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From "Leo Neumeyer (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (S4-4) Dynamic App Loading
Date Mon, 17 Oct 2011 01:56:11 GMT

    [ https://issues.apache.org/jira/browse/S4-4?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13128569#comment-13128569
] 

Leo Neumeyer commented on S4-4:
-------------------------------

I played with MultiClassLoader and ran a HelloApp test. Seems like a good approach at least
for milestone 1:

https://github.com/leoneu/s4-piper/blob/modules/subprojects/s4-core/src/main/java/io/s4/core/Server.java

To run the test you need to create a HelloApp class with a default package. The class should
extend class and print something in the start() method. Move HelloApp.class to /tmp/HelloApp.impl
for testing. (Different extension to make sure it is not a standard class found in the classpath.)

I propose using this approach for now. What we need to do is pass an app path in the properties
file and read apps from there every X seconds. When the app is removed, then we should stop
it and unload.

Jars loaded by the framework will be available to apps.
                
> 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 =)

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