WebLogic Server Domains

WebLogic Server Domains

The basic administrative unit for a WebLogic Server installation 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 only one instance of WebLogic Server called the Administration Server. The Administration Server serves as a central point of contact for server instances and system administration tools. A domain may also include additional WebLogic Server instances called Managed Servers.
You can configure some or all of these Managed Servers to 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 and JTA that are hosted on another server instance in the cluster. Your applications are also deployed and managed as part of a domain.
A Managed Server can also function as a virtual host.
You can organize your domains based on criteria such as:
  • Logical divisions of applications. For example, a domain devoted to end-user functions such as shopping carts and another domain devoted to back-end accounting applications.
  • Physical location. Domains for different locations or branches of your business.
  • Size. Domains organized in small units that can be managed more efficiently, perhaps by different personnel.
For more information about WebLogic Server domains, see:
    WebLogic Server Domain
  • Overview of WebLogic Server Domains
  • Creating and Configuring Domains Using the Configuration Wizard
  • Configuring Domains in the Administration Console 
  • Figure 1-1 WebLogic Server Domain

     
Figure 1-1 depicts a possible configuration of a WebLogic Server Domain—one of many possible configurations.
In the depicted domain, there are three physical machines:
Machine A hosts one instance of WebLogic Server, the Administration Server. The System Administration Tools communicate with the Administration Server to perform configuration and monitoring of the servers and applications in the domain. The Administration Server communicates with each Managed Server on behalf of the System Administration Tools. The configuration for all the servers in the domain is stored in the configuration repository, the config.xml file, which resides on the machine hosting the Administration Server.
Machines B and C each host two instances of WebLogic Server, WebLogic Servers 1 through 4. These instances are called Managed Servers. The Administration Server communicates with an instance of Node Manager running on each machine to control startup and shutdown of the Managed Servers.
WebLogic Servers 2 and 4 are part of a WebLogic Cluster (outlined in red). This cluster is running an application that responds to HTTP requests routed to the cluster from a hardware load balancer. (An instance of WebLogic Server or a third-party Web server with one of the WebLogic Server plug-ins can also provide load balancing.) The load balancer processes HTTP requests from the Internet after they have passed through a firewall. The load balancer and firewall are not part of the domain. A replicated copy of objects such as HTTP sessions is passed between the two cluster members to provide failover capability.
WebLogic Server 1 runs an application that uses Java Database Connectivity (JDBC) to access a database server running on another physical machine that is not part of the WebLogic Domain.
Note: The pictured domain is only intended to illustrate the concepts of a WebLogic Server domain and how you manage the domain. Many possible configurations of servers, clusters, and applications are possible in a WebLogic Server domain.


System Administration Infrastructure

System administration infrastructure in WebLogic Server is implemented using the Java Management Extension (JMX) specification from Sun Microsystems. The JMX API models system administration functions with Java objects called MBeans. Knowledge of this implementation as described in this discussion of system administration infrastructure is not necessary to manage a WebLogic Server domain.
There are three types of MBeans used to manage a WebLogic Server domain: administrationconfiguration, and runtime Mbeans.
Administration Mbeans contain a set of attributes that define configuration parameters for various management functions. All attributes for administration MBeans have pre-set default values. When the Administration Server starts, it reads the domain configuration file (called config.xml) and overrides the default attribute values of the administration MBeans with any attribute values found in the config.xml file.
The config.xml file, located on the machine that hosts the Administration Server, provides persistent storage of Mbean attribute values. Every time you change an attribute using the system administration tools, its value is stored in the appropriate administration MBean and written to the config.xml file. Each WebLogic Server domain has its own config.xml file.
Each time the Administration Server starts successfully, and each time you modify the configuration, a backup configuration file is created. You can configure the number of back up copies ofconfig.xml retained by the Administration Server. For more information, see Backing up config.xml.
If you set any configuration attributes on the command line when you start the Administration Server with -D arguments, these values override the values set by the defaults or those read from the config.xml file. These overridden values are also persisted to config.xml file by the Administration Server. For more information about these command-line arguments, see Configuring Servers in the Administration Console Online Help.
Configuration Mbeans are copies of Administration Mbeans that each Managed Server uses to initialize its configuration. When you start a Managed Server, the server receives a copy of all the administration MBeans configured on the Administration Server and stores them in memory as configuration MBeans. If you override any configuration attributes when starting a Managed Server, those values override the values received from the Administration Server but are not written to the config.xml file. For more information about starting a Managed Server, see Starting Managed Servers in the Administration Console Online Help.
Runtime Mbeans contain sets of attributes consisting of runtime information for active WebLogic Servers instances and applications. By retrieving the values of attributes in these runtime MBeans, you can monitor the running status of a WebLogic Server domain.
Mbeans may also contain operations used to execute management functions.
Although users with a knowledge of these Mbeans and the JMX API can create their own customized management system, most users prefer to use the system administration tools provided with WebLogic Server to perform these tasks. These tools do not require knowledge of the JMX API. For more information, see System Administration Tools.