Felix Meschberger created FELIX-4149:
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Summary: Do not directly support modifying service registration
properties
Key: FELIX-4149
URL: https://issues.apache.org/jira/browse/FELIX-4149
Project: Felix
Issue Type: Task
Components: Declarative Services (SCR)
Affects Versions: scr-1.6.2
Reporter: Felix Meschberger
With FELIX-3377 we implemented a feature for a component to easily influence
its service registration properties. This sounds very useful upfront but has
some intricate implications:
* If a component is instantiated multiple times as a result of being a factory
service (instantiation per using bundle) or even as a new prototype service
(instantiation per retrieval from the service registry) each such instance may
independently modify the service registration properties of the shared service
registration. This may have strange implications.
* If changing the service registration properties may cause changes in using
the service, it may be that a loop may be created: Consider a component C1
being registered as a delayed service. When this service is retrieved using a
service filter, the component is activated and may change the registration
properties. This may cause the consumer's service filter to not match any more
and hence to unget the service immediately. This may cause the Component C1 to
be deactivated and change back the service registration property. Now the
consumer's filter matches again and C1 is retrieved and activated again ...
Now, granted both situations may also happen if DS would not support setting
service registration properties. But the chances of falling into the trap is
higher if it would be too simple to do. Also explaining the why's and pro's and
con's etc. is somewhat complicated. On the other hand the use case is not one
happening very often -- in our application of 100s of components we had it once
or twice.
So the OSGi CPEG decided to not add this feature to the specification.
I am aware, that we unfortunately already released a version of Felix DS
supporting this feature. So it will be a hard decision to revert that.
I propose to disable the feature by default and allow it to be explicitly
enabled using some configuration (similar to what we did with the
non-spec-compliant handling of factory configuration for Component Factory
components).
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