Why is os gi dynamic.pptx

Why is OSGi dynamic ?
Christian Campo, compeople AG
EclipseCon Europe 2012,

Overview

• OSGi is...
• Lifecycle
• Services / Extensions
• Sample
• Workaround
• Others

compeople AG | EclipseCon Europe 2012 | Why are OSGi dynamic ? | 12.09.2012 | 2

OSGi is ...

... a module system and service platform for
the Java programming language that implements a
complete and dynamic component model.

Applications or components (coming in the form of
bundles for deployment) can be installed, started,
stopped, updated, and uninstalled without requiring a
reboot.

The service registry allows bundles to detect the
addition of new services, or the removal of services,
and adapt accordingly.

*Wikipedia 2012

OSGi is ...

Bundle Service Extension

started started installed
stopped stopped uninstalled
installed
uninstalled


OSGi Lifecycle ...

Bundle

Service

stopped
started

Extension

uninstalled
installed
uninstalled
stopped
started


OSGi Service Usage ...

Bundle
IService

started

Bundle Bundle

Service MyComponent

?
stopped
started serviceRef

stopped
started started


Extensions

ExtPoint

?
Bundle Bundle

Extension MyComponent
createExecRef
uninstalled
installed

uninstalled
stopped
started started


Lazy activation

Bundle
IService

started
stopped

Bundle Bundle

Service MyComponent

stopped serviceRef

stopped started
stopped

Who starts this bundle so the Service can be
accessed ?


Which means ...

• A service references can become invalid anytime during
runtime
• Services are uncoupled from their consumer (no
dependency)
• Services need to be started before the lookup
• Bundles are started lazy by default
›  you cannot rely on a bundle being active


Sample

Bundle
ISearchService

started

Bundle Bundle

SearchService MySearch

started search

started started


Which means ... (API approach)
MySearch
ref = context.getServiceReference(ISearchService.class.getName());
if (ref!=null) {
ISearchService search = (ISearchService)context.getService(ref);
if (search!=null) {
search.findByName("Bill");
...
...
// is search still valid here ???
}
context.ungetService(ref);
}


Which means .... (ServiceTracker)
MySearch
ServiceTracker tracker =
new ServiceTracker(ctx, ISearchService.class.getName(), null);
tracker.open();
ISearchService search = ((ISearchService )tracker.getService());
if (search!=null) {
...
...
// is search still valid here ???
}


Which means ... (Declaritive approach)

<component name= "MySearch">
<implementation class="org.example.MySearch"/>
<reference bind="setCustomerSearch"
cardinality="1..1"
interface="org.eclipse.search.ISearchService"
name="Search"
policy="dynamic"
unbind="unsetCustomerSearch"/>
</component>

Note: The declarative approach requires that DS controls the lifecyle
of the target component. (MySearch).
If that lifecycle is out of your control, you need an API.
Both components should be DS to make sense.


Which means ... (Declaritive approach)

public class MySearch {
public void setCustomerSearch(ISearchService search) {
this.search = search;
}

public Customer[] listCustomer(String name) {
return search.findByName(name);
}

public Customer findCustomer(String number) {
return search.findByNumber(number);
}

public void unsetCustomerSearch(ISearchService search) {
this.search = null;
}
}


Which means ... (e4 approach)


@Inject
private ISearchService search;

}

}

}


That things can break ...

• NPE when accessing service that are injected
›  Service was not started
›  Service was stopped
›  No bundle in the setup implements the Service
• Service instance exists, but Service-Bundle is stopped
• Lazy activation does not work once a bundle is stopped
• At startup no way to check that all dependencies are there.


Are you ready ... ?

• Is our code always ready that a reference to a service
instance is no longer valid ?
• In your RCP application, do you often stop and start
bundles ?
• Do you swap-in a new Service implementation at runtime ?

OR do you need that dynamic ?


Eclipse IDE...

• A typical usecase for dynamic OSGi is installation of
bundles in the Eclipse IDE
• Did you notice that the "Apply changes" option from
Eclipse 3.7 is gone in 3.8 and 4.2 ?
• Did it ever work for you ?
• Test with Eclipse IDE
›  install bundle in 3.7 and in 4.2 (no apply in 4.2)
›  stop a bundle in the console and see how the Eclipse
IDE reacts


stopped egit.ui


Not everyone wants/needs dynamic ...

• Often there is nothing reasonable to do when an injected
service is not there.
›  SearchDialog without SearchService
›  SearchService without DatabaseConnectorService
›  Car without BrakeService


Work-arounds

• Frameworks on top of OSGi implement their own way of
starting bundles EAGER
• OSGi based servers want to start everything immediately
• Applications manually start all necessary bundles and
service at startup and never stop them


The advantage ...

• A missing service is not "normal", don't try to handle it.
• Detect missing components at startup and fail instantly
• Services don't appear at some time. They are either there
or not.
• A Service cannot disappear while you are using it.


There are exceptions...

• Some Services need the dynamic (=flexibility)
›  i.e. change ServiceImplementation if you switch from
online to offline
• So document that decision and add the additional checks


Which means ... (API approach)

ref = context.getServiceReference(ICustomerSearch.class.getName());
if (ref!=null) {
ICustomerSearch search = (ICustomerSearch)context.getService(ref);
// if (search!=null) {
...
...
// is search is valid here !!!
// }
context.ungetService(ref); // ???
}


Which means ... (e4 approach)


@Inject
private ISearchService search;

}

}

}


Others ...

OSGi != !=

no dynamic lifecycle per component


OSGi

•  Modules as Bundles cool

•  Bundles /w public API & private Impl cool

cool
•  OSGi Service
•  Extensions cool

•  Lazy Activation be careful

•  Start / Stop Services on condition use with care

•  Ability to stop Bundles at Runtime avoid

•  Ability to replace Bundles at Runtime avoid


Q&A


Why is os gi dynamic.pptx

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