[hibernate-dev] Concept of "service availability"
mailing at bibbernet.org
mailing at bibbernet.org
Mon Aug 27 15:32:04 EDT 2012
OSGI has two classes/interfaces for dealing with Service Tracking -
ServiceTracker and ServiceTrackerCustomizer, which could be used to
create a OSGI-auto-discovery (and auto-cleanup)-Helper for hibernate:
A simple skeleton could look like:
package org.bibbernet.osgi;
import org.osgi.framework.ServiceReference;
import org.osgi.util.tracker.ServiceTrackerCustomizer;
public class MyServiceTrackerCustomizer implements
ServiceTrackerCustomizer {
@Override
public Object addingService(ServiceReference reference) {
// a new service matching the criteria of the ServiceTracker
can be added to the ServiceTracker
// get the service object
Object service =
reference.getBundle().getBundleContext().getService(reference);
// TODO: -> add service to hibernate service registry or if we
we don't want if to get tracked,
// return null
// reference.getBundle() could be used to
return service;
}
@Override
public void modifiedService(ServiceReference reference, Object
service) {
// nothing todo - only service properties are modified
}
@Override
public void removedService(ServiceReference reference, Object
service) {
// TODO: -> remove service from hibernate service registry
// release this service reference
reference.getBundle().getBundleContext().ungetService(reference);
}
}
-------------------------------------
package org.bibbernet.osgi;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.Constants;
import org.osgi.framework.Filter;
import org.osgi.util.tracker.ServiceTracker;
public class MyActivator implements BundleActivator {
private ServiceTracker myServiceTracker;
@Override
public void start(BundleContext context) throws Exception {
// Create a LDAP-like-Filter which matches the interested services
// or better the interested interfaces the service should implement
Filter filter = context.createFilter("(" + Constants.OBJECTCLASS
+ "=xxx.yyy.zzz)");
myServiceTracker = new ServiceTracker(context, filter,
new MyServiceTrackerCustomizer());
// start tracking
myServiceTracker.open();
}
@Override
public void stop(BundleContext context) throws Exception {
// close ServiceTracker to ensure all service references get
released
if (myServiceTracker != null) {
myServiceTracker.close();
}
}
}
The bundle implementing the service then simple needs to announce a
service with one of the watched interfaces - once the service gets
started, hibernate gets announced and could use either the service
object itself or the bundle defining the service - with this approach
you could also use the bundle's classloader to load classes with
bundle.loadClass() or access resources within the bundle with
bundle.findEntries(), etc.
- Martin
On 08/27/2012 07:34 PM, Steve Ebersole wrote:
> Not sure this will actually help with OSGi in terms of auto-discovery
> at all after thinking about it some more. The problem is that in order
> for auto-discovery to happen, Hibernate would need to have visibility
> into the jar defining the service anyway in order to auto-discovery it.
>
> On the bright side, I realized another benefit. It would finally be
> possible to report the available types of a particular
> service/strategy. For example, show me all the available
> ConnectionProviders; all the available Dialects; etc...
>
>
> On Fri 24 Aug 2012 08:51:09 PM CDT, Steve Ebersole wrote:
>> Would be nice to consolidate the notions of
>> "ServiceAvailabililtyNotifier" and "ServiceContributor" (that I just
>> added on metamodel). Such that a ServiceContributor would be able to
>> regsiter the short names as well.
>>
>>
>> On Thu 23 Aug 2012 03:58:10 PM CDT, Steve Ebersole wrote:
>>> Ok, going to start working this up on master tomorrow. We will just
>>> tackle the "going away" problem if/when it actually arises.
>>>
>>> On Tue 21 Aug 2012 05:55:31 PM CDT, Steve Ebersole wrote:
>>>> Everyone else ok with this idea?
>>>>
>>>> On Tue 21 Aug 2012 08:27:25 AM CDT, Steve Ebersole wrote:
>>>>> Not so concerned about shutdown situations.
>>>>>
>>>>> More, imagine a custom ConnectionProvider implementation provided by
>>>>> user. And the use case of upgrading that implementation "in flight".
>>>>> I think thats the OSGi use case. And not so sure Hibernate should be
>>>>> implementing this self healing. I guess it depends how deeply we want
>>>>> to support the OSGi model above and beyond JSE/JEE
>>>>>
>>>>> Obviously a used ConnectionProvider just going away is going to render
>>>>> the SessionFactory using it broken.
>>>>>
>>>>> On Tue 21 Aug 2012 08:22:11 AM CDT, Scott Marlow wrote:
>>>>>> On 08/20/2012 11:19 PM, Steve Ebersole wrote:
>>>>>>> This ties together a few different discussions that have been
>>>>>>> going on
>>>>>>> simultaneously on the mailing list that I think are all related.
