What exactly is the application container Apache Karaf?


Apache Karaf is a small OSGi-based runtime environment that provides a lightweight container capable of hosting various components and applications. Karaf offers numerous features familiar to those who use application containers based on Java EE or Spring.

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Those encompass the support for the Java Authentication and Authorization Service (JAAS), different dependency injection frameworks such as OSGi blueprint and Spring, the support for Java Persistence API (JPA) and Java Transaction API (JTA) and the offering of clustering, monitoring, and cloud integration utilities.

This article is the first of a series covering topics on the development and operation of OSGi based applications with Karaf.

Overview of Core Features

As already mentioned in the introduction, Karaf offers a comprehensive set of core features. These core features are structured along the areas of provisioning and deployment, logging, dynamic configuration, administration and management and last but not least OSGi framework support.

Provisioning and Deployment

Karaf provides different options for application provisioning and deployment which allows deploying artifacts as feature sets. This enables the structuring of artifacts into larger deployment units, which is a great way to increase the re-use of existing functionality and therefore reduces the overall footprint of your application.

Even if  feature deployment is a great way to structure OSGi based applications, Karaf’s deployment options are not limited to this approach. The container allows also to deploy so-called “Karaf archives” or even more commonly used web archives.

Logging

Additionally, Karaf provides a broad support of logging frameworks by integrating the Pax Logging project. Pax Logging integrates many of today’s popular logging frameworks and closes the gap that emerged by the OSGi community’s decision to discontinue their logging service due to the fact that numerous competing logging frameworks were already available and commonly used.

Dynamic Configuration

While it has been a challenge for other application containers to realize dynamic configuration

capabilities, Karaf offers the opportunity to interact with configuration changes at runtime. In order to utilize dynamic configurations, an application needs to register with the management service with its reference. The management service is then able to pass in any configuration change back to the application bundle.

Although this is a great way to make configuration changes without restarting the application, it is the application’s responsibility to react to these changes, i.e. by destroying and restarting an existing thread.

Administration and Management

Karaf offers several ways to administer and manage the container itself and all application artifacts executed within the container runtime. The most comprehensive administration interface is the extensible command-line interface that is remotely accessible via an incorporated SSH server. In addition to the command-line interface, Karaf offers also a comprehensive web UI that exposes all basic management features to your browser.

OSGi Framework Support

Finally, the container supports different OSGi frameworks. Per default, Apache Felix is pre-configured, but Karaf supports also Eclipse Equinox while it is theoretically possible to run on any OSGi environment.

History of Origins

The roots of Apache Karaf reach back to the kernel development of the Apache ServiceMix project. The aim

of the ServiceMix subproject was to develop a simple-to-use command-line interface for the administration and management of OSGi artifacts.

The development team announced in spring 2008 publicly that they completed the third milestone of the kernel project. It took only until the September 18th, 2008  to release the first version of the kernel as GA.

Even if ServiceMix was a great home for the kernel project during the early days, the project moved in spring 2009 under the Apache Felix project umbrella and got renamed to “Karaf”. The developer community made that decision to increase the attention towards the project within the broader OSGi community and grow the developer base.

Moving the project eventually paid off and Karaf received a promotion to become a top-level Apache project. Since that time, the project has continued to improve the runtime environment and added numerous features.

Summary

Given the fact that Karaf is around for almost eight years, has maintained a solid developer community with most founding members still on the team and has grown a comprehensive feature set, it is fair to say that Karaf is a stable application container to host OSGi based applications and more. The structure of the container offers a large variety of application areas, covering all areas of application development and its adaptability serves as a foundation for many operational scenarios.

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