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