Monday, February 8, 2016

Branch by Abstraction and OSGi

Inspired by my friend Philipp Suter who pointed me at this wired article which relates to Martin Fowler's Branch by Abstraction I was thinking: how would this work in an OSGi context?

Leaving aside the remote nature of the problem for the moment, let's focus on the pure API aspect here. Whether remote or not really orthogonal... I'll work through this with example code that can be found here:

Let's say you have an implementation to compute prime numbers:
public class PrimeNumbers {
  public int nextPrime(int n) {
    // computes next prime after n - see details
    return p;
And a client program that regularly uses the prime number generator. I have chosen a client that runs in a loop to reflect a long-running program, similar to a long-running process communicating with a microservice:
public class PrimeClient {
  private PrimeNumbers primeGenerator = new PrimeNumbers();
  private void start() {
    new Thread(() -> {
      while (true) {
        System.out.print("First 10 primes: ");
        for (int i=0, p=1; i<10; i++) {
          if (i > 0) System.out.print(", ");
          p = primeGenerator.nextPrime(p);
        try { Thread.sleep(1000); } catch (InterruptedException ie) {}
  public static void main(String[] args) {
    new PrimeClient().start();
If you have the source code cloned or forked using git, you can run this example easily by checking out the stage1 branch and using Maven:
.../primes> git checkout stage1
.../primes> mvn clean install
... maven output
[INFO] ------------------------------------------------
[INFO] ------------------------------------------------
Then run it from the client submodule:
.../primes/client> mvn exec:java -Dexec.mainClass=\
... maven output
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
... and so on ...
Ok so our system works. It keeps printing out prime numbers, but as you can see there is a bug in the output. We also want to replace it in the future with another implementation. This is what the Branch by Abstraction Pattern is about.

In this post I will look at how to do this with OSGi Services. OSGi Services are just POJOs registered in the OSGi Service Registry. OSGi Services are dynamic, they can come and go, and OSGi Service Consumers dynamically react to these changes, as well see. In the following few steps will change the implementation to an OSGi Service. Then we'll update the service at runtime to fix the bug above, without even stopping the service consumer. Finally we can replace the service implementation with a completely different implementation, also without even stopping the client.

Turn the application into OSGi bundles

We'll start by turning the program into an OSGi program that contains 2 bundles: the client bundle and the impl bundle. We'll use the Apache Felix OSGi Framework and also use OSGi Declarative Services which provides a nice dependency injection model to work with OSGi Services.

You can see all this on the git branch called stage2:
.../primes> git checkout stage2
.../primes> mvn clean install
The Client code is quite similar to the original client, except that it now contains some annotations to instruct DS to start and stop it. Also the PrimeNumbers class is now injected instead of directly constructed via the @Reference annotation. The greedy policyOption instructs the injector to re-inject if a better match becomes available:
public class PrimeClient {
  private PrimeNumbers primeGenerator;
  private volatile boolean keepRunning = false;
  private void start() {
    keepRunning = true;
    new Thread(() -> {
      while (keepRunning) {
        System.out.print("First 10 primes: ");
        for (int i=0, p=1; i<10; i++) {
          if (i > 0) System.out.print(", ");
          p = primeGenerator.nextPrime(p);
        try { Thread.sleep(1000); } catch (InterruptedException ie) {}
  private void stop() {
    keepRunning = false;
The prime generator implementation code is the same except for an added annotation. We register the implementation class in the Service Registry so that it can be injected into the client:
public class PrimeNumbers {
  public int nextPrime(int n) {
    // computes next prime after n
    return p;
As its now an OSGi application, we run it in an OSGi Framework. I'm using the Apache Felix Framework version 5.4.0, but any other OSGi R6 compliant framework will do.
> java -jar bin/felix.jar
g! start
g! start file:/.../clones/primes/impl/target/impl-0.1.0-SNAPSHOT.jar
g! install file:/.../clones/primes/client/target/client-0.1.0-SNAPSHOT.jar
Now you should have everything installed that you need:
g! lb
ID|State |Level|Name
0|Active | 0|System Bundle (5.4.0)|5.4.0
1|Active | 1|Apache Felix Bundle Repository (2.0.6)|2.0.6
2|Active | 1|Apache Felix Gogo Command (0.16.0)|0.16.0
3|Active | 1|Apache Felix Gogo Runtime (0.16.2)|0.16.2
4|Active | 1|Apache Felix Gogo Shell (0.10.0)|0.10.0
5|Active | 1|Apache Felix Declarative Services (2.0.2)|2.0.2
6|Active | 1|impl (0.1.0.SNAPSHOT)|0.1.0.SNAPSHOT
7|Installed | 1|client (0.1.0.SNAPSHOT)|0.1.0.SNAPSHOT
We can start the client bundle:
g! start 7
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
... and so on ..
You can now also stop the client:
g! stop 7
Great - our OSGi bundles work :)
Now we'll do what Martin Fowler calls creating the abstraction layer.

