Currently karaf features are done in a way that each feature is
consistent in itself.
While this makes the features easy to install I think the big problems
is that this creates a coupling between features that should not be
directly
related.
As an example consider the transaction and jpa features.
transaction -> persistence-api [1.1,2)
jpa -> persistence-api [1.1,2)
So both of them depend on the persistence-api feature. All three
features currently are in the enterprise feature shipped with karaf.
The problem is that this creates a tight coupling between the karaf
version and the aries jpa and aries transaction versions. The problem is
similar with other
features like hibernate, openjpa, ...
Currently we provide several feature versions in the same feature file
to give the user a bigger choice in what to install. The big problem
there is though that we can not
include all the possible versions and of course we can not include
versions that are not yet released. So when a customer hits a bug in any
of the bundles he can not easily upgrade to a new version of say aries jpa.
So what I would like to do is create features in the place where the
software is released and have as loose coupling between the modules as
possible.
So for example I would like to create a the aries transaction feature
file in the aries transaction module and the aries jpa feature file in
the aries jpa module. So whenever a release of these is done there is
also a new feature file that makes the newest release available to
users. So they do not need to wait until a new karaf version picks up
the new aries jpa release.
So in the aris jpa feature file I would put the jpa feature but I do not
want to have a dependency on the aries jpa feature file.
At the moment this is not possible as I need to define a reference to
the persistence-api feature to make the aries jpa featur consistent.
So I wonder if we could allow features to kind of have open
requirements. Like the aries jpa feature would have an open requirement
on the package javax.transaction [1.1,2) possibly also a requirement for
a TransactionManager service. These requirements would not even have to
be defined as the bundles contain all the needed information.
Then when I try to install the jpa feature in karaf the feature service
could tell me that there are the unresolved requirements above. It could
then even go through all other features and propose the ones that offer
the respective capabilities. So it could work like this:
> feature:install jpa
Unresolved requirements:
Package: javax.persistence [1.1,2)
Service: javax.transaction.TransactionManager
The requirements can be resolved when also installing
1. transaction transaction-api/1.1
2. transaction transaction-api/1.2
In an interactive mode the user could then select the features to also
install from the list. In a non interactive mode there would just be an
error and the user would have to add the features to the command.
In the same way it would work for a whole set of features:
> feature:install jpa/2.1.1 hibernate/4.3.6.Final
Unresolved requirements:
Package: javax.persistence [1.2,2)
Service: javax.transaction.TransactionManager
The requirements can be resolved when also installing
1. transaction transaction-api/1.2
In this case as there is only one solution the feature service could
even auto install the necessary features without them being mentioned in
any of the features to be installed. So it would act a bit like and OBR
on feature level.
I think it would make much more sense to handle features like this than
how we do it right now. It would allow to use the new requirements and
capabilties and the new resolver much better. At the same time the
features themselves would become much more loosely coupled and easier to
maintain.
WDYT?
Christian
--
Christian Schneider
http://www.liquid-reality.de
Open Source Architect
http://www.talend.com
