[jira] [Comment Edited] (FELIX-4417) Circular references detected but not resolved if one of the references in the cycle has optional cardinality

["Markus Kaufhold (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FELIX-4417?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14128201#comment-14128201 ] 

Markus Kaufhold edited comment on FELIX-4417 at 9/10/14 10:27 AM:
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David: Your comment is right, nevertheless the original complaint is still valid.
"However, if one of the references in the cycle has optional cardinality SCR must break the cycle. The reference with the optional cardinality can be satisfied and bound to zero target services. Therefore the cycle is broken and the other references may be satisfied."
Felix SCR isn't compliant to that OSGi part.

We do have the following use case, using that scheme:
Component B is responsible for a certain functionality, and registers an interface "b" as a service that provides access to some information of that functionality. As the information of that functionality can change during runtime, component B also defines another interface "c", which can be implemented and registered by anybody that is interested in those changes. Component B is then binding "c" as 0..n (optional).
Component A is interested in the information of "b", and needs to be informed about changes during runtime, thus A is then binding "b" unary (1..1) and providing "c" as a service.

According OSGi chapter 112.3.7 this is a valid use case, to be supported by the SCR.
With the current implementation the Felix SCR does not differentiate between mandatory and optional references within the cycle detection. From implementation point of view this is not easy to solve, as in the context of component A it is not known that component B is binding "c" in an optional way.


was (Author: markuska):
David: Your comment is right, nevertheless the original complaint is still valid.
"However, if one of the references in the cycle has optional cardinality SCR must break the cycle. The reference with the optional cardinality can be satisfied and bound to zero target services. Therefore the cycle is broken and the other references may be satisfied."
Felix SCR isn't compliant to that OSGi part.

We do have the following use case, using that scheme:
Component B is responsible for a certain functionality, and registers an interface "b" as a service that provides access to some information of that functionality. As the information of that functionality can change during runtime, component A also defines another interface "c", which can be implemented and registered by anybody that is interested in those changes. Component B is then binding "c" as 0..n (optional).
Component A is interested in the information of "b", and needs to be informed about changes during runtime, thus A is then binding "b" unary (1..1) and providing "c" as a service.

According OSGi chapter 112.3.7 this is a valid use case, to be supported by the SCR.
With the current implementation the Felix SCR does not differentiate between mandatory and optional references within the cycle detection. From implementation point of view this is not easy to solve, as in the context of component A it is not known that component B is binding "c" in an optional way.

> Circular references detected but not resolved if one of the references in the cycle has optional cardinality
> ------------------------------------------------------------------------------------------------------------
>
>                 Key: FELIX-4417
>                 URL: https://issues.apache.org/jira/browse/FELIX-4417
>             Project: Felix
>          Issue Type: Bug
>          Components: Declarative Services (SCR)
>    Affects Versions: scr-1.8.2
>            Reporter: Victor Antonovich
>         Attachments: CircularReferenceTest-trunk.patch, CircularReferenceTest.zip
>
>
> Looks like current Apache Felix SCR implementation doesn't conform fully to Declarative Services Specification, which says (http://www.osgi.org/download/r4v43/osgi.cmpn-4.3.0.pdf, 112.3.7):
> "Circular references must be detected by SCR when it attempts to satisfy component configurations and SCR must fail to satisfy the references involved in the cycle and log an error message with the Log Service, if present. However, if one of the references in the cycle has optional cardinality SCR must break the cycle. The reference with the optional cardinality can be satisfied and bound to zero target services. Therefore the cycle is broken and the other references may be satisfied."
> In case of two components, A and B, where A is delayed with 1..1 static reference to B, and B is immediate with 0..n dynamic reference to A, Felix SCR should:
> 1) activate B with dynamic reference to A satisfied and bound to zero services
> 2) activate A with satisfied static reference to B
> 3) bind dynamic reference to B in component A
> But it seems current Felix SCR implementation can't handle this kind of circular dependency correctly and is failing with message "Circular reference detected".



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