Introduce the Abstraction Layer: the OSGi Service

Go to the branch stage3 for the code:
.../primes> git checkout stage3
.../primes> mvn clean install
The abstraction layer for the Branch by Abstraction pattern is provided by an interface that we'll use as a service interface. This interface is in a new maven module that creates the service OSGi bundle.
public interface PrimeNumberService {
    int nextPrime(int n);
Well turn our Prime Number generator into an OSGi Service. The only difference here is that our PrimeNumbers implementation now implements the PrimeNumberService interface. Also the @Component annotation does not need to declare the service in this case as the component implements an interface it will automatically be registered as a service under that interface:
public class PrimeNumbers implements PrimeNumberService {
    public int nextPrime(int n) {
      // computes next prime after n
      return p;
Run everything in the OSGi framework. The result is still the same but now the client is using the OSGi Service:
g! lb
   ID|State      |Level|Name
    0|Active     |    0|System Bundle (5.4.0)|5.4.0
    1|Active     |    1|Apache Felix Bundle Repository (2.0.6)|2.0.6
    2|Active     |    1|Apache Felix Gogo Command (0.16.0)|0.16.0
    3|Active     |    1|Apache Felix Gogo Runtime (0.16.2)|0.16.2
    4|Active     |    1|Apache Felix Gogo Shell (0.10.0)|0.10.0
    5|Active     |    1|Apache Felix Declarative Services (2.0.2)|2.0.2
    6|Active     |    1|service (1.0.0.SNAPSHOT)|1.0.0.SNAPSHOT
    7|Active     |    1|impl (1.0.0.SNAPSHOT)|1.0.0.SNAPSHOT
    8|Resolved  |    1|client (1.0.0.SNAPSHOT)|1.0.0.SNAPSHOT
g! start 8
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
You can introspect your bundles too and see that the client is indeed wired to the service provided by the service implementation:
g! inspect cap * 7
org.coderthoughts.primes.impl [7] provides:
service; org.coderthoughts.primes.service.PrimeNumberService with properties: = 0 = org.coderthoughts.primes.impl.PrimeNumbers
   service.bundleid = 7 = 22
   service.scope = bundle
   Used by:
      org.coderthoughts.primes.client [8]
Great - now we can finally fix that annoying bug in the service implementation: that it missed 2 as a prime! While we're doing this we'll just keep the bundles in the framework running...

Fix the bug in the implementation whitout stopping the client

The prime number generator is fixed in the code in stage4:
.../primes> git checkout stage4
.../primes> mvn clean install
It's a small change to the impl bundle. The service interface and the client remain unchanged. Let's update our running application with the fixed bundle:
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
First 10 primes: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
g! update 7 file:/.../clones/primes/impl/target/impl-1.0.1-SNAPSHOT.jar
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
Great - finally our service is fixed! And notice that the client did not need to be restarted! The  DS injection, via the @Reference annotation, handles all of the dynamics for us! The client code simply uses the service as a POJO.

The branch: change to an entirely different service implementation without client restart

Being able to fix a service without even restarting its users is already immensely useful, but we can go even further. I can write an entirely new and different service implementation and migrate the client to use that without restarting the client, using the same mechanism. 

This code is on the branch stage5 and contains a new bundle impl2 that provides an implementation of the PrimeNumberService that always returns 1. 
.../primes> git checkout stage5
.../primes> mvn clean install
While the impl2 implementation obviously does not produce correct prime numbers, it does show how you can completely change the implementation. In the real world a totally different implementation could be working with a different back-end, a new algorithm, a service migrated from a different department etc...

Or alternatively you could do a façade service implementation that round-robins across a number of back-end services or selects a backing service based on the features that the client should be getting.
In the end the solution will always end up being an alternative Service in the service registry that the client can dynamically switch to.

So let's start that new service implementation:
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
g! start file:/.../clones/primes/impl2/target/impl2-1.0.0-SNAPSHOT.jar
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
g! stop 7
First 10 primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
First 10 primes: 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
First 10 primes: 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
Above you can see that when you install and start the new bundle initially nothing will happen. At this point both services are installed at the same time. The client is still bound to the original service as its still there and there is no reason to rebind, the new service is no better match than the original. But when the bundle that provides the initial service is stopped (bundle 7) the client switches over to the implementation that always returns 1. This switchover could happen at any point, even halfway thought the production of the list, so you might even be lucky enough to see something like:
First 10 primes: 2, 3, 5, 7, 11, 13, 1, 1, 1, 1
I hope I have shown that OSGi services provide an excellent mechanism to implement the Branch by Abstraction pattern and even provide the possibility to do the switching between suppliers without stopping the client!

In the next post I'll show how we can add aspects to our services, still without modifying or even restarting the client. These can be useful for debugging, tracking or measuring how a service is used.

PS - Oh, and on the remote thing, this will work just as well locally or Remote. Use OSGi Remote Services to turn your local service into a remote one... For available Remote Services implementations see

With thanks to Carsten Ziegeler for reviewing and providing additional ideas.

