Basic concepts of WebLogic Server

•  Domain

Domain is a logically related group of WebLogic Server resources that you manage as a unit. A domain always includes only one instance of WebLogic Server called the Administration Server. Administration Server serves as a central point of contact for server instances and system administration tools.

A domain may also include additional WebLogic Server instance called Managed Servers.

•All domain configuration information is stored in config.xml.Attributes in config.xml are stored a ConfigMbeans within the memory of Admin and managed server.

• Server startup:
Admin server: The server as part of its startup process, would parse the config.xml and create Mbeans.
For eg: <Server ListenPort=”7001″ Name=”myserver”/>
From the above entries, myserver instance would create a ConfigMbean of type ServerMbean object with attributes ListenPort=7001 and name=myserver

 •Administration Server

Admin server is just another server instance but with added unctionality.Admin server is equipped with the functionality to drive the deployment (master deployer), monitor the managed servers etc. All the weblogic.Admin utility commands are initiated from Admin server. Admin server would act as central point for managing the domain.

• Managed Server

Managed server would connect to admin server via http connection and gets a copy of config.xml file and creates the related Mbeans. These Mbeans exists in memory of server for the life time.Once managed server has started up successfully, we do not need Admin server. The application hosted on managed servers can be accessed irrespective whether or not Admin server is up and running. Admin server would be needed for deployments, any administrative activity like accessing the console etc.

• Managed Server independence Mode

Managed server can also start without Admin server if MSI mode is enabled. In this mode, managed server would look for file “msi-config.xml” under its root directory. It creates Mbeans out of msi-config.xml to startup successfully.

msi-config.xml will be created/copied to managed server by admin server (If MSI replication is enabled- from Admin console)

• Production Vs Development modes

Weblogic Server can be started in development mode or production mode.In dev mode, there is an application poller.This is a simple utility that polls the “<domain>/applications” directory every 3secs (default).If it finds a different version of application in the directory, then weblogic would redeploy the app automatically.Obviously there is a little bit of overhead involved, but this is  justified for dev  environment.In production mode application poller is turned off.User would either have to deploy from console or run  weblogic.Deployer utility.

• Domain-Wide Administration Port

This is an optional feature available that would help in segregating the admin traffic with application related traffic.A different port has to be configured and all admin tasks (including accessing Admin console) are picked up from the configured port.
The cons for this setup are:

1.The Administration Server and all Managed Servers in your domain must be configured with support for the SSL protocol.
2.After enabling the administration port, all Administration Console traffic must connect via the administration port.

• Admin Server recovery

If Admin server machine is down and if we need to migrate to another machine:

1) Backup Config.xml file
2) Backup security data (ldap directory & SerializedSystemIni.dat)

Replace these files in the new domain and start the Admin server.The new Admin server will automatically reconnect with running managed servers Also,

•Managed Servers within the domain can be at different Service Pack levels as long as the Admin Server is at the same Service Pack Level or higher than its Managed Servers.
•All servers that are part of cluster should be of same service pack level.

• WebLogic internal applications

WebLogic has some internal applications deployed by default to perform some of internal activities.
console.war – weblogic Admin console is a webApplication deployed only on Admin server.

FiledistributionServlet: This servlet takes care of communication activities between Admin and managed server.(Deployment, managed server startup etc)

Eg:During deployment, this servlet takes care of copying the application on to all managed servers staging directory.