You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 8 Next »

Overview

The OSGi framework supports deploying bundles into a flat and basically globally bundle space. The idea behind this approach can be summarized as, "the deployed set of bundles is your application configuration." This approach has performed well over the years; however, as OSGi technology is used in more and more complicated scenarios, this approach is not always sufficient. For example, when trying to run multiple applications in a single framework instance or when applications become so large that sets of bundles start mapping onto logical subsystems. In these types of situations, it is possible for the configurations of different applications or subsystems to interfere with each other.

To address some of these issues, this proposal introduces a composite bundle concept built on top of virtual bundles. The main goal of this proposal is to provide an isolation mechanism for groups of bundles, while still allowing collaboration among those groups and to manage everything as a layer above the framework.

Use cases

Some potential use cases for composite bundles:

  • Large application subsystems can be modeled as a composite bundle.
  • Different applications running in the same framework instance can be isolated from each other inside of composite bundles.
  • Web servers could model EARs and composite bundles.
  • Groups of bundles needing lifecycle management as a whole can be modeled as a composite bundle.

This list is not intended to be exhaustive.

Terminology

The following terms are used in this document:

  • Composite bundle - a bundle whose contents is actually a set of bundles that appear to be running inside of another framework instance.
  • Parent framework - the framework in which a composite bundle is installed.
  • Composite framework - the framework running inside the composite bundle (this term may not be necessary).

Technical approach

The overall technical approach is to build a layer on top of the virtual bundle concept (proposed separately) to manage composite as a layer above the framework. The specific approach is to forgo an API-based approach to support a simple, declarative approach. Therefore, the technical approach is actually divided into two halves: composite declaration and composite lifecycle management.

NOTE: The goal is not to completely rule out any API, but to keep things simple until some real-world experience is gained at which point API could potentially be introduced.

Composite declaration

A composite bundle is declared using a set of manifest-like headers, which is familiar to bundle developers and fits well with the virtual bundle proposal, where virtual bundles are installed with a given set of headers. Many existing headers are reused to declare a composite bundle, but not all are applicable (e.g., Bundle-ClassPath, Bundle-Activator, Bundle-NativeCode, Bundle-ActivationPolicy). Other than explicitly disallowed headers, all other headers maintain their normal meaning. For example, code dependencies are handled by:

  • Import-Package - the packages imported by the composite bundle from the parent framework.
  • Require-Bundle - the bundles required by the composite bundle from the parent framework.
  • Export-Package - the packages exported by the composite bundle to the parent framework.

For service dependencies, this proposal resurrects the following headers:

  • Import-Service - the services imported by the composite bundle (exact syntax is yet to be defined, but a list of filters is a reasonable starting point).
  • Export-Service - the services exported by the composite bundle (exact syntax is yet to be defined, but a list of filters is a reasonable starting point).

A new header is introduced to declare the composite's constituent bundles:

  • Include-Bundle - A comma-separate list of bundle URLs.

To simplify matching a composite's exported packages to its contained bundles, this proposal introduces a new from directive for Export-Package, which is used to specify the symbolic name of the providing bundle. Consider the following composite declaration:

Bundle-ManifestVersion: 2
Bundle-Name: Paint Program
Bundle-SymbolicName: org.foo.paint.composite
Include-Bundle: \
 file:/Users/rickhall/Projects/book-trunk/code/chapter04/paint-example/bundles/shape-4.0.jar, \
 file:/Users/rickhall/Projects/book-trunk/code/chapter04/paint-example/bundles/paint-4.0.jar, \
 file:/Users/rickhall/Projects/book-trunk/code/chapter04/paint-example/bundles/circle-4.0.jar, \
 file:/Users/rickhall/Projects/book-trunk/code/chapter04/paint-example/bundles/square-4.0.jar, \
 file:/Users/rickhall/Projects/book-trunk/code/chapter04/paint-example/bundles/triangle-4.0.jar
Export-Package: org.foo.shape; from:=org.foo.shape; version="4.0"
Import-Package: org.osgi.service.log; version=1.0.0
Import-Service: org.foo.shape.SimpleShape

This composite contains five bundles and exports org.foo.shape from the bundle with the symbolic name org.foo.shape. Further, it also imports the log service package and any services implementing the org.foo.shape.SimpleShape interface from the package it exports.