Tuesday, May 19, 2015

My favourite free Mac OSX apps

I have been a Windows guy for many, many years, starting with Windows 2.x in the late 1980's all the way up to Windows 7. Then, about 4 years ago I gave up. Not because Windows was so bad (I actually quite liked Windows 7) but because at the time I could not find any decent laptop for it! So I got a Mac in 2011. And, over the years I got to like it, especially the hardware. I was always a big fan of opensource and freeware/shareware which was always widely available for Windows. Over the past years a collected some good legal free software for the Mac and I thought I'd share with everyone what I like. The interesting bit here is that most of this stuff is not actually distributed via the Apple Appstore...

Here's my list in no particular order, starting with a bundle of freely available utilities...


Great video converter tool. Convert any videos that you might have into any other format. BTW if you're looking for good settings that give you videos that play/stream on most devices and are relatively modest in size, I use the following settings (everything not mentioned is left at default):
  • Format: MP4, Web Optimized
  • Picture: Size: source = destination, Cropping: automatic, Anamorphic: none, Filters: all off
  • Video: Codec: H.264, FPS: Same as source-variable, Quality: RF 22, Use Advanced Options
  • Audio: AAC Stereo, Samplerate auto, Bitrate 128
  • Advanced: Reference Frames: 6, CABAC Entropy Encoding: yes, 8x8 transform: yes, Weighted P-Frames: yes, Subpixel ME & Mode: 2  SATD qpel, Trellis: off, No DCT Decimation: off.
Download from


The video player that plays everything. If you install the right drivers it can even play Blu Ray discs, something the native DVD player on OSX cannot do. Download from


Previously called XBMC. Another media player. This one is really good for streaming over the network and has tons of plugins. You can also create a catalog that runs on a central server and stream that to all the devices in your house, for example. I wrote some details on how I use my Raspberry PI as such a media server here: Download from


This tool has a lot of features for the touchpad, but the main reason why I use it is not on the touchpad. I don't like the click feature of the touchpad, the fact that you have to press the touchpad itself. I prefer pressing a button to act as a mouseclick. On my UK keyboard there is a key that I never use, just above the TAB key there is a key with the § sign. The above screenshot shows how I used the BetterTouchTool to remap that key to be the mouseclick. I still use the trackpad to position my mouse, but no need to press it down, I do that with my other hand by pressing that mapped key. Download from here:
BetterTouchTool also has Windows 7-like window snapping functionality, which is really handy too. Although I personally use Breeze for that, which has more features in this area. It's not a free tool so it doesn't belong in this list, but its very cheap so worth checking out :)


This is a very handy little tool, that simply extends the clipboard with a history of up to 25 entries. Instead of cmd-V, use shift-cmd-V to cycle through all your past clipboard contents. Extremely useful. This one is free on the apple appstore:


This is a free WinZip-like tool for the Mac. It doesn't have as many features as WinZip, but I tend to find it very good for quickly peeking in a .zip or .jar file. And the program icon is just really cute! Download from:


I'm a VIM guy and this Mac version is really good! Download from


I'm relatively new to password management software but the recent vulnerabilities and attacks made me move to it. I'm a bit weary of storing my passwords in a cloud-based solution. I'd rather have a local file that's properly encrypted, so that it doesn't matter who actually sees that raw file. I also want something that automatically works with web browsers so that passwords are automatically entered. KeePass is a great Windows-based app that does all that. There's an Android version (KeePassDroid) that even works on my BlackBerry Z10. And there are Chrome (chromeIPass) and Firefox (PassIFox) integrations with the KeePass HTTP server that seamlessly enter your passwords in the appropriate websites if you approve this action.
There are a number of opensource projects to get this working on the Mac too. However, they seem to be in various stages of incompleteness. There is the KeePassX project, but it doesn't have support for the HTTP server (yet) that you need to integrate with the browser. There is a new project called MacPass which is another KeePass clone, but it also doesn't support the HTTP server (yet). Some people use Mono to run the Windows KeePass on OSX, but I found that not very satisfying and at some point it just started crashing continuously. In the end I found Keith Bennett's fork of KeePassX 2.0Alpha5 that has HTTP server support. The UI is a bit buggy, but it's the best I could find in the opensource arena for password management on OSX, so this is what I'm using right now. Looking forward to having a more stable version of KeePass on the Mac with HTTP server support.

PDF Split and Merge

I love PDFs :) (hey I work for Adobe). And there are tons of tools available for creating PDFs both from Adobe and from other vendors. PDFSam is a really nice little tool that helps you work with PDFs once they are created. Rearrange pages, split chapters off, merge multiple PDFs together, that kind of thing. I find myself using this tool pretty often. You can get it from here:

OSX Screen Saver runner

Ok, should maybe not be in this list, because it's not freeware or opensource, as it's shipped as part of OSX itself but it's free in that everyone who has OSX already has it, but you may not be aware of its existence. This is just a really simple way to lock your screen, if you need to walk away from your computer, for example. You simply find it on your local disk at /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/
and I normally have it locked to my dock, so I can just click it to lock my screen.


There are also a few larger applications where I think there are some great opensource products available that I use on a daily basis.