>>>>>>>
>>>>>>> Right now to configure certain services (select one impl over
>>>>>>> another)
>>>>>>> users generally give the FQN for that impl Class. For example to
>>>>>>> use
>>>>>>> C3P0 connection pooling users would say:
>>>>>>>
>>>>>>> hibernate.connection.provider_class =
>>>>>>> org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> We have discussed why this is bad even before any of the OSGi
>>>>>>> discussions and the solution we wanted to shoot for was that of
>>>>>>> naming
>>>>>>> "selectors" such that the user would instead say:
>>>>>>>
>>>>>>> hibernate.connection.provider_class = c3p0
>>>>>>>
>>>>>>> And "c3p0" here would be interpreted to mean "instantiate and
>>>>>>> configure
>>>>>>> the
>>>>>>> org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider
>>>>>>>
>>>>>>>
>>>>>>> Class". But that still means a limited set of short name values
>>>>>>> *and*
>>>>>>> still gives us a problem (iiuc) under OSGi due to visibility.
>>>>>>>
>>>>>>> So what I propose instead is a way for service implementors to be
>>>>>>> registered under a short name via discovery. The main piece to
>>>>>>> this is
>>>>>>> the "registry" (which is also a service under the
>>>>>>> BootstrapServiceRegistry):
>>>>>>>
>>>>>>> interface AvailableServiceRegistry extends Service {
>>>>>>> public<T> Class<? extends T>
>>>>>>> findAvailableServiceImplementor(Class<T> serviceRole, String
>>>>>>> selector);
>>>>>>> }
>>>>>>>
>>>>>>> class AvailableServiceRegistryImpl
>>>>>>> implements AvailableServiceRegistry,
>>>>>>> ServiceRegistryAwareService {
>>>>>>> private Map<Class,Map<String,Class>> availableImplementors =
>>>>>>> ...;
>>>>>>>
>>>>>>> @Override
>>>>>>> public<T> Class<? extends T>
>>>>>>> findAvailableServiceImplementor(Class<T> serviceRole, String
>>>>>>> selector) {
>>>>>>> // purposely simplistic :)
>>>>>>> return availableImplementors.get( serviceRole ).get(
>>>>>>> selector );
>>>>>>> }
>>>>>>>
>>>>>>> @Override
>>>>>>> public void injectServices(ServiceRegistryImplementor
>>>>>>> serviceRegistry) {
>>>>>>> final LinkedHashSet<ServiceAvailabililtyNotifier>
>>>>>>> notifiers =
>>>>>>> serviceRegistry.getService( ClassLoaderService.class
>>>>>>> ).loadJavaServices(
>>>>>>> ServiceAvailabililtyNotifier.class );
>>>>>>> for ( ServiceAvailabililtyNotifier notifier : notifiers
>>>>>>> ) {
>>>>>>> for ( ServiceAvailabililty availability :
>>>>>>> notifier.getAvailabilities() ) {
>>>>>>> // again, purposely simplistic
>>>>>>> Map<String,Class> serviceImplementors =
>>>>>>> availableImplementors.get( availability.getRole() );
>>>>>>> serviceImplementors.put(
>>>>>>> availability.getSelector(),
>>>>>>> availability.getImplementor()
>>>>>>> );
>>>>>>> }
>>>>>>> }
>>>>>>> }
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Outstanding question... Especially in OSGi, where service bundles
>>>>>>> can be
>>>>>>> added/removed, how do we best account for cleaning up no-longer
>>>>>>> valid
>>>>>>> references (even more importantly perhaps, what the hell does it
>>>>>>> mean to
>>>>>>> Hibernate when a ConnectionProvider implementation, for example,
>>>>>>> that is
>>>>>>> in use gets taken away?!?). Perhaps this is just where an
>>>>>>> OSGi-specific
>>>>>>> Hibernate ServiceRegistry implementation would come into play.
>>>>>>>
>>>>>> Adding Jesper as we were talking about how to handle "quiescence
>>>>>> shutdown" at the AS level, which sounds related. Once we take the
>>>>>> ConnectionProvider away, I would expect the Hibernate
>>>>>> session(s)/session factory to be broken. If/when the
>>>>>> ConnectionProvider comes back, Hibernate would need to re-establish
>>>>>> it. I'm thinking that we need a neutral (autonomic) API/SPI for
>>>>>> attempting to re-establish the ConnectionProvider.
>>>>>>
>>>>>> For the most part, a "quiescence shutdown" of the AS, would mean
>>>>>> keeping the ConnetionProvider alive until the end (of the planned
>>>>>> shutdown). I'm thinking that being able to re-establish the
>>>>>> ConnectionProvider would still be useful (for AS "quiescence
>>>>>> shutdown"), especially if something goes wrong during the shutdown
>>>>>> and
>>>>>> manual intervention is needed.
>>>>>>
>>>>>> To me, the process of re-establishing the ConnectionProvider,
>>>>>> could be
>>>>>> labeled "self healing" (with the help of an autonomic API/SPI).
>>>>>>
>>>>> --
>>>>> steve at hibernate.org
>>>>> http://hibernate.org
>>>> --
>>>> steve at hibernate.org
>>>> http://hibernate.org
>>> --
>>> steve at hibernate.org
>>> http://hibernate.org
>> --
>> steve at hibernate.org
>> http://hibernate.org
> --
> steve at hibernate.org
> http://hibernate.org
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