The OSGi Subsystems specification is now published. Subsystems provide a way of structuring large systems comprising many OSGi bundles - a "no brainer" for architects who want to build a modular system. But what are the practical considerations when using Subsystems? We'll look at the trade-offs involved and indicate some patterns and anti-patterns. Eclipse Virgo was one of the technologies that provided input to the Subsystem specification. We'll use Virgo as a source of examples in discussing the practical trade-offs in the use of Subsystems. This presentation was given at the OSGi DevCon at EclipseCon Europe 2012.

1. Practical OSGi Subsystems
Glyn Normington

2. Agenda
• Background
• Subsystem types
• Nested subsystems
• Subsystem content and dependencies
• Getting started

3. Subsystems
• New in OSGi Enterprise 5.0 spec
• Implementation underway in Apache Aries
• Based on input from several projects
• Aries, Geronimo, Glassfish, Karaf,
Paremus Service Fabric,Virgo, WebSphere
• Best practices...

4. Why Subsystems?
• Many bundles
• Named, versioned grouping
• Management
• Scoping

5. Scoping
• Bundles
• Packages
• Capabilities
• Services

6. Subsystem Types

7. Feature Subsystems
Feature
Scoping:
bundle bundle
• None
bundle bundle

8. Application Subsystems
Scoping: Application
• No exports bundle bundle
• Automatic imports
bundle bundle

9. Composite Subsystems
Scoping: Composite
• Configurable exports bundle bundle
• Configurable imports
bundle bundle

10. Subsystem Types in
Practice

11. Feature Subsystems
• Useful for grouping dependencies
• More controlled than auto-detection
• Helps avoid uses constraint failures
• Useful for:
• Common infrastructure
Feature
bundle bundle
• Shared bundles
bundle bundle

12. Application Subsystems
• Useful for top layer of the system
• Application or independent parts of an app
• Avoid including infrastructure bundles
• Reduces sharing Application Application
• Makes update more difficult bundle bundle bundle bundle
• Can lead to resolution failures
Feature
bundle bundle

13. Composite Subsystems
• Essentially a systems programming feature
• Useful for kernels and special infrastructure
• Not recommended for general, widespread
application use
• Complex Composite
• Can duplicate infrastructure bundle bundle
• extenders
• management bundles bundle bundle

14. Composite
Subsystems: Kernels
Root (kernel)
Composite (user region)
bundles packages services

15. Subsystem Content and
Dependencies

16. Subsystem Content
• By value in a subsystem archive
• By reference to a repository

17. Dependencies
• Satisfy the requirements of a subsystem
• But not included in the subsystem content
• Automatically installed from a repository

18. Subsystem Content and
Dependencies in
Practice

19. Use of Repositories
• Subsystem archive content
• Easy to deploy
• May limit sharing
• Harder to update
• Repository content
• Convenient during development
• Avoids bloated archives
• Generally preferable

20. Nested Subsystems

21. Nested Subsystems
• So far all subsystems have been children of
the root subsystem
Root
Application Application
bundle bundle bundle bundle
Feature
bundle bundle

22. Nested Subsystems
• Subsystems can be nested to any depth
• Sharing between parent and child is
controlled by the child

23. Nested Subsystems in
Practice

24. Nested Subsystems
• Features useful for grouping other subsystems
Root
Feature
Application Application
bundle bundle bundle bundle
bundle bundle bundle

25. Nested Subsystems
• Nested features useful for logical grouping
Root Root
Feature
Application
Feature
bundle bundle bundle bundle
Feature
bundle bundle bundle bundle

26. Nested Subsystems
• Nested applications useful for scoping
Root
bundle
Application
Application
bundle bundle bundle bundle

27. Getting Started

28. Trying it out
• Apache Aries support in development
• Some Virgo equivalents available

29. Aries Subsystems
• This example runs on Virgo Kernel
• (Live demo)
• You can run it yourself in 5 minutes
• https://github.com/glyn/aries-subsystems-
on-virgo-kernel
• README.md contains full instructions

30. Virgo Equivalents
Standard Function Virgo Equivalent
Feature Unscoped plan
Application - scoped using a PAR or scoped plan - scoped
region using metadata rewriting
Composite - scoped using a
User region
region
Nested plans
Nesting
PARs, plans, user region
Repository Repository
Deployment Contexts -

31. Thanks
I am indebted to John Ross for his help with
Aries Subsystems

32. Further Information
• OSGi R5 Enterprise Specification
• http://www.osgi.org/Download/Release5
• Aries Subsystems on Virgo Kernel
• https://github.com/glyn/aries-subsystems-on-virgo-kernel
• Virgo User & Programmer Guides
• http://www.eclipse.org/virgo/documentation/