Openoffice / Libreoffice

Personally I prefer this office suite to the commercial ones available. Great features, great usability and it works on all the platforms that I use outside of OSX, including Linux. Download from

Thunderbird and Firefox

These two need no further introduction. Just an excellent webbrowser and a great email client. Get them at


Still the best IDE around for Java and other programming languages. Available from

That's it - these are my favourite opensource / freeware / shareware apps that I use all the time. Enjoy :)

Wednesday, May 28, 2014

Running bndtools OSGi builds with Maven - bnd-maven-plugin now released!

The project provides some really great tooling for building OSGi bundles. This includes help for generating the bundle manifest, templates for working with Declarative Services, testing support and more. There's tooling for resolving deployments against repositories, and last but certainly not least - automatic semantic versioning. The tooling for semantic versioning will tell you whether the version of your exported packages is correct, by comparing the content of your APIs with the previous release of the bundle.
I won't go into the full feature set of bndtools here, have a look at the bndtools website for all the goodness. Another thing to say about bndtools is that it has a really active community, which is always a good thing to know when you're deciding on using an opensource project.

Example IDE screen, taken from
The bndtools project itself focuses on providing an Eclipse-based graphical IDE for developing OSGi bundles. However you normally also want to be able to build your bundles in a headless build. For example as part of a Jenkins/Hudson process or simply to build it from the command-line yourself.
The bndtools project provides headless build options using Ant and Gradle, but for many people being able to use Maven for builds is essential.

Most OSGi development in Maven is done these days using the maven-bundle-plugin or using Eclipse Tycho. Tycho focuses on building OSGi bundles using a manifest-first approach which is supported by the Eclipse PDE tooling. 
The maven-bundle-plugin uses the pom.xml file as the source of information for the bundle and while it uses bnd under the covers, just like the bndtools project, maven-bundle-plugin is a little different in that it uses the pom.xml as the source of information for the build. The bndtools IDE uses bnd.bnd files for this.

This is where the bnd-maven-plugin comes in. Toni Menzel started it a while ago, Peter Kriens worked on it, and I have been putting bits and pieces in recently. The bnd-maven-plugin basically builds a bndtools project exactly like the bndtools IDE would build it, but then from Maven. As of today the first version of the bnd-maven-plugin  (called 1.0.2 - mea culpa ;) has been released to Maven Central, so you can easily use it in your Maven builds.

Precise instructions on how to use it can be found in the project readme, but you can easily try it out by looking at the bnd-maven-plugin samples in github. They show what your pom should look like. The samples contain the following test projects:
TestBundle - this is a simple default-layout bndtools project that can be built with Maven.
TestBundle2 - a bndtools project that follows Maven conventions for directory locations of source and output files. It can be built using Maven and the bndtools IDE. This project also contains an example unit test.
TestBundle3 - this project contains an OSGi Framework integration test, which can be run both from Maven and Bndtools.
Additionally, the sample structure contains
ParentPom - a module that contains a parent pom used by all the other submodules.
MBPBundle - a module that builds a bundle using the maven-bundle-plugin, showing that it can be used alongside the bnd-maven-plugin, although within a single module you need to choose one or the other.

To run it with Maven you can simply go into the parent directory and execute mvn install:
$ .../samples/sample-projects $ mvn install
... lots of maven output ...
[INFO] --- maven-surefire-plugin:2.17:test (default-test) @ TestBundle2 ---
[INFO] Surefire report directory: /Users/David/clones/bnd/bnd-maven-plugin-parent/samples/sample-projects/TestBundle2/target/surefire-reports
 T E S T S
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.003 sec - in
Results :
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] --- bnd-maven-plugin:1.0.2:integration-test (default-integration-test) @ TestBundle3 ---
[INFO] Running the Bnd OSGi integration tests
Tests run  : 1
Passed     : 1
Errors     : 0
Failures   : 0
No Errors
[INFO] Finished running the Bnd OSGi integration tests
[INFO] ----------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] ParentPom ......................................... SUCCESS [0.185s]
[INFO] TestBundle ........................................ SUCCESS [4.134s]
[INFO] TestBundle2 ....................................... SUCCESS [0.361s]
[INFO] TestBundle3 ....................................... SUCCESS [1.041s]
[INFO] MBPBundle ......................................... SUCCESS [0.842s]
[INFO] sample-projects ................................... SUCCESS [0.041s]
[INFO] ----------------------------------------------------------------------
[INFO] ----------------------------------------------------------------------
[INFO] Total time: 11.707s

The above has built all modules, ran the unit tests in TestBundle2 and the integration tests in TestBundle3.

Use them with the bndtools IDE!

