This document discusses modularity in Java and OSGi, focusing on multi-bundle modules and scoping mechanisms. It describes how modules are represented in Java through classes, packages, JAR files, and classloaders. In OSGi, bundles provide modularity, and the document explores approaches to multi-bundle modules like composite bundles, framework hooks, and region digraphs. It introduces subsystems as a proposed OSGi standard for defining and scoping multi-bundle modules.

1. Multi-Bundle
Scoping in
OSGi
Glyn Normington
50 µm lead sulphide
assemblies each containing
~108 nanoparticles
Copyright VMware Inc. 2011. Licensed under the Eclipse Public License

2. Agenda
•Modules in Java
• Multi-bundle modules
• Scoping mechanisms
• Subsystems

3. Modules in Java

4. Modularity
the degree to which a system’s components
may be separated and recombined
the tightness of coupling between components
the degree to which the system architecture
controls the mixing and matching of components

5. Modularity
the degree to which a system’s components
information
may be separated and recombined
the tightness of coupling between components
the degree to which the system architecture
hiding
controls the mixing and matching of components

6. Modules in Java
• Class
• Package
• JAR
• Class loader } OSGi bundle
• Module - Java 8
• Larger scale modules?

7. Multi-Bundle Modules

8. Information to Hide
• Bundle
• Package
• Generic capability
• Service

9. Example 1
Feature
bundle bundle
bundle bundle

10. Example 2
Scoped App
bundle bundle
bundle bundle

11. Example 3
Kernel
bundle bundle
bundle bundle

12. Scoping Mechanisms

13. Scoping Mechanisms
• x-internal/x-friends
• metadata rewriting
• composite bundle
• framework hooks
• region digraph

14. Eclipse x-* Directives
• Export-Package directives
• Enforced by framework
• x-internal
• x-friends
• Non-standard
• Naming friends is brittle
➡ Standard, maintainable mechanism

15. Metadata Rewriting
Hide internals:
• Decorate referents
• Bundles and packages
• Generic capabilities
• Fix references
• Use service registry hooks

16. Metadata Rewriting
in Virgo
Demo ...

17. Metadata Rewriting
Issues:
• Suitable for isolated applications only
• Intrusive
• Complicates bundle update
• Can be coded round
• Non-standard
➡ Standard scoping in the framework

18. Composite Bundle
• OSGi draft spec
• Prototyped in Equinox, deprecated
• Uses framework instance
• Surrogate bundle represents “parent”
• Used in Virgo 2.1 to isolate the kernel

19. Composite Bundle
Issues:
• Implemented in the framework
• Cannot expose constituent bundles
• Hard to spec portably
➡ Superseded

20. Framework Hooks
• Part of OSGi 4.3 core
• Control visibility
• Enable framework to be partitioned
• 5 main hooks:
• resolver hook
• bundle find/event hooks
• service find/event hooks

21. Framework Hooks
Issues:
• Low level
• Consistency of hooks
• Coexistence of sets of hooks
➡ Higher level API needed

22. Region Digraph

23. Graph Theory Interlude

24. Tree

25. DAG

26. Digraph

27. Graph

28. Region Digraph
• Framework hook abstraction
• Bundles partitioned into regions
• Regions connected by filters
• Bundles
• Packages
• Generic capabilities
• Services

29. Example Digraph
region A
p3
"allow p1"
p1, p2
region B

30. Example Digraph
region A
visible: p1, p3
p3
"allow p1"
p1, p2
visible: p1, p2
region B

31. Example Digraph 2
region X
"allow p1, p3"
p1, p2 "allow p3, p4" p3, p4
region Y region Z

32. Example Digraph 2
region X
visible: p1, p3
"allow p1, p3"
p1, p2 "allow p3, p4" p3, p4
region Y region Z
visible: p1, p2, p3, p4

33. Region Digraph
• Equinox bundle
• Runs on standard framework hooks
• Persistent
• JMX

34. Region Digraph Uses
user region
• Virgo 3.0 kernel/user region
p3
s3, s4
b3
• Apache Aries “subsystems” "allow s3" "allow p1, s1, b1"
prototype (service of p1 type)
• Standalone: p1, p2
s1, s2
b1, b2
“We're using [the region digraph] kernel
essentially for a fully multi-tenant
plugin system to Web applications.”
Robert Sauer

35. Region Digraph in Virgo
Demo ...

36. Region Digraph API
public interface RegionDigraph ... {
Region createRegion(String regionName) throws BundleException;
RegionFilterBuilder createRegionFilterBuilder();
...
}
public interface Region {
void connectRegion(Region headRegion, RegionFilter filter)
throws BundleException;
...
}

37. Region Digraph
Issues:
• Too low level for application use
• Non-standard
➡ Standard, high-level programming model

38. Subsystems

39. Why Subsystems?
5 projects have multi-bundle constructs
• Poor portability
• Inconsistent terminology
• Diverse function
➡ Standard

40. Subsystems
• OSGi Enterprise Expert Group
• Scoping based on composite bundles
framework hooks
• Public draft available (“RFC 152”)

41. What is a Subsystem?
• Group of constituents with dependencies
• Constituents: bundles, subsystems, ...
• Dependencies: loosely coupled pre-reqs
• Named and versioned
• Defined lifecycle

42. Subsystem Scoping
Subsystem type Scoping
application contents hidden
composite configurable
feature none

43. Example 1
Application
Feature

44. Example 2
Feature A
Feature C
Feature B

45. Example 3
Application A
Application B
Composite C

46. Scoping and Regions
Subsystem scoping can be
• defined in terms of
• implemented using
a region digraph

47. Complex Scoping 1
App3
FeatureQ
FeatureR
FeatureP
CompositeC

48. Region Digraph
Region A3
App3 FeatureQ
FeatureR
FeatureP
App3
FeatureQ
FeatureR
Region C CompositeC FeatureP
CompositeC

49. Complex Scoping 2
Root
Application A
Application B
Composite C
Composite D
Application E

50. Region Digraph
Root
App A Comp D App E
Root
Application A
Application B
App V Comp C Composite C
Composite D
Application E

51. Dependencies
FeatureX
App1 App2
FeatureY
CompositeF

52. Installing Dependencies
Root
"accept
transitive"
CompositeF
App1 App2

53. Installing Dependencies
Root
"accept
transitive"
CompositeF
App1 App2 FeatureX FeatureY

54. Region Digraph
Region Region F Region
A1 A2
FeatureX
App1 App2
FeatureY
CompositeF FeatureX
App1 App2
FeatureY
CompositeF

55. Conclusion
Modularity in Java:
• Class, package, JAR, class loader
• Modules in Java 8
• Larger modules?
Modularity in OSGi:
• Bundles
• Multi-bundle modules
• Subsystems

56. Further Information
• Public draft of subsystems (RFC 152)
• http://www.osgi.org/Download/File?url=/download/osgi-early-draft-2011-09.pdf
• Region digraph
• http://underlap.blogspot.com/2011/05/equinox-digraph-ready-for-use.html
• Eclipse Virgo and the region digraph
• https://bugs.eclipse.org/bugs/show_bug.cgi?id=330776
• Apache Aries use of region digraph
• https://issues.apache.org/jira/browse/ARIES-644
• Multi-module modules and Java 8
• http://mail.openjdk.java.net/pipermail/jigsaw-dev/2011-October/001564.html

57. Thanks to...
• Wikipedia image courtesy of Wikipedia.org
• Field of Flowers by Paul Podsiadlo and Elena
Shevchenko, courtesy of Argonne National
Laboratory