Since the Eclipse Virgo web site is seeing more new visitors than ever, it seemed like a good time to summarise the benefits of Virgo.
Essentially Virgo is the culmination of 3 years of work in developing an application server based entirely on OSGi and exposing OSGi for use by applications. Virgo started inside SpringSource as the dm Server project which was licensed under the GNU Public License and shipped two major versions. About a year ago we started donating the codebase to Eclipse as the Virgo project licensed under the much more liberal Eclipse Public License. The transfer of the runtime code to Eclipse is now complete, we have satisfied the rather stringent IP checks required of all Eclipse projects, and Virgo is nearing its first formal release by the end of October.
Virgo is an ideal OSGi server runtime, possibly the ideal OSGi server runtime, from several perspectives. Firstly, it was designed from the ground up as OSGi bundles so it's extremely modular and extensible. For example, the core of the runtime is the Virgo kernel and the Virgo web server is constructed by configuring a web layer, an admin console, and some other utilities on top of the kernel. The kernel protects itself from interference from such additions, and applications, by running them in a nested framework. This nested framework approach also enables applications to use new versions of the Spring framework (regardless of the fact the kernel depends on Spring 3.0.0). All the major Java EE application servers are now built on top of OSGi, but only Virgo was designed for OSGi and didn't need OSGi to be retrofitted.
Secondly, Virgo is an ideal OSGi server runtime because it exposes OSGi in a usable way to applications. In addition to all the standard features of OSGi — encapsulated modules or bundles, a powerful service registry, versioning, class space isolation, sharing of dependencies, dynamic refreshing and updating, and much more — Virgo provides a multi-bundle application model to simplify the deployment and management of non-trivial applications. Virgo also provides a repository which can store dependencies such as OSGi bundles which are "faulted in" when needed. This results in cleaner definitions of applications and a small footprint compared to traditional Java EE servers which pre-load many features just in case an application needs them. The major Java EE application servers are beginning to follow suit in exposing an application model, so SpringSource is working with other vendors in the OSGi Alliance to produce a standard multi-bundle application construct.
Thirdly, Virgo is an ideal OSGi server runtime because it enables existing Java libraries to run successfully in an OSGi environment. The Virgo kernel supports thread context class loading, load time weaving, classpath scanning, and a number of other features which are commonly used by persistence providers and other common Java utilities. Essentially, we observed the commonly-occurring problems when people attempted to migrate to OSGi and implemented general solutions to those problems early on.
Fourthly, Virgo is an ideal OSGi server runtime because it integrates OSGi with Spring and Tomcat. Virgo uses Spring DM to wire application contexts to the OSGi service registry. Spring beans can be published as OSGi services and can consume OSGi services, both with minimal effort. The embedded form of Tomcat is used as a servlet engine in Virgo's web support and is configured and managed just like standard Tomcat.
Fifthly, Virgo is an ideal OSGi server runtime because of its excellent diagnostics:
- An admin console lets you examine the state of all artifacts deployed in Virgo as well as explore the state of bundles in the OSGi framework.
- Virgo provides multiple types of diagnostics: event logging aimed at administrators, diagnostics logging aimed at developers, as well as various types of diagnostic dumps.
- Virgo builds on Logback to support highly configurable and efficient logging.
- When an application is deployed, Virgo first resolves the application in a "side state" and, if resolution fails, no changes are committed to the OSGi framework and the side state is dumped to disk so that it can be analysed using the OSGi state inspector in the admin console.
- If a resolution failure occurs, Virgo also analyses the resolver error, including the root causes of any uses constraint violation, and extracts a meaningful message to include in an exception which is then thrown.
- Virgo adds advanced diagnostics to Spring DM to track the starting of bundles and their application contexts and issue error messages on failure and warnings when dependencies are delayed.
- Virgo automatically detects deadlocks and generates a thread stack dump.
Footnote: The bulk of this article was originally posted to springsource.org. Reproducing here for the benefit of Planet Eclipse and others.