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Apache Stanbol - Dependency Management

Dependency Management

Apache Stanbol is a modular software stack. Technically, this modularization is implemented by the underlying OSGi component infrastructure. Each Apache Stanbol component is implemented as an OSGi bundle that can be deployed in an OSGi environment. Apache Stanbol uses the OSGi implementation from Apache Felix for this.

Apache Stanbol's dependency management is organized at three levels:

  1. Build Dependency Management
  2. Runtime (OSGi) Dependency Management
  3. Launcher Configuration Management

Build Dependency Management

Build dependencies are configured in the Apache Maven pom.xml (POM) files. We distinguish between internal and external dependencies.

Internal dependencies are managed separatly for each component. So each component defines in its own POM which other Apache Stanbol components in which versions it requires. This makes it possible that different components could rely on different versions of another Apache Stanbol component. This is a requirement to be able to release single Apache Stanbol components independently.

External dependencies to third party components are managed at a central place to ensure that all Apache Stanbol components rely on the very same version of a third party component. The central configuration for this can be found in the dependency management section of the Apache Stanbol parent POM. As all Apache Stanbol components define this central POM as their parent, it is ensured that all components use the same version of an external dependency. Changing the version of an external dependency in the parent POM affects all Apache Stanbol components that rely on this dependency.

In consequence it is not allowed that Apache Stanbol components define their own version of an external dependency in their POM. External dependencies are configured in the parent POM only! If there is any Apache Stanbol component that does not apply to this rule, it should be considered as a bug or a good reason for this should be documented. Relying on the same version of external dependencies is essential to ensure a working Apache Stanbol stack.

Runtime (OSGi) Dependency Management

Apache Stanbol components are OSGi bundles. Therefore, the runtime dependencies between such bundles are managed by the exported and imported packages of those bundles. What a bundle requires to import and what it exports is configured by using the Maven Bundle plugin of the Apache Felix project. Its configuration is done inside the POM. The Maven Bundle plugin ensures that the OSGi specific meta-information are generated correctly for the resulting JAR.

Configuring the OSGi imports and exports is in most cases analogue to configure the build dependencies in the POM. We can not go into the details of OSGi here and refer to the OSGi Alliance and the Apache Felix project for further information.

Launcher Configuration Management

Launchers represent the binary distribution of Apache Stanbol. An Apache Stanbol launcher is a packaged OSGi runtime environment. It includes a set of Apache Stanbol components and required third party components to build up the runtime stack. Additionally, a launcher instantiates an Apache Stanbol HTTP endpoint to provide the RESTful interface and documentation.

Basically, a launcher packages everything together in a single executable JAR that is needed to run Apache Stanbol. To configure which bundles are part of this launcher and to ensure that all required third party components are in place, the launchers are composed of so called 'partial bundlelists'.

A partial bundlelist is defined by each Apache Stanbol component and it lists all required internal and external dependencies that have to be included in a launcher in order to make this component work. The dependencies are configured in a 'src/main/bundles/list.xml' file of each partial bundlelist artifact. The dependencies listed in such a file of a partial bundlelist are more or less the same as the ones defined for the build dependencies in the POM. But remember that the partial bundlelists define the dependencies needed at runtime and there may be other/more dependencies needed at runtime than at build time.

Apache Stanbol pre-configures a set of launchers in the /launchers directory. The most important one is the 'full' launcher which includes all available Apache Stanbol components. This launcher is also used when executing the integration tests. In a launcher's POM you just have to declare dependencies to all required partial bundlelists that the launcher should include.