Intro

WebLogic Server Instance

A WebLogic Server instance is a Java Virtual Machine (JVM) process that runs the Java code. The instance is the actively working component, receiving client requests and sending them on to the appropriate components, and sending the processed requests back to the originating clients. The server instance manages the resources necessary for applications, such as the JTA and JDBC services, to function. In each domain, one instance serves as the Administration Server, which is your primary means of managing the domain.

WebLogic Server Domain

A domain is a set of WebLogic Server instances that you manage with the Administration Server, which itself is nothing but another WebLogic Server instance, albeit a special one. Any configuration changes you make to a domain will apply to all members of that domain. Domains offer you ease of administration—for example, you can apply configuration changes on a domain-wide basis that apply to all the management servers that belong to that domain. Every domain has exactly one Administration Server, which is used to configure and manage that domain. 

Administration Server

A server is an instance of WebLogic Server that runs in its own JVM, and the Admin Server is a special instance of WebLogic Server designed for managing the domain rather than running applications. There is a one-to-one relationship between domains and the Admin Server—an Admin Server belonging to Domain A can't manage Domain B.

You can deploy applications on the Admin Server, but unless you're operating in a purely developmental environment, use the Admin Server strictly for performing management tasks, not for deploying any applications. Although you can deploy applications on the Admin Server in a development environment, it's a best practice not to do so in a production environment. For one thing, you don't want application work to compete with administrative work in a production environment. You also want to firewall the Admin Server separately so external clients can't access it.

The Admin Server is critical to the functioning of a WebLogic Server domain, since it manages the domain configuration, including the servers that are part of the domain, as well as all the applications and services you deploy to the various servers. Apart from this management of the domain configuration information, the Admin Server has all of the functionality of a Managed Server; in fact, an Admin Server runs the same code and is managed internally the same way as a Managed Server. The Admin Server hosts the Administration Console, which is a web application front end used for configuring, monitoring, and managing a domain. You can access the Administration Console with any supported browser that can access the Admin Server. All WebLogic system administration tools and APIs interact with the Admin Server. If you install the optional Node Manager service, the Admin Server communicates with the Node Manager service on each machine to talk to the Managed Servers running on that machine.

Managed Server

Managed servers are the workhorses of WebLogic Server. Any additional servers you create after the creation of the default Admin Server are Managed Servers. The Managed Server contacts the Admin Server, only when you start it up, to get the configuration and deployment settings. For this reason, you should always start up the Admin Server before you start a Managed Server. Once a Managed Server starts running, it operates completely independent of the Admin Server.

 A Managed Server hosts your Java EE applications, as well as all related resources and services such as Java Database Connectivity (JDBC) connection pools and data sources, Java Transaction API (JTA) transaction services, and Java Messaging Service (JMS) connection factories that are necessary to support application deployments. As mentioned earlier, upon its startup, a Managed Server will contact the Admin Server to retrieve any configuration changes since the Managed Server was last shut down. However, a Managed Server can continue to run, and it's even possible to start it up, in the absence of an Admin Server.

WebLogic Server Cluster

A WebLogic Server cluster is a group of WebLogic Server instances consisting of multiple Managed Servers that run simultaneously. The multiple Managed Servers work together to provide replication services for one another, and the Admin Server is not generally a part of any cluster. Most production deployments use clusters to increase reliability and scalability through load distribution and high availability. To achieve the high availability capability, you deploy resources and services in a homogeneous fashion on each of the Managed Servers that are part of a cluster. Clusters host applications that respond to HTTP requests that are routed to the cluster through a hardware load balancer. You can also set up load balancing on a WebLogic Server instance or a third-party Web Server with the help of plug-ins supplied by WebLogic Server. The load balancer handles the HTTP requests after the requests pass through a firewall. Cluster members pass replicated copies of objects such as HTTP sessions among themselves to provide the failover capability for the cluster.

Machine

A machine in the WebLogic Server context is the logical representation of the computer that hosts one or more WebLogic Server instances (servers). The Admin Server uses the machine definitions that you create to start remote servers through the Node Managers that run on those servers. A machine could be a physical or virtual server that hosts an Admin or Managed Server that belongs to a domain. You'll see later in the book that you must define a machine first if you want the Admin Server to use the Node Manager service to monitor, start, and stop the Managed Servers running on a server. In a sense, a machine in a WebLogic Server environment is more or less equivalent to an instance of a Node Manager and this is essentially the concept that a machine represents.

Node Manager

The Node Manager is an optional process that runs on a machine and manages the availability of all servers that run on that machine. Node Managers help you remotely start, stop, suspend, and restart Managed Servers. The Node Manager works with the Admin Server using a secure channel and lets you manage the availability, as well as monitor the health, of all Managed Servers in a single domain. The Managed Servers that the Node Manager controls can be independent servers or they can be members of a cluster. The Node Manager monitors remote Managed Servers and is capable of automatically restarting them when they fail. It also kills Managed Servers that exhibit unstable behavior. It is recommended that you install a Node Manager service on each machine that hosts a Managed Server

Security Realm

You use security realms to protect the WebLogic Server resources. A security realm is simply a logical container for your user, group, roles, security policies, and security providers. It's the security realm that authenticates users and determines which resources they can access. WebLogic Server uses a default security realm by the name myrealm. In the default security realm, the Admin Server stores the domain security data in an LDAP server, but you can also choose an RDBMS store for this instead. The Managed Servers replicate this LDAP server, and when the Admin Server fails it can use their copy of the LDAP server for providing security services to the deployed applications.

When you create a domain, the username/password credentials you provide are used by the Configuration Wizard to seed the security realm myrealm. The username you provide will be the initial administrative user in myrealm. When you start up the server, it uses the default security realm to authenticate usernames. You can configure the server to use other security realms, but you must always specify one of them as the default security realm.