High-level architecture
Apache Geronimo consists of a lightweight kernel that provides services such as Lifecycle Management, Dependency Management, Repository, and Configuration Management to the other services that are deployed on it. As we saw earlier in the chapter, the basic manageable unit in Apache Geronimo is the GBean. GBean is a manageable wrapper object with attributes, references, and lifecycle callback methods. It also has a static GBEAN_INFO attribute, which is, a description used by the kernel to instantiate, manage, and modify the GBean state.
The following diagram shows the Apache Geronimo kernel and all of the services that it provides, as well as the other services that are layered on top of the basic kernel services:
This architecture is the key for Geronimo's extensibility. It adds new services or plugins as services deployed on the server, which are in the form of GBeans whose state, dependencies, and lifecycle are managed by the kernel. A GBean may have a reference to another GBean, and the kernel makes sure that the GBean has access to a properly initialized and started instance of that GBean. The Configuration Manager is a component in the Apache Geronimo kernel that takes care of starting the configurations in the requisite order and also of providing the plumbing required to connect related and dependent configurations together so that they can consume the services of other configurations.
The Apache Geronimo kernel also provides the Repository and configuration store that contains configurations of the existing modules as well as the required libraries. Any new module that you add will also be added to the Repository. If it is a configuration archive, then it will be added to the configuration store as well.
By the Geronimo kernel, we mean the core of Geronimo that provides the services shown in the previous diagram. We will also be referring to the org.apache.geronimo.kernel.Kernel interface. This is the core interface whose implementations will actually manage GBeans. Whenever we refer to the Kernel implementation, we mean implementations of this interface. We will be referring to both of these, and you should keep in mind the subtle difference between them.
We will now see each of the services that the kernel provides, in detail:
The repository is the place where Apache Geronimo stores the majority of the artifacts that it contains. The repository consists of two types of artifacts:
Libraries: The libraries are utilized by Apache Geronimo as well as the applications that are deployed on it. The libraries are usually JAR files that contain a number of classes. The libraries may be standalone or be included in a configuration. An example of a commonly-used library is log4j for logging. The log4j library will be present in the repository in the directory <GERONIMO_HOME>/repository/log4j/log4j/1.2.14/log4j-1.2.14.jar.
Configurations: These are the configurations that are deployed in Apache Geronimo. A configuration represents a set of services that can be loaded and brought online by the Geronimo kernel. Once you deploy a module, the configuration information is serialized to the disk in a subdirectory of <GERONIMO_HOME>/repository, along with the libraries that provide the functionality of that module. The serialized configuration information generally consists of the information in the deployment plan as well as the values for different GBean attributes, and queries for different GBean references. It is stored in the META-INF directory directly inside the root directory of the configuration. The path of the root directory of the configuration will consist of the group ID of the configuration, with dots replaced by slashes. A configuration will always have a directory name ending with .car. For example, the openEJB configuration will be present in the repository in the <GERONIMO_HOME>/repository/org/apache/geronimo/configs/openejb/2.1.4/openejb-2.1.4.car directory as its artifactId is org.apache.geronimo.configs/openejb/2.1.4/car.
The kernel manages all of these tasks, from creating new configurations to linking them to their parents and children, serializing them to disk, reading them back, as well as managing the attributes of the GBeans. There is a configuration manager GBean which manages these tasks. An additional task that the kernel performs is to make MBean wrappers for the GBeans so that they can be managed remotely over JMX.
This is another service that is provided by the kernel. This actually includes providing lifecycle management to all of the services that are deployed in Apache Geronimo. The Kernel implementation provides Lifecycle Management through GBeans. The Kernel implementation is actually aware of GBeans only, and it can Start, Stop, Load, and Unload GBeans. GBeans that implement the org.apache.geronimo.gbean.GBeanLifecycle interface will provide implementations of callback methods that will be called by the Kernel after starting (doStart), before stopping (doStop), and in case of failure (doFail). The kernel internally has a LifecycleMonitor that will be sent notifications whenever a GBean is loaded, starting, running, stopping, stopped, failed, or unloaded. The notifications are sent by a LifecycleBroadcaster. The LifecycleMonitor will have a number of instances of LifecycleListener that are registered with it, and they are configured to perform tasks corresponding to the state notifications that they receive.
Dependency Management
Finally, the Apache Geronimo Kernel provides Dependency Management—that is, the management of dependencies for the services deployed in the server. The dependencies for a configuration or module are specified in the deployment plan for that module. So the kernel provides the plumbing that will link a configuration together with all of its dependencies. Dependency Management also includes making sure that all of the services that a configuration depends on are started before the configuration is started. Again at the Kernel implementation level the dependencies are between GBeans, and the Kernel manages starting all of the dependencies of a GBean before it is started, as well as making them available to the GBean at runtime.