WEBLOGIC DOMAIN STRUCTURE

The basic administrative unit for a WebLogic Server is called a domain. A domain is a logically related group of WebLogic Server resources that you manage as a unit. A domain always includes the minimum of one instance called the Administration Server. The Administration Server is a central point of contact for server instances and system administration tools. A domain may also include additional WebLogic Server instances called Managed Servers and clusters.

A Managed Server might be part of a WebLogic Server cluster. A cluster is a group of WebLogic Server instances that work together to provide scalability and high-availability for applications. A Managed Server in a cluster can act as a backup for services such as JMS that are hosted on another server instance in the cluster.

The domain can run in two modes:

Development mode: A domain running in development mode does not require a password during start-up of the Administration Server. You also can do auto deploy of your applications, as some other parameters are not that strictly configured.

Production mode: Running in production mode needs passwords, encryption of files at a higher level, and some parameters within the resources (such as JDBC, JMS) are strictly configured.

You can have multiple domains based on:
• Physical location
• Logical division of applications (backend application domain, frontend application domain)
• Size (smaller manageable units instead of one huge domain)

ADMIN SERVER 

Each WebLogic domain has an Administration Server. As its name suggests, it's the base administration unit to handle all kinds of administration tasks within the WebLogic domain. An Administration Server does all its tasks within the Java Virtual Machine (JVM). The Administration Server can be accessed through a web browser. You can deploy your application on an Administration Server but it is recommended to create Managed servers and deploy your application in a Managed server and leave the Administration Server for configuration and maintenance purposes only. There will allways be only one Administration Server per domain. 

 MANAGED SERVERS

Other instances running in their own JVM than the Administration Server that are within a WebLogic domain are called Managed Servers. As stated, it runs in its own JVM. A Managed server can start independently from an AdminServer, if configured so.

 Two or more Managed servers can be configured as a WebLogic Server cluster to increase scalability and availability. In a WebLogic Server cluster, most resources and services are deployed identically on each Managed Server, enabling failover and load balancing. A single domain can contain multiple WebLogic Server clusters, but can also contain nonclustered Managed Servers.

The main difference between clustered and nonclustered Managed Servers is the support for failover and load balancing handled by the WebLogic domain (one can configure clustering outside WebLogic of course). These features are available only in a cluster of Managed Servers.