As the TestBundle, TestBundle2 and TestBundle3 projects are bndtools Eclipse projects, they can be imported via File -> Import -> Existing Projects into Workspace (along with the bnd cnf project):

Then you can use the IDE for all your dev needs, e.g. to execute the OSGi integration tests:

This is only the first release, but should be good enough to start playing with it. See if it works for you. Give us feedback! Help developing it further! You can fork, file issues and patches in the bndtools/bnd project on github. Enjoy :)

Tuesday, March 25, 2014

Apache Felix Framework 4.4.0 provides full OSGi Core R5 support

Felix Framework 4.4.0 was released today ( and now provides for OSGi Core R5 support. From a features point of view, it means that the following R5 features are now fully supported:
  • New org.osgi.framework.UnfilteredServiceListener interface
  • New org.osgi.framework.VersionRange class
  • The Resource API and it's use in the Wiring API
  • New osgi.identity namespace
  • New value and new default for org.osgi.framework.bsnversion framework property
  • support for static valueOf methods in the Filter
  • Enhanced Bundle.adapt() to support AccessControlContext
  • Updates to the Bundle Hook Specification (Bundle Collision Hook)
But the main benefit as I see it that we can now run OSGi Subsystems, such as the Apache Aries implementation, on Apache Felix! For more details on that see this blog post:

Tuesday, January 28, 2014

How I learned to stop worrying (about power cuts) and love my Raspberry PI

As many other hobbyists I got myself a Raspberry PI a few months ago. It's very cool that such a functional piece of hardware can be so cheap. I'm using mine as a backup server for my photos, documents, videos etc, it is my Samba Server, FTP server and, most importantly an XBMC network server.

I'll outline some of the things that I did to make mine working the way I wanted to, but first I'd like to address the biggest issue that I experienced with my Raspberry PI: the fact that it doesn't deal well with power cuts. The PI itself isn't really to blame for that, its a general issue with Linux and other unix-type operating systems: sudden power losses can make their boot/os disk unreadable. With Linux running on a laptop you don't really have that issue, as your laptop has a built-in UPS, but my PI doesn't have that. Which means that I often happened to find my device in a state that it wouldn't boot up again. Apparently we had a power blip.
While some companies are starting to sell UPS solutions for the PI, one of the main issues with that is that the UPS often costs more than the PI itself. If you don't want to spend that extra money, I have used a different setup that gave me the capability to survive PI boot disk corruptions without too much work or cost.

Backup your PI boot SD card

Some people suggested that the best way to prevent your PI from corrupted boot disks is to make the disk readonly. While certainly a good idea, this doesn't really work for me as I often experiment with my PI and change its installed packages regularly. So I didn't want to restrict myself that much.
What I did instead was use Billw's cloning script to make a backup copy of the main SD card on a second one. I used an old SD card that I still had lying around and got a USB SD card adapter, which sells on ebay for about £1 including shipping. Using that script I make regular backups of my main SD card onto its clone. When experiencing one of those regular power blips and the PI doesn't want to start up, I simply swap the SD cards - the one from the USB goes into the main SD card port and vice-versa! That will allow me to start up the PI again with all my configuration as I had it. Then simply run Billw's script again to clone the SD card that's still working back on the corrupted one at which point I'll have two working SD cards again!
I've used this mechanism for a few months now and it works pretty well. Certainly much cheaper than a UPS and although a little bit of work (you need to ensure that the backup SD-card stays up-to-date) I found that it worked great. You can also automate the cloning so that it happens automatically every night or so...

In addition to the backup script there are a few other things that I tweaked for my PI. They might be a little different than what you're used to on Linux, so I'm outlining them here.

Get a Hard Disk with power-save

If you're planning to use your PI as a file server, like I do, get a USB harddrive that has a power-save feature. This means that you can have your system running 24/7 but that there won't be any spinning parts if nobody's using it, and very low power consumption too in that case. I got this Seagate 2TB one for about £60 which works great! The only thing you need to do is enable the power save feature once, which you have to do on a Windows PC using the program that comes on the drive. Once power-save enabled, you can connect it to the PI and it works perfectly. Note that you do need to sudo apt-get install ntfs-3g to use most pre-formatted USB drives.

Use UUIDs to mount your drives

Another issue that I was experiencing was that my USB drives didn't always get the same disk identifier after a PI reboot. One disk would sometimes be sdb and other times it would be sdc, for example. This is problematic if you use the standard 
  /dev/sda1 /disks/x1 auto noatime 0 0
in your /etc/fstab to mount your drives to a directory, as your mounted directory would sometimes be mounted to one disk and sometimes to the other...
A good solution to that is to mount your drives by UUID instead. If you look in /dev/disk/by-uuid/ you can see the UUIDs of all the available disks. Find the right one and use that to mount your disk. The following makes sure that always the same disk is mounted to /disks/x1:
  /dev/disk/by-uuid/5A22DB2D2DB0CBF /disks/x1 auto noatime 0 0

Rotate the logs

If you're doing a lot of file transfer to-from your PI, the Samba/FTP log files can get pretty big. If you, like me, use a crappy old 4GB SD card to run off, it might make sense to keep the size of the log files down. Logrotate allows you to do this, and the standard Raspbian distribution comes with this installed. I just tweaked it to keep the log files smaller, by only remembering 1 week of logs. To do this, you tweak /etc/logrotate.conf, I'm using these settings:
  # rotate log files weekly

