The following sections introduce WebLogic Server domains and their contents:
A WebLogic Server administration domain is a logically related group of WebLogic Server resources. Domains include a special WebLogic Server instance called the Administration Server, which is the central point from which you configure and manage all resources in the domain. Usually, you configure a domain to include additional WebLogic Server instances called Managed Servers. You deploy Web applications, EJBs, Web services, and other resources onto the Managed Servers and use the Administration Server for configuration and management purposes only.
You can use a single WebLogic Server installation to create and run multiple domains, or you can use multiple installations to run a single domain. See Figure 2-1.
How you organize your WebLogic Server installations into domains depends on your business needs. You can define multiple domains based on different system administrators’ responsibilities, application boundaries, or geographical locations of the machines on which servers run. Conversely, you might decide to use a single domain to centralize all WebLogic Server administration activities.
Depending on your particular business needs and system administration practices, you might decide to organize your domains based on criteria such as:
For development or test environments, you can create a simple domain that consists of a single server instance. This single instance acts as an Administration Server and hosts the applications that you are developing. The
wl_server domain that you can install with WebLogic Server is an example of this type of domain.
Figure 2-2 shows a production environment that contains an Administration Server, three standalone Managed Servers, and a cluster of three Managed Servers.
Although the scope and purpose of a domain can vary significantly, most WebLogic Server domains contain the components described in this section.
The Administration Server operates as the central control entity for the configuration of the entire domain. It maintains the domain’s configuration documents and distributes changes in the configuration documents to Managed Servers. You can also use the Administration Server as a central location from which to monitor all resources in a domain.
To interact with the Administration Server, you can use the Administration Console, WLST, or create your own JMX client. Seein Overview of WebLogic Server System Administration to modify the domain’s configuration.
Each WebLogic Server domain must have one server instance that acts as the Administration Server.
The failure of an Administration Server does not affect the operation of Managed Servers in the domain but it does prevent you from changing the domain’s configuration. If an Administration Server fails because of a hardware or software failure on its host machine, other server instances on the same machine may be similarly affected. However, the failure of an Administration Server itself does not interrupt the operation of Managed Servers in the domain.
If an Administration Server for a domain becomes unavailable while the server instances it manages—clustered or otherwise—are up and running, those Managed Servers continue to run. Periodically, the Managed Servers attempt to reconnect to the Administration Server. If the domain contains clustered server instances, the load balancing and failover capabilities supported by the domain configuration remain available, even if the Administration Server fails.
You can start a Managed Server even if the Administration Server is not running. In this case, the Managed Server uses a local copy of the domain’s configuration files for its starting configuration and then periodically attempts to connect with the Administration Server. When it does connect, it synchronizes its configuration state with that of the Administration Server.
For information on starting a Managed Server without a running Administration Server, seein Managing Server Startup and Shutdown. For information on re-starting an Administration Server, see in Managing Server Startup and Shutdown.
Managed Servers host business applications, application components, Web services, and their associated resources. To optimize performance, Managed Servers maintain a read-only copy of the domain's configuration document. When a Managed Server starts up, it connects to the domain's Administration Server to synchronize its configuration document with the document that the Administration Server maintains.
For production environments that require increased application performance, throughput, or high availability, you can configure two or more Managed Servers to operate as a cluster. A cluster is a collection of multiple WebLogic Server server instances running simultaneously and working together to provide increased scalability and reliability. In a cluster, most resources and services are deployed identically to each Managed Server (as opposed to a single Managed Server), enabling failover and load balancing. A single domain can contain multiple WebLogic Server clusters, as well as multiple Managed Servers that are not configured as clusters. The key difference between clustered and non-clustered Managed Servers is support for failover and load balancing. These features are available only in a cluster of Managed Servers. For more information about the benefits and capabilities of a WebLogic Server cluster, seein Using WebLogic Server Clusters.
In addition to the Administration Server and Managed Servers, a domain also contains the resources and services that Managed Servers and deployed applications require.
Managed Servers can use the following resources:
Applications can use the following resources and services:
In designing your domain configuration, note the following restrictions:
If you have created multiple domains, each domain must reference its own database schema. You cannot share a configured resource or subsystem between domains. For example, if you create a JDBC data source in one domain, you cannot use it with a Managed Server or cluster in another domain. Instead, you must create a similar data source in the second domain. Furthermore, two or more system resources cannot have the same name.