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From Achim Nierbeck <bcanh...@googlemail.com>
Subject Karaf 4.0 : new behaviour of the feature resolver
Date Wed, 11 Feb 2015 18:08:12 GMT
Hi,

as Guillaume and I have been discussing this on IRC and we didn't come up
with a real solution we decided to move this little "rambling" to the dev
list.

First of all please take a look at the issue KARAF-3520 [1]

This issue leaves us with a strange situation I'd like to briefly describe:
- Installation of a bundle with a require-capabilties header does work
- Installation of a feature containig the same headers doesn't work

Reason, the Feature Installer now does have a lifecycle and therefore
checks the Meta-Information and doesn't find the required Capability even
though the service is available and running.
Ergo, it fails on installing the exact same bundle with the error which is
in the issue. [1]


According to Guillaume, this is intentional since the feature service needs
to rely on the meta-information available from all bundles, that are going
to be installed with the feature (including transitive features/bundles)
and already installed features. BUT if one bundle doesn't contain those
"meta-data" the feature install will fail because of the missing meta-data
but not necessarily the "requested" capability isn't there.

I fear this is a specialty of services at this point.


The big Issue I currently see at this point, therefore this writing.
The user experience will suffer tremendously because installations that did
work before < Karaf 4
will now break. The next big discrepancy people will see, if I install the
bundle itself it works but not the features.

Therefore we somehow need a solution that basically fits both needs, the
user-experience and the management of transitive dependencies through the
feature service. The way it is working right now, is a major breaking issue
(yes Karaf 4 is a major release therefore we are safe).
Especially with new bundles being build by the maven-bundle-plugin those
require headers are generated more often.

My first Impression had been to make sure that the feature service isn't as
strict anymore.
For example to verify that the Required Service isn't available through the
meta-information verify it via the service registry.

Guillaume mentioned to me on IRC that this isn't as easy as I imagined, due
to the restrictions on the Resolver which is used from the framework. As
benefit with the way the features resolving works right now, transitive
dependencies which are declared as "dependency='true'" will be removed
during the installation of another feature in case it's not needed any
longer.

So it boils down to, if one bundle declares a Required-Capabilities those
need to be fullfilled by the means of Provide-Capabilities. Otherwise it
doesn't install the feature.

regards, Achim

[1] - https://issues.apache.org/jira/browse/KARAF-3520

-- 

Apache Member
Apache Karaf <http://karaf.apache.org/> Committer & PMC
OPS4J Pax Web <http://wiki.ops4j.org/display/paxweb/Pax+Web/> Committer &
Project Lead
blog <http://notizblog.nierbeck.de/>
Co-Author of Apache Karaf Cookbook <http://bit.ly/1ps9rkS>

Software Architect / Project Manager / Scrum Master

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