Hi I am a newbee to osgi . Could anyone please help me understand that whether osgi is a container or framework? Can anybody please explain the difference between the two as well. Thanks it will be a great help.
It depends what you mean by those terms, there's no universally-accepted definition of either.
From my perspective, OSGi is not really a container since it does not wrap around your code or change the execution model. So I prefer the term framework, and indeed most of the documentation refers to it as "the OSGi Framework" or sometimes "the OSGi Service Platform".
For example: you can create a running and useful OSGi Framework in just 5-10 lines of Java code.
I would define a framework as a library that you code against and that has control over the execution. So e.g. spring is a framework and OSGi in the same way.
A container is a generic application you can start up and where you can deploy executeable artifacts. OSGi by default does not allow you to deploy artifacts. It mainly provides and API that allows you to load and start bundles. In practice though pretty much all OSGi implementations allow deployments like from a certain directory. So at least the implementations are also containers.
Like Neil wrote my definition is not the only one so the above may not be universally true.
Related
Is there any documentation / articles identifying best practices / rules for when should one use JPA in an ejb container vs. when is it appropriate to use JPA in a web container?
At a high level - couple of things that I can think about is if we have JPA in WAR then the transactional code will need to be managed by the developer in UserTransactions and it cannot be reused by other modules or even the EJB in the same application.
I typically use JPA for either Java SE applications or in a Container. The SE application is really only playing around or doing database loads or setup. I have setup JPA in Tomcat, but I wasn't really pleased with the end result and felt there was some issues that couldn't really be resolved.
EDIT: That said, consider Spring/Hibernate runs in a servlet container under your HTTP server of choice, and seems to be a pretty popular and mature platform, but also quite vendor specific. It will do just about anything I can think of that a Java EE certified container will do and perhaps more.
I think your considerations come down to what kind of application you're building which, in turn, provides a lot of input about what database to use.
I'm in the process of exposing some of the data generated in one OSGi bundle with a REST interface so it can be consumed with typical http requests afterwards by anyone. To do this I'm using a combination of apache-cxf, JAX-RS and Jetty (for testing purposes mostly).
My whole application is distributed in two ways:
Core: Runs a set of minimal bundles as an OSGi framework.
Full: Core + plus a set of plugins to be used within Eclipse, in a graphical way, to put it that way.
While in the Core launch configuration I can make this work without any problems, have a look at the MANIFEST:
Require-Bundle: cxf-dosgi-ri-singlebundle-distribution;bundle-version="1.3.1",org.eclipse.core.runtime, org.eclipse.osgi.services;bundle-version="3.3.0", org.mortbay.jetty.server;bundle-version="6.1.23", org.mortbay.jetty.util;bundle-version="6.1.23"
When I try to launch the Full distribution, jetty server does not seem to start and thus, the RESTful service is not working properly. Full distribution comes with a lot more plugins, mostly related with wizards, perspectives and so on. Root of this problem is somehow related with the cxf-dosgi-ri-singlebundle-distribution bundle, and can be tackled by "forcing" auto-start of this bundle to true, instead of default (check image below):
While this is OK, my question is ...why is this happening? And second and most important, I distribute the Full distribution via an update site, so users can download it, open perspective and start using it, that said, will this affect the final distribution? Is there a way I can control this auto-start=true for the apache-cxf bundle in the update site?
Hope I can make myself clear...
Thanks!
I don't like answering my own questions, but since I'm getting no feedback and maybe someone else has the same problem...here it goes
I decided to change the whole design of my restful bundle, by not using apache-cxf, since it is really not needed. All I need to do what I want is the embedded Jetty server and javax for dealing with HttpRequests and HttpResponses. Thus, I get rid of dependecies with third party libraries, leveraging the launching problems at the same time.
Got the ideas from these official sites in Eclipe:
Embedding an HTTP server in Equinox
Writing a bundle-based server application
Besides, a more hands-on tutorials can be also found here:
OSGi as a Web Server application
Extending Eclipse: displaying HTML content from a bundled archive
Special thanks to both authors for the enlightenment :).
I'm trying to design the architecture for a new LOB MVVM project utilising Caliburn Micro and nHibernate and am now at the point of looking into DI and IOC.
A lot of the examples for bootstrapping Caliburn Micro use MEF as the DI\IOC mechanism.
