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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