Oracle R12 MIDDLE TIER Concepts

OC4J Instances

 An OC4J instance is a logical instantiation of the OC4J implementation in Oracle Application Server. This implementation is Java 2 Enterprise Edition (J2EE) complete and written entirely in Java. It executes on the standard Java Development Kit (JDK) 1.4 Java Virtual Machine, which is installed with OracleAS (JDK 1.3 is supported). It has a lower disk and memory footprint than the previous Oracle Application Server Java environment and competitive Java application servers. Note that each OC4J instance can consist of more than one JVM process where each process can be executing multiple J2EE containers. The number of JVM processes can be specified for each OC4J instance using the Oracle Enterprise Manager Application Server Control GUI.

Oracle Application Server allows several OC4J instances to be clustered together for scalability and high availibility purposes. When OC4J instances are clustered together, they have the same configuration and applications deployed amongst them. A more in-depth discussion on clustering is found in the section "Oracle Application Server Support for High Availability and Load Balancing" below.

 Oracle Process Manager and Notification Server (OPMN) Server

 Each OracleAS instance has an OPMN server which performs monitoring and process management functions within that instance. This service communicates messages between the components in an OracleAS instance to enable startup, death-detection and recovery of components. This communication extends to other OPMN services in other OracleAS instances belonging to the same cluster as well, thereby allowing other instances in a cluster to be aware of active OC4J and Oracle HTTP Server processes in other OracleAS instances (in the same cluster).

The OPMN service also communicates and interfaces with Application Server Control to provide a consolidated interface for monitoring, configurating, and managing Oracle Application Server. Oracle Application Server components, Oracle HTTP Server, OC4J instances, and Distributed Configuration Manager (described below), use a subscribe-publish messaging mechanism to communicate with the OPMN service. For failover and availibility, the process that implements the OPMN service has a shadow process that restarts the OPMN process if it fails.

 Distributed Configuration Manager (DCM)

 In order to manage and track configuration changes in the various components in each OracleAS instance, a DCM process exists in each OracleAS instance to perform those tasks. Each configuration change made to any of the components in a OracleAS instance is communicated to the DCM. DCM in turn takes note of the change and records it in the Oracle Application Server Metadata Repository in the Infrastructure database. This repository contains the configuration information for all the OracleAS instances connected to it through their respective DCMs. All OracleAS instances connecting to the same infrastructure repository in this way belong to the same OracleAS Farm. If any of the OracleAS instances fail, the configuration information can be retrieved from the repository for purposes of restarting the instance.

Each DCM also communicates with the OPMN in their respective instances to send notification events on changes in repository data. This allows OPMN to make the corresponding adjustments to the Oracle Application Server components.