[
https://issues.apache.org/jira/browse/S4-4?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13126697#comment-13126697
]
Adam Wojtuniak commented on S4-4:
---------------------------------
Hi Leo,
We can't avoid class loading issue even if we want only to load apps
dynamically. Every app is going to have dependencies to s4 (plus s4
dependencies) and also can happen that app can have their own dependencies, all
these dependencies must be visible to app classloader. One solution is to
expose all these dependencies as a system packages then they will be visible to
all bundles in the osgi container but that is a hack which are going to lead to
maintenance nightmare (every time is new dependency it must be added to system
package). Class.forName is another problem, for example kryo lib is loading
classes which in the container aren't visible to kryo until kryo will be
bundlized with DynamicImport-Package: *. These problems can't be avoided and is
no matter what solution is used (JBossModules or Felix etc.). One solution is
to find proper bundlized version of some s4 dependencies or wrap them using
http://www.aqute.biz/Code/Bnd and put for some of them DynamicImport-Package
: *.
Cheers,
Adam
> Dynamic App Loading
> -------------------
>
> Key: S4-4
> URL: https://issues.apache.org/jira/browse/S4-4
> Project: Apache S4
> Issue Type: New Feature
> Reporter: Leo Neumeyer
>
> We want to be able to load and unload applications dynamically. Here is some
> background information:
> Once a server is started, we expect it to run indefinitely until there is a
> failure or we need to upgrade the framework software or we change the cluster
> configuration. In S4, the cluster is symmetric, that is, all the nodes are
> identical, with the same code base and the same configuration.
> Applications are loaded dynamically after the cluster is started. If app1
> depends on app2, then app2 needs to use an EventSource to publish events.
> Apps must be able to find EventSource and subscribe to it during
> initialization.
> When an app is unloaded, it will close its EventSource(s). This in turn, will
> close all the streams that are subscribed. The apps that own the streams are
> responsible for taking action once their dependencies are gone.
> Possible roadmap:
> M1:
> - Server starts with no apps.
> - Apps are loaded during initialization, for example by searching an
> app directory.
> - Apps are initialized (no dependencies among apps).
> - Apps are started.
> M2:
> - Add dependencies among apps using EventSource.
> M3:
> - Add runtime loading/unloading functionality
> Leo implemented a solution using JBOSS Modules which seems simpler than using
> OSGI. Modules seems to work but is not well documented which makes it hard to
> use without understanding the low level details. See the commented out code
> here:
> https://github.com/leoneu/s4-piper/blob/master/subprojects/s4-core/src/main/java/io/s4/core/Server.java
> From Adam:
> Yes I have a few ideas but I need couple days to implement them.
> First, is to build S4 and examples as proper OSGi bundles using gradle osgi
> plugin http://gradle.org/osgi_plugin.
> Its using bnd tool http://www.aqute.biz/Bnd/Bnd.
> Then embed osgi container (using standard osgi api) in Controller instead of
> JBoss modules see
> http://felix.apache.org/site/apache-felix-framework-launching-and-embedding.html.
> and configure directory from which felix should auto deploy our examples
> http://felix.apache.org/site/apache-felix-framework-usage-documentation.html#ApacheFelixFrameworkUsageDocumentation-autodeploy.
> For dynamic deployment we can use file install bundle
> http://felix.apache.org/site/apache-felix-file-install.html, it will watch
> directory from which it loads new bundles.
> To configure properly examples we can use ipojo
> http://felix.apache.org/site/apache-felix-ipojo.html or declarative services
> http://felix.apache.org/site/apache-felix-service-component-runtime.html,
> they both using service component model and are not intrusive in the code.
> That will be enough to load properly the apps.
> Later we have to think how we want to use osgi in s4, just to load apps or
> properly use service management. Bundles are hiding the internals from other
> bundles and should communicate through well defined services. And also I
> don't have clear picture how s4 is going to be distributed (have to look more
> at https://github.com/s4/s4). Maybe you can write me some small description
> of s4 concept.
> ...
> I've implemented ideas which I sent you before. Now I can load the apps using
> embedded osgi container but there is still class loading issue
> (com.esotericsoftware.kryo.SerializationException: Unable to find class:
> io.s4.example.counter.UserEvent). To be able to run s4 I had to put plenty
> system packages in configuration file for felix.
> Around 15 jars defined in libraries are not osgi bundles (don't have proper
> manifest file). The biggest issue are dependencies and classloading (for
> example Kryo is trying to load UserEvent class but that class is not visible
> for him so Class.forName is not going to work).I will try to fix these
> problems, in the meantime you can have a look at my repo
> https://github.com/adamwojtuniak/s4-piper. I added task createRepo, it will
> create bundles directory with all dependencies and project jars. When you
> launch Main class from s4-core, it launches embedded felix which will auto
> deploy all bundles located in bundles directory. In the console you will see
> felix gogo shell ( http://www.packtpub.com/article/apache-felix-gogo ).
> From Henry:
> Ugh, I was actually trying to avoid OSGI if I can =)
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