  # keep 1 weeks worth of backlogs
  rotate 1

  # create new (empty) log files after rotating old ones

  # compress log files

Serve XBMC throughout the house

Many people use the PI to run XBMC directly. XBMC is a wonderful media playback application that gives you a really nice interface to your media collection. And a PI connected to an audio/video system can certainly be used for running it. However, I was looking for something different. I wanted to use my video library on any device in the house. Maybe a laptop, maybe our multimedia pc which is connected to the tv, maybe even on an Android smartphone. XBMC has features that allow you to do this. To get this working, you don't actually need XBMC on the Raspberry PI at all! You need the video files on it, a network file access protocol, such as Samba, and an installation of MySQL which is what the XBMC installations on the client computers communicate with to show your video library etc. The whole setup is really nice as it allows you to watch your videos on any device in the house and it even remembers where you left off if you stopped halfway, so that you can easily continue watching on another computer. Once you've installed Samba and MySQL for XBMC on the PI, you need to install XBMC on all the client devices that you want to use. There are builds available for most platforms. Next you need to configure XBMC to use MySQL. I actually use different profiles to organize my video files, for example you may have holiday videos in one and educational videos in another profile (or any other separation of your choice ;). You can do this by putting the advancedsettings.xml file in the .../userdata/profile/<Profile Name> directory and name the database in it to separate the content, like this:
The name tag in the configuration is used to connect to separate MySQL databases running in the same database server. That allows you to serve different media depending on the XBMC profile you're in. In the Holidays profile it lists my holiday videos, in the Education profile it has my educational ones.

Finally, when you add videos to your database, make sure to select add the directory on the PI using a network protocol that works on all your devices. For example a Samba URL: 
And voilà, you will get all your movies on all your XBMC devices.

Ok, so this was a random collection of bits and pieces that I did to get my Raspberry PI to do what I wanted. If you have a nugget of PI goodness, leave a comment to share :)

Monday, January 13, 2014

OSGi Subsytems on Apache Felix

Note: more up-to-date information on getting started with Apache Aries Subsystems can be found here:

Last year I blogged about running the OSGi Subsystems implementation from Apache Aries. At the time Equinox was the only OSGi framework that had the full Core R5 implementation supported, which is needed in order to run OSGi Subsystems.

With some recent work on the Apache Felix container, it is now very close to supporting the full OSGi R5 specification. One of this first things that I tried with that is to run the Aries Subsystems implementation on it. And it works :)

OSGi Subsystems are a great way to package a number of OSGi bundles together, for example if you want to distribute an OSGi-based application. Subsystems provide a really convenient deployment model, without compromising the modularity of your application. For more background on Subsystems, see my earlier post about how to use OSGi Subsystems to deploy your applications.

What I'm doing here is take the example from that blog post and run it on Apache Felix with Aries Subsystems.

First of all you need the latest and greatest Felix Framework. I always build it as follows:

  svn co felix
  cd felix/framework
  mvn install 
  cd ../main
  mvn install 

This will give you a framework runtime in the felix/main folder.
Then add the following bundles to the bundle subdirectory:

All of the bundles above have links behind them where they can be downloaded, except for the Felix Coordinator implementation, as this one is very new. You can just build it from the coordinator directory of your Felix checkout. Or, if you prefer to use released components, you can also use the Equinox implementation org.eclipse.equinox.coordinator_1.1.0.v20120522-1841, but in order to run that on Felix you also need the Equinox supplement bundle org.eclipse.equinox.supplement_1.5.0.v20130812-2109.
Finally, I'm adding my little gogo command bundle as described in my older post, to add subsystem:list, subsystem:install, subsystem:uninstall, subsystem:start and subsystem:stop commands to Gogo. You can also download that as a bundle here: subsystem-gogo-command-1.0.0.jar

Ok, let's start up Felix and look what's inside:
.../felix/main $ java -jar bin/felix 
... some log messages appear ...
Welcome to Apache Felix Gogo
g! lb
   ID|State      |Level|Name
    0|Active     |    0|System Bundle (4.3.0.SNAPSHOT)
    1|Active     |    1|Apache Aries Application API (1.0.0)
    2|Active     |    1|Apache Aries Application Modelling (1.0.0)
    3|Active     |    1|Apache Aries Application Utils (1.0.0)
    4|Active     |    1|Apache Aries Blueprint Bundle (1.1.0)
    5|Active     |    1|Apache Aries Proxy Bundle (1.0.1)
    6|Active     |    1|Apache Aries Subsystem API (1.0.0)
    7|Active     |    1|Apache Aries Subsystem Core (1.0.0)
    8|Active     |    1|Apache Aries Util (1.1.0)
    9|Active     |    1|Apache Felix Bundle Repository (1.6.6)
   10|Active     |    1|Apache Felix Configuration Admin Service (1.8.0)
   11|Active     |    1|Apache Felix Coordinator Service (0.0.1.SNAPSHOT)
   12|Active     |    1|Apache Felix Gogo Command (0.12.0)
   13|Active     |    1|Apache Felix Gogo Runtime (0.10.0)
   14|Active     |    1|Apache Felix Gogo Shell (0.10.0)
   15|Active     |    1|Apache Felix Resolver (1.0.0)
   16|Active     |    1|Region Digraph (1.1.0.v20120522-1841)
   17|Active     |    1|slf4j-api (1.7.5)
   18|Resolved   |    1|slf4j-simple (1.7.5)
   19|Active     |    1|Subsystem Gogo Command (1.0.0)
   20|Active     |    1|org.osgi.service.subsystem.region.context.0 (1.0.0)

g! subsystem:list
0 ACTIVE org.osgi.service.subsystem.root

Looking at the bundles, you can see the Gogo command line ones that come with Felix, as well as its Bundle Repo. You can also see bundle 20, which is a synthesised bundle that the Subsystems implementation added and which represents the root subsystem. The subsystem:list command reports that there is one subsystem: the root one.

