of objects in the Web Logic Clustered deployment.
• Remote Method Invocation (RMI) objects
• Java Messaging Service (JMS) destinations
Web logic Server provides the support for Servlets and JSPs in the Clustering Environment by replicating the HTTP Session State of the Client that accesses servlets through clustered environment.
The HTTPS Sessions State can be maintained in the memory, a file based system or in the database by the Web Logic Server as per the configuration.
For the Enterprise Java Beans and Remote Method Invocation (RMI) objects, the Web Logic Server uses the Replica-Aware Stubs which can locate the instances of the object within the clustered environment.
In the object fails for EJBs or RMIs then the the stub detects the failure and retries the call on another replica
Which Objects are NOT considered as part of Web Logic Clustered Environment?
File services with File Shares and the Time Related Services will not be considered as part of Load Balancing or Failover techniques but can still be utilized in the Web Logic Clustered Environment as independent services.