Details
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New Feature
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Status: Resolved
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Major
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Resolution: Fixed
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None
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None
Description
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 =)