OSGi: Open Services Gateway Initiative

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OSGi Open Services Gateway Initiative

• Richard Chapman
• 5 Sept. 2002

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Original Intention

• To allow services to be remotely deployed and
  administered onto home network gateways such as
  set-top boxes and DSL modems

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Current Intention

• Expanded to be a language and OS-neutral way of
  providing and administering services
• Targets include divserse platforms such as
  automotive components, comsumer electronics,
  mobile phones

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OSGi Appropriate where

• Lifecycles are managed
• Uptimes are long
• Management is done remotely
• Resilience, robustness matters
• (this is the marketing hype)

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What is it

• A Java-based environment consisting of the J2RE
  and the OSGi framework in which downloadable
  bundles of services managed by a registry can
  be run

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Supposedly plays well with other services

• Insert picture from the overview here

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Definitions

• Service framework execution environment for
  downloaded services
• Bundles groups of downloadable services
  (related in some way)
• Registry exhange services between bundles in
  some manner

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OSGi

• Insert picture from p. 7-282

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OSGi Service Specifications

• Three version 1 services
• Logging service
• Web server
• Device access

• Version two services added
• Configuration svc
• Preferences svc
• User admin svc
• Permission admin svc
• Pkg admin svc

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Framework

• Allows the downloading, execution, and removal of
  bundles
• M anages bundle installation and updates
  dynamically (scalably?)
• Consistent programming model for bundle
  developers
• Bundles can select an available implementation at
  runtime through the framework registry

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Bundles

• JAR files
• May include other resources (html, help files,
  icons, etc)
• Contains manifest
• States dependencies on other resources
• Bundle activator class

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Bundles, contd

• Framework itself is a bundle (system bundle)
• Management bundles implement policies (e.g. from
  what location is a bundle to be loaded)
• Each bundle has a classloader with it, providing
  bundle with own namespace, permitting sharing of
  packages between bundles

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Native Code Libraries

• Specified in manifest header
• Certain permissions required to load native code
  library
• Can specify different native code for different
  OS and processor (in the manifest header)

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Bundle Object

• Identifier
• B undle location
• Bundle state (installed, active, starting,
  stopping, active, etc)

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Bundle Context

• Created by framework when a bundle is started
• Represents execution environment of a single
  bundle
• Acts as proxy to underlying framework
• Provides private persistent storage area
• Maintains environment properties

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Services

• Bundle is a set of cooperating services available
  from shared service registry (bundle does the
  registering)
• Services provided in framework as service
  objects
• Service interface defines semantics of a service
• Framework manages dependencies between services
  provided and used by different bundles

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Service Interfaces

• Service interface is the specification of the
  services public methods
• Some defined as standards by OSGi EGs (expert
  groups)
• Can define your own interfaces (? )
• When requesting a service, you specify the name
  of the service interface you are interested in
  implementations of

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Registering services

• Service object is registered with registry, along
  with key/value pairs used in searching (like
  Jini)
• Every registered service object has unique
  ServiceRegistration object (provided only to the
  bundle that registered the svc) and some
  ServiceReference objects that refer to the
  registered service (used to acquire the code when
  you are ready to use the service)

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Obtaining Services

• Bundle asks for services implementing an
  interface, gets ServiceReference Objects back
  from the registry
• Use the ServiceReference object to actually get
  the service object (you need permission to do
  that anyone can get the reference object)

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What have I not covered

• Events
• Security
• Using on JDK1.1
• Particular service specifications
• What runs on what machine?
• LAN, WAN, what?