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OSGi: An Overview of Its Impact on the Software Lifecycle
Getting ready for OSGi will involve changes to how you write applications and how you deploy and manage those applications
By: Scott Cranton
Mar. 25, 2011 01:45 PM
OSGi technology brings a number of much needed benefits to the Java enterprise application market, and is disruptive in that it impacts the software development, deployment, and management practices of many organizations. OSGi impacts deployment given the shared, modular nature of OSGi, meaning application code must be written differently to capitalize on the benefits of OSGi. Equally important, application management processes need to be adjusted, given the highly shared nature of OSGi modules across many applications. This article provides a high-level overview of OSGi, and the impact this framework is having on the software lifecycle.
What Is OSGi?
The OSGi framework covers three major areas: Bundles, Lifecycle, and Services. The Bundle layer is the most visible, and most used part of the OSGi framework. In short, the Bundle layer is represented as a JAR file (Java ARchive) that includes extra information about which parts of its content can be used by other applications, as well as its dependencies. The Lifecycle layer defines a sequence of steps that the bundles (your code) go through when installed, started, updated, stopped, and uninstalled. Having the lifecycle explicitly defined allows the code in the bundle to start managing its own resources. Equally important, the Lifecycle layer helps administrators catch issues early as OSGi mandates that all external dependencies be resolved before a bundle can be used. If the dependencies cannot be resolved, an error is logged before the bundle starts. Clearly, this is a much better process than getting a phone call in the middle of the night during a critical process run. The Services layer exposes services running code objects that can be called from other code running in the OSGi server. The big difference for OSGi Services is that the framework allows the service implementation to change at runtime - the concept of dynamic updating referenced earlier.
The positive impacts of OSGi on the application development process are clear, but what about application deployment and management? And why is deployment and management even important to the developer?
Impact on Developing Applications
There is also version information about imports and exports, which allows multiple versions of Java libraries (e.g., hibernate or log4j) to be deployed on the same server at the same time and not cause of the traditional conflicts which developers encountered.
As developers get more sophisticated with OSGi, they will better understand the critical importance of the Lifecycle layer. At the most basic, it's a great place to get (start) and release (stop) external resources such as database connections. This eliminates the need to generate the often complicated code required to safely manage connections in the middle of your code.
Some will eventually look at the OSGi Service layer; most to access existing capabilities such as configuration management and logging. Some will share and consume code in different bundles (modules) using OSGi services to take full advantage of runtime versioning of components. Imagine the ability to install a patch to an application module in a running system without needing to write lots of fancy code into your application. That's what OSGi services offer the enterprise developer.
Deploying an OSGi Application
Several OSGi servers, such as Apache ServiceMix (http://servicemix.apache.org) or the productized distribution Fuse ESB (http://fusesource.com), allow OSGi bundles to be installed and upgraded from Apache Maven repositories. Maven is a build system that can keep software artifacts within a set of locations (repositories) with a number of build time advantages for automatically downloading all dependent code libraries such that a code build can run. For deployment teams, this allows for much closer coordination between themselves and the development team as all team members are working against a common repository. This allows deployment teams to update running systems with a single command once a patch has passed all QA tests.
Managing an OSGi Application
Another advantage of OSGi-based applications is the ability to leverage the OSGi Configuration Admin service, which allows systems administrators to package, change and audit changes to the configuration of bundle properties. For example, the host name and port of the database that a given bundle should access can be managed as OSGi configuration properties giving the administrator a common way to see those properties, make runtime changes, and audit changes.
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