Re: Apache Karaf info

[Shawn Jiang <ge...@gmail.com>]

On Fri, Sep 4, 2009 at 3:43 AM, Jarek Gawor <jg...@gmail.com> wrote:

> Hey,
>
> Apache Karaf was mentioned a couple times on this list as something we
> want to consider for using in/for Geronimo 3.0. So, I've been looking
> at it for last few days to get a better understanding how it works,
> what it does, etc. and thought maybe others want to know what I
> learned about it. Overall, I think it fits pretty well for what we
> want to do with Geronimo.
>
> What does Karaf have?
>
> - JMX (rfc 139): it actually uses the RI from OSGi Alliance
>

I noticed there's rfc 139 RI(ASL2) in Karaf too, I'm wondering if geronimo
can use this RI as rfc 139 implementation directly.

- Blueprint (rfc 124): it uses Geronimo implementation. It actually
> uses Blueprint to deploy and configure all of the services it
> provides. For example, the JMX connector, rmi registry, mbean server,
> gshell commands, all are configured via Blueprint. It also uses the
> Blueprint features that were deleted from the official spec but we
> still kept it in the Geronimo impl - i.e. the Configuration Admin
> service support and namespace handler extensions.
>




> - GShell: provides commands for osgi operations (install bundle,
> uninstall bundle, stop, start, etc.), adding/changing/deleting
> configurations (for Configuration Admin service support), some basic
> shell commands (exec, grep, etc)., some commands for creating Karaf
> instances (same idea as in Geronimo), some for inspecting logs, and
> some ssh commands. I think you can install ssh daemon and ssh into the
> console but I have not tried that.
> - Logging service: provided by Pax Logging, exposes all the logging
> API (slf4j, commons logging, etc.)
> - Additional url handlers for mvn and wrap urls. That's provided by
> Pax Url bundles. The mvn: url handler allow you to specify a url in
> mvn://group/artifacts/version/type format, and the handler will go out
> to a specified set of repositories (local or remote) and resolve the
> bundle and install it. The wrap: url handler allows transform and
> install non OSGi jar files into OSGi bundles. See Pax Url for more
> details.
> - It also comes with Preference Service and Configuration Admin
> Service - both provided by Felix.
> - "File Install" bundle provided by Felix. It provides hot
> deployment/update functionality. For example, you can drop bundles
> into the "deploy" directory and they will be automatically deployed
> (just like in Geronimo) and also, it can watch configuration files
> (those in "etc" directory) and update the corresponding Configuration
> objects (from Configuration Admin service). More on this below.
> - Some JAAS security support which I haven't looked at yet.
>
> How does it work?
>
> Karaf can work on top of any OSGi framework and in fact comes with
> Equinox and Felix jars but by default it boots Felix. Karaf starts by
> instantiating the OSGi framework and than reads etc/startup.properties
> file which contains an ordered list of bundles (and their run level)
> to start. The bundles in the startup.properties file are specified in
> mvn format. That's one of the places where Pax Url is used. Pax Url is
> actually configured (see etc/org.ops4j.pax.url.mvn.cfg) to use the
> "system" directory as a default local maven repo. It's also configured
> with a few remote repos. The "system" directory is in mvn layout and
> has all the bundles that Karaf needs (mentioned above). However,
> during startup Karaf actually installs the bundles from the "system"
> bundle into the framework. And with Felix, when you install a bundle
> it actually copies the bundle jar file into its own "data" directory.
> So after starting Karaf for the first time, you pretty much will have
> the jars in the "system" directory duplicated in the "data" directory.
> I guess that's the price for being OSGI framework independent. But we
> might be able to improve it. Also, if you install a bundle
> dynamically, it goes into the Felix's "data" directory and not the
> "system" directory.
>
> After all the bundles are installed and framework started, all the
> rest of the work is done via Blueprint.
>
> Configuration
>
> In OSGi things are configured via Configuration Admin service.
> ConfigAdmin service provides Configuration objects which basically
> have multiple name=value properties and each Configuration object has
> an unique id. In Karaf and with the help of "File Install" bundle
> these Configuration objects are represented as simple text
> configuration files under the "etc" directory with ".cfg" extension.
> The name of the file corresponds to the id of the Configuration
> object. And if you change the .cfg file at runtime the Configuration
> object should be updated as well. Geronimo Blueprint implementation
> has Configuration Admin support so all this nicely integrates
> together.
>
> Features
>
> Karaf has this concept of "features". Features can have dependencies
> on other features, they contain a list of bundles, and can have some
> configuration data. So installing a feature, can trigger installation
> of multiple bundles. For example, installing "http" feature installs
> Geronimo Servlet spec jar bundle, Jetty bundle, and Pax Web bundle.
> All are automatically downloaded from the maven repos specified in
> etc/org.ops4j.pax.url.mvn.cfg.
>

This is like current plugin and plugin group of geronimo.

>
> There is also a "webconsole" feature that installs Felix's webconsole
> bundle and some Karaf specific console extensions. And there are other
> predefined features besides "http" or "webconsole".
>
> As far I can tell right now, features are just external metadata and
> are not usually embedded within bundles. However, feature metadata can
> be deployed at runtime and actually during deployment it is turned
> into a bundle.
>
> Anyway, that's what I learned about Karaf so far. Of course, I might
> have missed a few things or not provided 100% complete descriptions.
> I'm sure Guillaume or others can correct me.
>
> Jarek
>
This is really a good summary of karaf, are there any further discussions on
how to reuse karaf(part or whole) in geronimo 3.0 ?


-- 
Shawn