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From "Leo Neumeyer (Created) (JIRA)" <>
Subject [jira] [Created] (S4-4) Dynamic App Loading
Date Wed, 12 Oct 2011 17:45:11 GMT
Dynamic App Loading

                 Key: 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:

- 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.

- Add dependencies among apps using EventSource.

- 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:

>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
Its using bnd tool
Then embed osgi container (using standard osgi api)  in Controller instead of JBoss modules
and configure directory from which felix should auto deploy our examples
For dynamic deployment we can use file install bundle,
it will watch directory from which it loads new bundles.
To configure properly examples we can use ipojo
or declarative services,
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 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
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
( ).

>From Henry:

Ugh, I was actually trying to avoid OSGI if I can =)

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