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An Introduction to Remote Method Activation (ROA)
by Anghel Leonard
The traditional way to develop most RMI applications is to base them on permanent remote objects. This technique works fine if you need to familiarize yourself with the RMI concept or if you're developing simple RMI architectures. But the real power of RMI is demonstrated in large, ROA-based architectures. This technique is available starting with Java 1.2 and it performances are in a continuously increase.
The main task of ROA (Remote Method Activation) is to economize system resources. Traditionally, a remote object will run for a long period of time waiting for potential clients, which takes up system resources—even if it has no clients for a long time. This implementation becomes a problem when the same machine is simultaneously running hundreds or thousands of remote objects—it's a waste of resources to keep all these objects "alive" all these objects, especially when there are no clients.
Using ROA solves this problem while also offering three main facilities: activation of remote objects on client demand, re-activation of remote objects on client demand (if an active object is de-activated, due to an absence of clients for a representative period of time, it can be re-activated on client demand), and permanent references for the remote objects that can be activated on demand.
The first part of this article will discuss the fundamentals of any ROA application. You will also see how to implement an activable object and how to create a setup program for it.
Both parts of this article will describe and use the classes from java.rmi.activation package and the RMID tool.
A Brief Introduction to ROA Terminology
Before learning about ROA, you'll need to be familiar with a few terms commonly used in this technology:
- Activation System is a system made of:
- rmid (the "core" of the activation system)
- the log directory
- activation services that acts as "processes or sub-processes" for rmid.
- rmid is the "core" of the activation system. It has two main tasks:
- It starts the activator and an intern registry on port 1098 (default)
- It registers, in this registry, an
ActivationSystem object, named java.rmi.Activation.ActivationSystem (ActivationSystem. This object is a public interface from the java.rmi.activation package that allows you to register activable remote objects and activable remote objects groups).
- Activable object: Any kind of remote object that can be activated on demand.
- Active object: an activable remote object that runs into a JVM (it was instantiated and exported into a JVM).
- Passive object: an activable remote object that isn't instantiated or exported into a JVM, but can still be activated/reactivated on demand.
- Activation: This means passing an activable remote object from the passive state to the active state (passive object—>activation—>>active object)
- Lazy activation: This means that the activable remote object will be activated by rmid (default). This activation is performed only when the first remote method call is intercepted.
- Activation identifier: an identifier that marks, in a unique way, an activable remote object (
java.rmi.activation.ActivationID).
- Group activation: an identifier that marks in the unique way an activation group (java.rmi.activation.ActivationGroupID).
- Activator: an entity supervises the activation process (
java.rmi.activation.Activator).
- Activation group: an entity that runs into a JVM. The activator calls it to initiate an activation request (
java.rmi.activation.ActivationGroup).
- Group descriptor: a descriptor that contains very important informations for create/recreate an activation group (
java.rmi.activation.ActivationGroupDesc).
- Activation descriptor: a descriptor that contains information necessary to activate an activable remote object (
java.rmi.activation.ActivationDesc).
- Activation on demand: This indicates that the client stub is sending the activator an activation request for an activatable remote object.
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