Now let's look at my example applications again. I have the following two subsystems:

Subsystems are a great way to distribute multi-bundle applications, and my two subsystems each contain 2 specific bundles as well as a shared bundle.

Let's install then and see what happens:

g! subsystem:install
Installing subsystem:
Subsystem successfully installed: subsystem1; id: 1
g! subsystem:start 1
g! lb
   ID|State      |Level|Name
   21|Active     |    1|SharedBundle (1.0.0)
   22|Active     |    1|BundleA (1.0.0)
   23|Active     |    1|BundleB (1.0.0)
The three bundles needed by subsystem1 are installed and all started with the subsystem:start command.

And let's add the second subsystem...
g! subsystem:install
Installing subsystem:
Subsystem successfully installed: subsystem2; id: 2
g! subsystem:start 2
g! lb
   ID|State      |Level|Name
   21|Active     |    1|SharedBundle (1.0.0)
   22|Active     |    1|BundleA (1.0.0)
   23|Active     |    1|BundleB (1.0.0)
   24|Active     |    1|BundleC (1.0.0)
   25|Active     |    1|BundleD (1.0.0)

As expected, the two new bundles for subsystem2 are now also installed. And, because we're talking about a feature subsystem here, where everything is shared, the SharedBundle (21) is not installed a second time, but rather reused from subsystem1. 

The topic of subsystems is much bigger. Subsystems can provide a certain level of isolation, they can work with Repositories for provisioning and you have a lot of options wrt to what you can put inside an .esa file: you can put all the bundles in there that your application needs, or you can just have a textual descriptor that declares your main bundles and let the OSGi Resolver and Repository find the dependencies and deploy these for you. Details about these various options can be found in the OSGi Subsystem spec (chapter 134 in the OSGi R5 Enterprise spec). 

All in all good news - Subsystems now works in Apache Felix as well. Right now you need to build the latest snapshot, but hopefully we'll have a Felix Framework release for this soon!

Tuesday, October 22, 2013

Role-based access control for Karaf shell commands and OSGi services

In a previous post I outlined how role-based access control was added to JMX in Apache Karaf. While JMX is one way to remotely manage a Karaf instance, another management mechanism is provided via the Karaf Console/Shell. Up until now security for console commands was very coarse-grained. Once in the console you had access to all the commands. For example, it was not possible to give certain users access to merely changing their own configuration without also giving them access to shutting down the whole karaf instance.

With commit r1534467 this has now changed (thanks again to JB Onofré for reviewing and applying my pull request). You can now define roles required for each shell command and even have different roles depending on the arguments used with a certain command. This is achieved by using a relatively advanced feature of OSGi: Service Registry Hooks. These hooks give you a lot of control on how the OSGi service registry behaves. I blogged about them before. They enable you to:
  • see what service consumers are looking for, so you can register these services on-the-fly. This is often used to import remote services from discovery, but only if there is actually a client for them.
  • hide services from certain service consumers
  • change the service properties the client sees for a service by providing an alternative registration
  • proxy the original service
Every Karaf command is in reality an Apache Felix Gogo command, registered as an OSGi service. Every command has two service registration properties: osgi.command.scope and osgi.command.function. These properties define the name of the command and its scope. With the use of the OSGi Service Registry hooks I can replace the original service with a proxy that adds the role-based security. 

When I originally floated this idea on the Karaf mailing list, Christian Schneider said: "why don't we enable this for all services?" Good idea! So that's how I ended up implementing it. I first added a mechanism to add role-based access control to OSGi services in general and then applied this mechanism to get role-based access control for the Karaf commands.

Under the hood

the original service is hidden by OSGi Service Registry Hooks
The theory is quite simple. As mentioned above you can use OSGi Service Registry hooks to hide a service from certain consuming bundles and effectively replace it with another. In my case the replacement is a proxy of the original service with the same service registration properties (and some extra ones, see below). It will delegate an invocation to the original service, but before it does so it will check the ACL for the service being invoked to find out what the permitted roles are. Then it checks the roles of the current user by looking at the Subject in the current AccessControlContext. If the user doesn't have any of the permitted roles the service invocation is aborted with a SecurityException.

How do I configure ACLs for OSGi services?