What I'm struggling with is that MEF seems to by reasonably popular but the idea of the Mef [Imports] annotations smells to me like another flavour of a Service Locator?
Am I missing something about MEF whereby almost all the examples I've seen are not using it correctly or have I completely not understood something about how it's used whereby it side steps the whole service locator issues?
MEF is not a Service Locator, on it's own. It can be used to implement a Service Locator (CompositionInitializer in the Silverlight version is effectively a Service Locator built into MEF), but it can also do dependency injection directly.
While the attributes may "smell" to you, they alone don't cause this to be a service locator, as you can use [ImportingConstructor] to inject data at creation time.
Note that the attributes are actually not the only way to use MEF - it can also work via direct registration or convention based registration instead (which is supported in the CodePlex drops and .NET 4.5).
I suppose if you were to just new up parts that had property imports and try to use them, then you could run into some of the same problems described here: Service Locator is an Anti-Pattern
But in practice you get your parts from the container, and if you use [Import] without the additional allowDefault property, then the part is required and the container will blow up on you if you ask for the part doing the import. It will blow up at runtime, yes, but unlike a run of the mill service-locator, its fairly straightforward to do static analysis of your MEF container using a test framework. I've written about it a couple times here and here.
It hasn't been a problem for me in practice, for a couple of reasons:
I get my parts from the container.
I use composition tests.
I'm writing applications, not framework code.
I am trying to get my head around OSGi Services. The main question I keep asking myself is: What's the benefit of using services instead of working with bundles and their exported packages?
As far as I know it seems the concept of Late Binding has something to do with it. Bundle dependencies are wired together at bundle start, so they are pretty fixed I guess. But with services it seems to be almost the same. A bundle starts and registers services or binds to services. Of course services can come and go whenever they want and you have to keep track of these chances. But the core idea doesn't seem that different to me.
Another aspect to this seems to be that services are more flexible. There could be many implementations for one specific Interface. On the other hand there can be a lot of different implementations for a specific exported package too.
In another text I read that the disadvantage of using exported packages is that they make the application more fragile than services. The author wrote that if you remove one bundle from the dependency graph other dependencies would no longer be met, thus possibly causing a domino effect on the whole graph. But couldn't the same happen if a service would go offline? To me it looks like service dependencies are no better than bundle dependencies.
So far I could not find a blog post, book or presentation that could clearly describe why services are better than just exposing functionality by exporting and importing packages.
To sum my questions up:
What are the key benefits of using OSGi Services that make them superior to exporting and importing packages?
Addition
I have tried to gather further information about this issue and come up with some kind of comparison between plain export/import of packages and services. Maybe this will help us to find a satisfying answer.
Start/Stop/Update
Both, bundles (hence packages) and services, can be started and stopped. In addition to that they can be kind of updated. Services are also tied to the bundle life cycle itself. But in this case I just mean if you can start and stop services or bundles (so that the exported packages "disappear").
Tracking of changes
ServiceTracker and BundleTracker make it possible to track and react to changes in the availability of bundles and services.
Specific dependencies to other bundles or services.
If you want to use an exported package you have to import it.
Import-Package: net.jens.helloworld
Would net.jens.helloworld provide a service I would also need to import the package in order to get the interface.
So in both cases their would be some sort of "tight coupling" to a more or less specific package.
Ability to have more than one implementation
Specific packages can be exported by more than one bundle. There could be a package net.jens.twitterclient which is exported by bundle A and bundle B. The same applies to services. The interface net.jens.twitterclient.TwitterService could be published by bundle A and B.
To sum this up here a short comparison (Exported packages/services):
YES/YES
YES/YES
YES/YES
YES/YES
So there is no difference.
Additionally it seems that services add more complexity and introduce another layer of dependencies (see image below).
alt text http://img688.imageshack.us/img688/4421/bundleservicecomparison.png
So if there is no real difference between exported packages and services what is the benefit of using services?
My explanation:
The use of services seems more complex. But services themselves seem to be more lightweight. It should be a difference (in terms of performance and resources) if you start/stop a whole bundle or if you just start and stop a specific service.
From a architectural standpoint I also guess that bundles could be viewed as foundation of the application. A foundation shouldn't change often in terms of starting and stopping bundles. The functionality is provided by services of this packages in some kind of dynamic layer above the "bundle layer". This "service layer" could be subject to frequent changes. For example the service for querying a database is unregistered if the database is going offline.