This proposal introduces one final header, which is:

  • Provide-Bundle - a comma-delimited set of symbolic names specifying the constituent bundles provided by the composite bundle to the parent framework.

The provided bundles will be manifested in the parent framework as virtual bundles themselves.

These declarative headers define the entire capabilities of a composite bundle. To summarize, these capabilities are: containing bundles, importing/exporting packages, requiring/providing bundles, and importing/exporting services.

NOTE: It is not clear if Import-Package should essentially support an "export as" directive where the composite creator explicitly specifies how the imported package gets converted to an export internally or if this should be somehow automatically derived from the actual injected wire. If the latter, then this relates to the rich wiring section in the open issues.

Composite lifecycle management

Since composite bundles are implemented as virtual bundles, access to their content and portions of their lifecycle are controlled by an external manager. As a result, their lifecycle handling is slightly different from normal bundles. This section describes various composite lifecycle management issues.

Composite manager

The composite manager results from the use of virtual bundles. The composite manager is largely responsible for actually realizing the capabilities embodied in the composite declaration headers. This means it is the composite manager's responsibility to:

  • Manage a composite bundle's constituent bundles.
  • Provide constituent bundles access to imported packages, required bundles, and imported services.
  • Provide the parent framework access to exported packages, provided bundles, and exported services.
  • Manage the overall lifecycle of composite bundles.

The precise approach the composite manager uses to accomplish these responsibilities is not specified.

Installing composites

If an "install hook" is introduced in the virtual bundle proposal, then the composite manager can use it to seamlessly install composite bundles via the BundleContext.installBundle() method, like any normal bundle. If install hooks are not proposed, then it could provide a simple service for installing composites. A composite is installed with a complete composite description, which forms the manifest of the installed virtual bundle. As such, composite installation is effectively atomic.

Resolving composites

The composite's wires for its required packages and bundles are injected into the composite's virtual module by the framework, like for all virtual bundles, which the composite manager can use for delegation purposes for the constituent bundles. If a composite is resolved, then it is possible to load classes from it. After a composite is resolved it is also possible to realize any provided bundles in the parent framework.

Starting and stopping composites

Starting a composite bundle starts all internal constituent bundles. Likewise, stopping a composite bundle stops all constituent bundles. Composite bundles do not have user-defined activators, although the composite manager may make use of an activator. For active composites, the composite manager must provide constituent bundles access to imported services and must make exported services available in the parent framework. Conversely, when a composite bundle is no longer active, it must stop providing access to services.

NOTE: For a provided bundle into the parent framework, it is not clear how we should tie its lifecycle to the corresponding constituent bundle and/or whether it should be controllable in the parent framework.

After a composite bundle is stopped, it should remain resolved and continue to provide access to its exported packages and provided bundles.

Refreshing a composite

When refreshing a composite, all constituent bundles are refreshed and the composite bundle returns to the installed state. Any provided bundles in the parent framework will also be refreshed and returned to the installed state and will not be resolvable until the composite is resolved.

NOTE: Another approach for provided bundles is to uninstall and refresh them.

Uninstalling a composite

The normal bundle uninstall operation does not directly mean that a bundle is no longer in use, since it is still possible to load classes from it. Further, the framework provides to additional callbacks or state changes to notify when it is really done with a bundle. As a result, if a composite is uninstalled, the composite manager must immediately refresh it to perform proper clean up.

NOTE: This could be improved with a VirtualModule.dispose() method, indicating that the framework is really done with the virtual module.

Relationship to composite manager lifecycle

Since the composite manager manages all aspects of the composite's content, its active lifetime scopes its managed composites. In other words, if the composite manager is stopped, then it explicitly causes all of its managed composites to refresh and return to the installed state.

Open issues

Rich wiring information

Currently, the wiring information provided by the virtual bundle proposal has been kept purposely simplistic. To fully implement aspects of composites, like requiring/providing bundles, it is necessary to get richer information from the wires, such as the type of capability. Further, the wiring information needs to be at the module-level (i.e., bundle revision level), not at the bundle level.

Considered alternatives

TBD

  • No labels