ACLs for OSGi services are defined in a way similar to how these are defined for JMX access: through the OSGi Configuration Admin service. The PID needs to start with org.apache.karaf.service.acl. but the exact PID value isn't important. The service to which the ACL is matched is found through the service.guard configuration value. Configuration Admin is very flexible wrt to how configuration is stored, but by default in Karaf these configurations are stored as .cfg files in the etc/ directory. Let's say I have a service in my system that implements the following API and is registered in the OSGi service registry under the org.acme.MyAPI interface:
  package org.acme;

  public interface MyAPI {
    void doit(String s);
If I want to specify an ACL to say that only clients that have the manager role can invoke this service, I have to do two things:
  1. First I need to enable the role-based access for this service by including it in the filter specified in the etc/ in the property:|(objectClass=org.acme.MyAPI)(...what was there already...))
    only services matching this property are enabled for role-based access control. Other services are left alone.
  2. Define the ACL for this service as Config Admin configuration. For example by creating a file etc/org.apache.karaf.service.acl.myapi.cfg:

    So the actual PID of the configuration is not really important, as long as it starts with the prefix. The service it applies to is then selected by matching the filter in the service.guard property.
There are some additional rules. There is a special role of * which means that ACLs are disabled for this method. Similar to the JMX ACLs you can also specify function arguments that require specific roles. For more details see the commit message.

Setting roles for service invocation

The service proxy checks the roles in the current AccessControlContext against the required ones. So when invoking a service that has role-based access control enabled, you need to set these roles. This is normally done as follows:
  import org.apache.karaf.jaas.boot.principal.RolePrincipal;
  // ... 
  Subject s = new Subject();
  s.getPrincipals().add(new RolePrincipal("manager"));
  Subject.doAs(s, new PrivilegedAction() {
    public Object run() {
      svc.doit("foo"); // invoke the service
This example uses a Karaf built-in role. You can also use your own role implementations by specifying them using the className:roleName syntax in the ACL.

Note however, that is a very powerful API. You should give bundles that import it extra scrutiny to ensure that they don't give access to clients that they shouldn't really have...

Applied to Shell Commands

Next step was to apply these concepts to the Karaf shell commands. As all the shell commands are registered with the osgi.command.function and osgi.command.scope properties. I enabled them in the default Karaf configuration with the following system property:*)(osgi.command.function=*))

The next thing is to configure command ACLs. However that presented a slight usability problem. Most of the services in Karaf are implemented (via OSGi Blueprint) using the Function interface. Which means that the actual method name is always execute. It also means that you need to create a separate Configuration Admin PID for each command which is quite cumbersome. You really want to configure this stuff on a per-scope level with all the commands for a single scope in a single configuration file. To allow this the command-integration code contains a configuration transformer which creates service ACLs as described above but based on command scope level configuration files.
The command scope configuration file must have a PID that follows this structure: org.apache.karaf.command.acl.<scope> So if you want to create such a file for the feature scope the config file would be in etc/org.apache.karaf.command.acl.feature.cfg:
  list = viewer
  info = viewer
  install = admin
  uninstall = admin
In this example only users with the admin role can do install/uninstall operations while viewers can list features etc... Note that by using groups (as outlined in this previous post) users added to an admin group will also have viewer permissions, so will be able to do everything. For a more complex configuration file, have a look at this one.

Can I find out what roles a service requires?

It can be useful to know in advance what the roles are that are required to invoke a service. For example the shell supports tab-style command completion. You don't want to show commands to the user that you know are not available to the user's roles. For this purpose an additional service registration property is added to the proxy service registration: org.apache.karaf.service.guard.roles=[role1,role2]. The value of the property is the Collection of roles that can possibly invoke a method on the service. Since each command maps to a single service, we can have a Command Processor that only selects the commands applicable to the roles of the current user. This means that commands that this user doesn't have the right roles for are automatically hidden from autocompletion etc. When I'm logged in as an admin I can see all the feature commands (I removed ones not mentioned in the config for brevity):
  karaf@root()> feature: <hit TAB>
  info            install         list            uninstall
while Joe, a viewer, only see the feature commands available to viewers:
  joe@root()> feature: <hit TAB>
  info            list

In some cases the commands have roles associated with particular values being passed in. For example the config admin shell commands require admin rights for certain PIDs but not all. So Joe can safely edit his own configuration but is prevented from editing system level configuration:
  joe@root(config)> edit
  joe@root(config)> property-set somekey someval
  joe@root(config)> update
So Joe can edit the PID, but when he tries to edit the jmx.acl PID access is denied:
  joe@root(config)> edit jmx.acl
  Error executing command: Insufficient credentials.

Where are we with this stuff today?

The first commits to enable the above have just gone into Karaf trunk and although I wrote lots of unit tests for it, more use is needed to see whether it all works as users would expect. Also the default ACL configuration files may need a bit more attention. What's there now is really a start, the idea is to refine as we go along and have this as a proper feature for Karaf 3.

The power of OSGi services

One thing that this approach shows is really the power and flexibility of OSGi services. None of the code of the actual commands was changed. The ability to build role-based access on top of them in a non-intrusive way was really enabled by the OSGi service registry design and its capabilities.