What's your opinion? Am I starting to get the whole point of services or am I still thinking the wrong way? Are there things I am missing that would make services far more attractive over exported packages?
Its quite simple:
Bundles are just providing classes you can use. Using Imports/Exports you can shield visibility and avoid (for example) versioning conflicts.
Services are instances of classes that satisfy a certain contract (interfaces).
So, when using Services you don't have to care about the origin of a implementation nor of implementation details. They may even change while you are using a certain service.
When you just want to rely on the Bundle Layer of OSGi, you easily introduce crosscutting dependencies to concrete implementations which you usually never want. (read below about DI)
This is not an OSGi thing only - just good practice.
In non OSGi worlds you may use Dependency Injection (DI) frameworks like Guice, Spring or similar. OSGi has the Service Layer built into the framework and lets higher level frameworks (Spring, Guice) use this layer. - so in the end you usually dont use the OSGi Service API directly but DI adapters from user friendly frameworks (Spring-->Spring DM,Guice-->Peaberry etc).
HTH,
Toni
I'd recommend purchasing this book. It does an excellent job explaining services and walking through the construction of a non-trivial application that makes use of OSGi Services.
http://equinoxosgi.org/
My company routinely builds 100+ bundle applications using services. The primary benefits we gain from using services are:
1) Loose coupling of producer/consumer implementation
2) Hot swappable service providers
3) Cleaner application architecture
When you start with OSGi, it is always easier to start with an export-package approach it certainly feels more java-like. But when your application starts growing and you need a bit of dynamicity, services are the way to go.
Export-package only does the resolution on startup, whereas services is an on-going resolution (which you may want or not). From a support point of view having services can be very scary (Is it deterministic? How can I replicate problems?), but it is also very powerful.
Peter Kriens explains why he thinks that Services are a paradigm shift in the same way OO was in its time. see µServices and Duct Tape.
In all my OSGi experience I haven't had yet the occasion to implement complex services (i.e. more than one layer), and certainly annotations seem the way to go. You can also use Spring dynamic module to ease the pain of dealing with service trackers. (and many other options like iPOJO, and Blueprint)
Lets consider the two following scenarios:
Bundle A offers a service which is an arithmetic addition add(x,y) return x+y. To achieve this, it exports "mathOpe package" with "IAddition interface", and registers a service within the service registry. Bundles B, C, D, ... consume this service.
Bundle A exports "mathOpe package", where we found a class Addition exposing an operation (x+y)<--add(x,y). Bundles B, C, D, ... import the package mathOpe.
Comparison of scenario 1 vs. scenario 2:
Just one implementation instance vs. many instances (Feel free to make it static!)
Dynamic service management start, stop, update vs. no management, the consumer is owning the implementation (the "service")
Flexible (we can imagine a remote service over a network) vs. not flexible
... among others.
PS: I am not an OSGI expert nor a Java one, this answer shows only my understanding of the phenomena :)
I think this excellent article could answer a lot of your questions:OSGi, and How It Got That Way.
The main advantage of using a service instead of the implementation class is that the bundle offering the service will do the initialization of the class itself.
The bundle that uses the service does not need to know anything about how the service is initialized.
If you do not use a service you will always have to call kind of a factory to create the service instance. This factory will leak details of the service that should remain private.
I currently have a home-made IoC container that I will soon replace with a new one. My home-made IoC container is configured using config files. From what I have read on the net, the ability to "configure from code" seems to be a very popular feature.
I don’t like the idea of having a class that knows every other class in the system in order to setup the IoC Container. Such class would have to be in an assembly that depends on the 80 other assemblies of my project.
Are there best practices on how to organize the code that configures the container?
I have read this post. Using conventions and auto-wiring is good when there are patterns in the types to be registered. But I have hundreds of types that are in different assemblies and that don’t have anything in common. How should I organize the code for those?
Regards,
Update: I chose an approach where the code that configures the container is decentralized. Each assembly in my system is given a chance to configure the container. The method at the entry points in my system (many .exe apps, the web app, the web services app and the unit test fixtures are all entry points) are responsible for calling each assembly to let them setup the container. I'm currently implementing that, I' not sure if it is going to be satisfactory. I will post another update soon.
Depending on your programming language (I use c#) you might want to look something like Autofac modules: http://code.google.com/p/autofac/wiki/StructuringWithModules