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Re: [equinox-dev] DS and bundle stop/start 


BJ Hargrave wrote:
Why doesn't DS just asynchronously process bundles which are lazy activated (not lazy started which is an incorrect term)? Then you have the same behavior (async processing) regardless of whether the bundle is lazily or eagerly activated.
The DS components of activated bundles are processed by DS when it receives BundleEvent.STARTED (for normal activated bundles) or BundleEvent.LAZY_ACTIVATION (for lazy activated bundles). These events are processed in DS by a SynchronousBundleListener. Actually DS processes synchronously the components in both cases. The difference comes with the fact that BundleEvent.STARTED is sent by the framework after the bundle's activator is started. We cannot modify DS to process only BundleEvent.STARTED because in this case lazy activated bundles will not be processed at all.
What we can do in DS is to process the DS components of a bundle when DS receives BundleEvent.STARTING event instead of BundleEvent.STARTED. This way DS will process the components before the framework calls the bundle activator's start method and the behavior would be similar to the one when processing lazy activated bundles. However, the DS specification always speaks of DS processing started bundles. So, I am not sure whether it is appropriate to process the DS components of a bundle before it is fully started. 

Stoyan



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*BJ Hargrave*
Senior Technical Staff Member, IBM
OSGi Fellow and CTO of the _OSGi Alliance_ <http://www.osgi.org/>_
__hargrave@xxxxxx.com_ <mailto:hargrave@xxxxxxxxxx> 	

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From: 	John Arthorne <John_Arthorne@xxxxxxxxxx>
To: 	equinox-dev@xxxxxxxxxxx
Date: 	2009/10/26 14:32
Subject: 	[equinox-dev] DS and bundle stop/start
Sent by: 	equinox-dev-bounces@xxxxxxxxxxx


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I came across an interesting problem today involving DS and expicitly starting/stopping bundles. After chatting with Tom he suggested I raise it here for general awareness and to discuss whether the behaviour makes sense.
In various places in p2 today we explicitly start bundles for various reasons. We typically use Bundle.start(Bundle.START_TRANSIENT) for this purpose. We are starting to use DS in p2 today, and we have a few places where a bundle acquires a service that it registered via DS in its own BundleActivator.start method. It turns out that DS only processes/starts service components synchronously for bundles that are lazy started. If you start a bundle explicitly the DS processing occurs asynchronously, and as a result the services provided via DS are not available at the time the bundle starts. The result is subtlely different bundle behaviour (or outright failures), if a bundle is started explicitly via Bundle#start versus implicitly via lazy activation: 

1) Lazy start case:
a) bundle's service component is processed and services provided by the component are registered by DS 
b) bundle activator starts, and can use services registered in 1a)

2) Activation via Bundle.start(Bundle.START_TRANSIENT):
 a) bundle's activator starts, and services are not yet available
b) bundle's service component is processed and services provided by the component are registered by DS
It turns out there is a coding pattern that can be used to make the explicit start case match the lazy start case: 

                       *final* Bundle bundle = ...;//get some bundle
                       bundle.start(Bundle.START_ACTIVATION_POLICY);
                       bundle.start(Bundle.START_TRANSIENT);
The call to start(Bundle.START_ACTIVATION_POLICY) causes DS to process the bundle and register its component services, but does not start the bundle. The second call to start with Bundle.START_TRANSIENT actually starts the bundle.
The moral of the story seems to be that we need to use this "double start" coding pattern anywhere we are starting bundles, because those bundles might be using DS and relying on the activation order. Or, perhaps someone has a suggestion for changes that can be made to the framework or DS so that these cases behave consistently... 

John_______________________________________________
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_______________________________________________
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