How to deploy OSGi apps and dependencies?

Question

OSGi seems to have an excellent benefit of having small deployable artifacts by not wrapping dozens of JAR dependencies into a lib directory. However, I can\'t find anything tha

Answer 1

There is a fundamental aspect of your question that is not yet addressed.

Glassfish is indeed a full fledged Application server like most modern application servers: these provide you with a Web container (where you would deploy WAR archives), a Java EE container (to deploy EJB's in JAR and EAR archives), as well as integrate an OSGI container. The later is then used for the application server's own internal start-up mechanisms.

In essence, you may target three containers. Modern IDE's and build tools provide you with means to pack your logic to target any of these. So the question becomes: how do I architect my application with all those possibilities?

There are a few very important technical issues not to lose from sight. (I focus here on objective and factual considerations, keeping out from any subjective choices, philosophy, strategy, and other context-dependent considerations that indeed may also weight much on your final decision):

1. An OSGI container does not provide you with Implicit or Declarative Thread Management, Persistence, or Transaction management services like the Web and Java EE containers. So, do plan to analyse these issues and produce the code to manage your threads and deal with transaction propagation over these threads for instance. Of course OSGi provides all necessary APIs to deal with those aspects, but does require coding (where AOP may help). On the other hand, in the Web and Java EE containers, you rely on container-managed services and/or use EJB annotations, deployment descriptors, and server managed objects like pools to simply declare how many threads you want in parallel, the sizes of connection pools, and which transaction attributes. There are advantages and drawbacks in either style (procedural in OSGi versus declarative or implicit in java app. servers).
2. OSGI provides an orderly way of loading your code, manage module dependencies, even deal with multiple coexisting versions of the same module, and dynamically add/remove and start/stop so-called bundles (OSGI deployment units), provided indeed your bundle contains the logic to handle potential start/stop issues like properly interrupting all launched threads on a stop -- threads which could have 'propagated' through other dependent modules. On the other hand, Java EE and Web containers will often embark copies of dependent JAR's that may yield more fat deployments unless you start to consider your application server's classloader hierarchy and take advantage of it to deploy 'shared libraries' either as plain POJO JARs, or as Java EE beans packed in JAR's. Anyhow, in the later cases, managing deployment dependencies becomes a concern which you will have to address at least at build time using frameworks like Maven indeed. Then, at run time you may have to script start/stop cascades according to dependencies; else, take advantage from specific application server extensions that address these dynamic deployment issues in Web and Java EE containers (e.g. Weblogic).
3. As already said, OSGI is now used by most application servers to manage their own start-up sequence. With the increasing complexity of platforms, multiplication of API's, increrase in the number of development teams to assemble a single final product, as well as use of numerous third party / open source components, OSGI becomes an indispensable server-start-up tool to ensure stable releases and a coherent set of compatible versions of all components. Think about the Eclipse IDE: with a catalogue of thousands of plug-ins and a high rate of new releases, this IDE would be a much brittle platform without OSGI as a base. Modern Application servers face just the same problem.
4. Based on above considerations, you may be much tempted to layer your code into some facilities which you could base into the OSGI layer, which in turn provides core services to a Java EE bean layer hosting business logic, and then a Web servlet layer to interface the whole... but two other questions pop up: (a) How do you make all these components communicate together? OSGI has its own repository mechanism and the deployed JAR's API's will not be visible by other modules unless explicitly published in OSGI. Web and Java EE containers use a completely different repository technology to access each-other components' interfaces, namely JNDI. Again, look at your specific application server documentation which may provide means to address Java EE beans from OSGI bundles and vice-versa (Glassfish for instance, since V3); but be cautious about thread management and transaction scopes. (b) How would you avoid interfering in the application server start-up sequence? OSGI tends to become a core system feature (under the vendor's governance), compared to the Web and Java EE containers naturally oriented to host your application code (under your governance). Upgrading your application server or installing a new release can interfere with your own OSGI deployments; you'll have to check the issue and organize your deployment scripts as a consequence.

The question is rich and its analysis is complex. Further consideration must take into account the nature of the application to build. Moreover, if you intend to use development frameworks like open source Spring and/or Camel, as well as vendor specific ones like Oracle Fusion SOA composites, JBoss Switchyard, etc. you will have many other technical constraints to take into consideration.

There is no "one-size-fits-all" answer in these matters and that, in essence, justifies the current plethora of overlapping technologies.

When that architecture question is first solved, then you can look to optimize productivity with a suitable configuration management and deployment repository.