Modula

Modula is a thought experiment in Scala modularity.

At the present time it is unfinished and highly likely to change so if you don't want to be hacking around with breaking changes this is not the project you're looking for.

Features

Modula is highly influenced by the OSGi specification but adds several unique features that make it interesting in it's own right.

The basic building blocks in Modula are:

Services - services are simply objects, they can be shared via a ModuleContext and act as building blocks to build more complex services
ModuleContext - a module context provides a mechanism to share services within a VM, it has methods to make it easy to register new services or discover existing ones
Module - a service that provides class and resource loading capabilities, modules may provide or require capabilities they are wired together via a Resolver
Framework - the framework is at the root of all Modula applications, it is a Module and provides a ModuleContext
ModuleProvider - a service that can be registered to install a new module within the framework.

It is worth restating one of the points from above, Modules are Services! This was pretty tricky to implement but the fact that it seems to be possible to implement a Modularity layer on top of a Services layer seems kinda cool :)

Background

I started to write Modula as an evening project to help me improve my knowledge of the Scala programming language. I've been using Scala as a hobby language on and off for a couple of years but have so far really only tinkered around the edges. However I find the best way to really learn something new is to set a hard problem and then figure out how to solve it. So after reading the rather excellent Programming In Scala last year, I decided to do something more interesting than the standard Hello World as an Xmas project.

Having worked extensively with OSGi in the past - I've worked on the specifications group and helped write a book on the subject - I wondered if the Scala language could improve on the existing OSGi API's. Given how well thought out the OSGi specification is, this sounded like a hard problem so I immediately started experimenting :)

After a couple of months of tinkering I have something that can mimic a several of the core features of OSGi including:

service discovery
dependency resolution
uses constraints
fragment attachement

However there are probably more missing features than completed ones, most notably uninstall is only implemented to a very basic level and does not currently re-resolve dependencies after a change.

I've decided to open source this project on Github as a way to see if there was any interest in this other than myself and hopefully solicit some help in implementing more features.

Project Structure

The source code for this project is broken down into several sub projects

api - Defines the Modula API
factory - Provides a way to instantiate an instance of a Modula framework
impl - Where the majority of the work is done
osgi - An experiment to see if it is possible to implement an OSGi framework on top of a Modula framework
main - Trivial project to start the Modula OSGi framework
tools - Various utilities I cooked up to help in testing.

Building

The project is built using SBT so to build and test the framework you can do:

$ sbt clean test

Launching

I've set up an assembly target on the main sub project to allow you to simply boot modula and point at some OSGi bundles as a test case.

$ sbt clean assembly
$ java -jar main/target/modula-main-assembly-1.0.jar <path-to-bundles>

Gogo

As a simple test case I've been booting the Gogo shell environment in Modula:

$ java -jar main/target/modula-main-assembly-1.0.jar gogo
g! ls gogo
/Users/dave/development/chronological-thought/modula/gogo/org.apache.felix.gogo.command-0.12.0.jar
/Users/dave/development/chronological-thought/modula/gogo/org.apache.felix.gogo.runtime-0.10.0.jar
/Users/dave/development/chronological-thought/modula/gogo/org.apache.felix.gogo.shell-0.10.0.jar
g! lb
Start Level service is unavailable.
   ID|State      |Name
    0|Active     |system.bundle (0.0.0)
    1|Active     |Apache Felix Gogo Command (0.12.0)
    2|Active     |Apache Felix Gogo Runtime (0.10.0)
    3|Active     |Apache Felix Gogo Shell (0.10.0)