A Weblogic server domain is an administrative grouping of servers and/or clusters. You configure, manage, monitor the domain from central location; this central location is the administration (admin) server.
Admin Server
Admin server is just a Weblogic Server instance which maintains a repository of configuration information for the domain. Admin server acts as a centralized application deployment server which provides browser based admin console for configure, manage and monitor all aspects of the domain.
Managed Server
A Managed server is a term for any other server in the domain other than the admin server. Managed Servers host the components and associated resources that constitute your applications – for example, JSPs and EJBs. When a Managed Server starts up, it connects to the domain’s Administration Server to obtain configuration and deployment settings.
Two or more Managed Servers can be configured as a WebLogic Server cluster (more about this in next blog) to increase application scalability and availability. In a WebLogic Server cluster, most resources and services are deployed to each Managed Server (as opposed to a single Managed Server,) enabling failover and load balancing.
Node Manager
Node Manager is a Java utility that runs as separate process from WebLogic Server and allows you to perform common operations tasks for a Managed Server, regardless of its location with respect to its Administration Server. While use of Node Manager is optional, it provides valuable benefits if your WebLogic Server environment hosts applications with high availability requirements.
If you run Node Manager on a machine that hosts Managed Servers, you can start and stop the Managed Servers remotely using the Administration Console or from the command line. Node Manager can also automatically restart a Managed Server after an unexpected failure.
WebLogic Server Cluster
A WebLogic Server cluster consists of multiple WebLogic Server server instances running simultaneously and working together to provide increased scalability and reliability. A cluster appears to clients to be a single WebLogic Server instance. The server instances that constitute a cluster can run on the same machine, or be located on different machines. You can increase a cluster’s capacity by adding additional server instances to the cluster on an existing machine, or you can add machines to the cluster to host the incremental server instances. Each server instance in a cluster must run the same version of WebLogic Server.
How Does a Cluster Relate to a Domain?
* A cluster is part of a particular WebLogic Server domain.
* A domain is an interrelated set of WebLogic Server resources that are managed as a unit.
* A domain includes one or more WebLogic Server instances, which can be clustered, non-clustered, or a combination of clustered and non-clustered instances.
* A domain can include multiple clusters.
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Courtesy:http://jamesminks.wordpress.com/2010/09/02/weblogic-server-basic-components/