Quick post… If you are not familiar with WebSphere at first you might get confused with its concepts: cell, deployment manager, node, node agent, cluster, server, …
First of all, lets start with the concept of a Cell:
A Cell is a virtual unit that is built of a Deployment Manager and one or more nodes. I guess a picture will help making things clearer:
But still there are a few concepts that need to be explained. The next obvious one is the Deployment Manager.
The Deployment Manager is a process (in fact it is an special WebSphere instance) responsible for managing the installation and maintenance of Applications, Connection Pools and other resources related to a J2EE environment. It is also responsible for centralizing user repositories for application and also for WebSphere authentication and authorization.
The Deployment Manager communicates with the Nodes through another special WebSphere process, the Node Agent.
The Node is another virtual unit that is built of a Node Agent and one or more Server instances.
The Node Agent it the process responsible for spawning and killing server processes and also responsible for configuration synchronization between the Deployment Manager and the Node. Extra care must be taken when changing security configurations for the cell, since communication between Deployment Manager and Node Agent is ciphered and secured when security is enabled, Node Agent needs to have configuration fully resynchronized when impacting changes are made to Cell security configuration.
Servers are regular Java process responsible for serving J2EE requests (eg.: serving JSP/JSF pages, serving EJB calls, consuming JMS queues, etc).
And to finish, Clusters are also virtual units that groups Servers so resources added to the Cluster are propagated to every Server that makes up the cluster, this will in fact affect usually more than a single Node instance.
Lets finish this post with another diagram to illustrate all those concepts.
http://itdevworld.wordpress.com/2009/05/03/websphere-concepts-cell-node-cluster-server/
The WebSphere Application Server network Deployment version 5.x (WAS ND) provides an infrastructure for you to centrally administer multiple WAS servers, resources and other elements of the application topology. It includes support for clustered servers with workload management and failover. In WAS ND, it introduces an object called Cell. Cells are arbitrary, logical groups of one or more nodes in a WebSphere Application Server distributed network.
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