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From Donald Whytock <dwhyt...@gmail.com>
Subject Re: Fun with OSGI
Date Tue, 09 Aug 2011 18:56:51 GMT
My experience with OSGi/Felix is comparatively limited compared to
some, but I think I'm making good use of its fundamental capability:
hot-swapping components.

My app is a framework for hooking a varying number of command
interpreters to a varying number of communication components, thus
allowing multiple users connecting over different services/platforms
to enter commands to the system and possibly share the results with
other users.

As a result, my primary need is to be able to conveniently add, update
and remove command interpreters and/or communication components at
will.  Felix has been excellent for this.  Because of this I've been
able to conveniently develop new parsers to provide myself with more
configuration options and debug Camel components.

Yes, there's an issue with non-OSGi-ready libraries, but in many cases
that's just a matter of packaging them with the right manifest.  The
biggest problem I've had with that is when jars depend on other jars,
sometimes four or five levels deep.  But mostly this encourages me to
keep my code very lean, with minimal external dependencies, and even
with those looking for open-source equivalents to large corporate
jars.

All told, with the required modularity thinking and the encouragement
to loose coupling, I believe working with OSGi has induced me to build
a better app.

Don

2011/8/9 Teemu Kanstrén <tkanstren@gmail.com>:
> Hello all,
>
>  I have used OSGI in a few projects, most recently in a research prototype
> for a sensor network data collection thingy.
>
>  Overall, I think OSGI is not hugely complex and provides some useful
> features. However, overall my feelings are a bit mixed. So I would like to
> ask others, what are your experiences in using OSGI vs other platforms. Some
> more specific experiences from my viewpoint:
>
> -Automated updates (or support for them) are commonly mentioned as something
> supported by OSGI. I see there is some basic support for this in
> loading/unloading services and bundles in the standard container.
> Additionally, there are things like Apache ACE that are commonly mentioned
> to take it further. But I fail to see how this really helps much, the big
> issue for me comes to transferring state from old services to new ones and
> managing all the dependencies between the elements as  change is rarely
> localized. While I have needed to support updates, I find it is easier to
> just deploy a complete new version and restart the software.
>
> -Service code is separated by OSGI through different classloaders. For me
> this has been really nice in keeping dependencies from spreading and forcing
> me to think about component boundaries in a more focused way.  But running
> an OSGI container just for this seems a bit heavyweight for me. The
> classloader separation also causes some big issues for me such as sharing
> libraries over services, such as web-services frameworks, where managing
> configuration files across services is just extra hard when the classloaders
> are separated, in addition to the usual OSGI classloader issues. When
> libraries are better supported, such as the Felix HTTP service with Jetty,
> it seems nice but actually is a wrapper that hides the configuration options
> under layers of abstraction (added complexity) that hide the more advanced
> configuration options from me and makes it hard if possible at all to use
> them.
>
> -Managing the framework and my application becomes more complex as the user
> has to understand the container caches, large number of directories,
> libraries, configuration files, etc. Things like configuring my app to run
> as a unix daemon are much more complex to manage and debug as I am not in
> direct control over the platform. Errors in application startup from remote
> deployments are harder since they are shown mainly in Felix and not in my
> application log files.
>
> -Using OSGI for me is a form of a local-level service abstraction (SOA). I
> have basically used Java interfaces to define the "interface" of each OSGI
> service. But this eliminates the navigation support my IDE in terms of
> static analysis and adds, what seems to me, unnecessary abstration. This is
> probably my failure in using too many interfaces but it feels to me as a
> "common practice" for OSGI apps. I was also looking for some support to hook
> and trace service interactions, which could justify some of this, but there
> is nothing like this.
>
> -Integration testing is difficult due to all the wiring required to get the
> overall system running. In simple cases it works OK with my own
> MockBundleContext, and the SOA approach makes the component composition even
> cleaner in this regard. However, in more complex interactions starting the
> whole container becomes a big burden for me.
>
> Overall, the approach of SOA at local level seems great for the overall
> architecture. But it seems to me currently there are just too many issues
> for me, and for that reason, I would prefer a more simple approach.
>
> Maybe I am just doing wrong or building wrong types of apps. Any other
> experiences?
>
> Thanks for your thoughts,
> Teemu
>

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