Time flies like an arrow (and fruit flies like a banana) ... One big disadvantage of organizing conferences is that it looks like time goes even faster. It is already time again to prepare for OSGi DevCon 2008 (a.k.a. EclipseCon). For OSGi DevCon we are looking for people that are using OSGi technology in unusual ways and that are willing to present it to their peers. You can present it in a lightning talk, a short talk, a long talk, or even a tutorial.
Finally, the OSGi technology is in the upswing after many years of preparation. It is wonderful to see how people apply OSGi tecnology in the most interesting ways. It is also very good to see how more and more tools and frameworks are being produced to simplify the use of OSGi technology.
There are three more weeks to submit your proposals. I really hope many people will take the effort to submit a proposal. You have time until the 19th of November.
Submit now!
Peter Kriens
OSGi Blog
Monday, October 29, 2007
Wednesday, October 17, 2007
iJAM, Formalized Class Loading
(This blog is adapted after comment from Victor, I had not see that iJam had an exception for java.*, excuses).
There is a paper floating on the net that proposes an alternative to the class loading strategy of JSR 277. Richard S. Hall pointed me to this paper and told me to write a blog about it. So I did.
Though the paper provides a formalization of the class loading strategy, the modification it proposes is actually quite simple. Standard Java class loading rules is parent first, then self. In a modern Java system, there can be many ancestors so this rule is supposed to be applied recursively until one reaches the system class loader. This is a simple rule that provides Sun with the guarantee that its classes can not be overridden at a lower level. This model implies that a module can never override anything available in its parents. For example, if a module wants to use a newer version of the XML parser in the platform then it would be nice if these classes could be overridden on application level to ensure the proper version. For this reason, iJAM proposes to change the parent first rule to local first, except for java.* and javax.* classes. This allows a module to override any class in an ancestor class loader.
I do agree with the problem but I disagree rather strongly with the solution, it is too simple. Let me explain why I came to this conclusion.
First, loading only javax.* and java.* from the parent is ignoring classes that come from the bootclasspath but do not start with java.* or javax.*. An example is org.xml.sax. If this package is loaded from a module, then the system classes will load their own instance of this package and modules will see another. This will cause class cast exceptions if you try to give your SAX Handler to the parser because they will use different class loaders for the same package.
Another problem is that many javax.* packages are prime candidates to be downloaded as bundles. Though there are logical reasons to treat java.* as special because overriding java.lang.Object is quite disastrous, there are no reasons to treat javax.* in the same way.
A module must be able to define its priorities in loading, there are clear use cases where overriding a platform provided class is crucial. Can this be done as an all or nothing decision on module level? Don't think so, only in simple cases are these decisions module wide, and when was the last time you did something simple and got paid for it? Sometimes you want to provide a default implementation in case the platform, or any of the other modules, does not provide an implementation. Other times you want to be sure to get your own version. A simple rule as local first can not distinguish between these cases, nor can a rule like parent first satisfy you all the time.
Another problem with the JSR 277 and iJAM rules is that it treats classes as standalone entities, not part of a cohesive package. If your module overrides one class of a larger package you have something we call a split package. Split packages are nasty. First, classes in the same package have package private visibility (the default). However, this rule only works when those classes come from the same class loader. Obviously, you can get some very hard to understand errors when two classes in the same package get access errors for a field that is package private. Really, split packages are evil and it is quite surprising that JSR 277 allows them, as much as iJAM proposes this same behavior. Obviously, there are many other errors that can occur when half of your classes are loaded from your module and the other half from some other module. Each half could have quite interesting assumptions about the other half. Unless you enjoy debugging very strange errors, split packages are just not a recommended strategy. A package is not just part of the name of a class, it is a grouping concept that should be respected.
So how does OSGi address this issue? Well, our class loading rules are extensive because this is a highly complex area that we were unfortunate enough to have to learn over the last 9 years.
Our first rule is to load any java.* class from the parent class loader. As I explained, very few java.* classes can be overridden without wreaking havoc in some way (java.sql.* is the only example that comes to mind).
It then looks at the imported packages of a bundle. In OSGi, the manifest explicitly tells the framework which packages are expected to come from another bundle. These packages are listed in an Import-Package header with explicit constraints on the exporter.
If the package is not imported, the Framework will look in bundles linked with Required-Bundle. If that fails, the framework then looks in the jar and attached fragments. The developer can control searching the JAR in detail with the Bundle-Classpath header.
The class loading strategy is obviously an example that Einstein must have had in mind when he said: "Things should be as simple as possible, but not simpler". We learned the hard way that in the end, the developer must be able to control what he gets from where, but not at the expense of a potentially unworkable system. Though I like French laissez-faire, when it comes to class loading I prefer the more deterministic Anglo-Saxon approach.
To conclude, I really like the formal work that the iJAM paper shows, it is one of my secret desires to ever have a similar Z specification of the OSGi module layer. If the authors of the iJAM papers want to work on this, please let me help. However, I think that the class loading strategy in this paper is, just like the class loading strategy of JSR 277, unfortunately too simplistic for the complexity of the real world.
Peter Kriens
There is a paper floating on the net that proposes an alternative to the class loading strategy of JSR 277. Richard S. Hall pointed me to this paper and told me to write a blog about it. So I did.
Though the paper provides a formalization of the class loading strategy, the modification it proposes is actually quite simple. Standard Java class loading rules is parent first, then self. In a modern Java system, there can be many ancestors so this rule is supposed to be applied recursively until one reaches the system class loader. This is a simple rule that provides Sun with the guarantee that its classes can not be overridden at a lower level. This model implies that a module can never override anything available in its parents. For example, if a module wants to use a newer version of the XML parser in the platform then it would be nice if these classes could be overridden on application level to ensure the proper version. For this reason, iJAM proposes to change the parent first rule to local first, except for java.* and javax.* classes. This allows a module to override any class in an ancestor class loader.
I do agree with the problem but I disagree rather strongly with the solution, it is too simple. Let me explain why I came to this conclusion.
First, loading only javax.* and java.* from the parent is ignoring classes that come from the bootclasspath but do not start with java.* or javax.*. An example is org.xml.sax. If this package is loaded from a module, then the system classes will load their own instance of this package and modules will see another. This will cause class cast exceptions if you try to give your SAX Handler to the parser because they will use different class loaders for the same package.
Another problem is that many javax.* packages are prime candidates to be downloaded as bundles. Though there are logical reasons to treat java.* as special because overriding java.lang.Object is quite disastrous, there are no reasons to treat javax.* in the same way.
A module must be able to define its priorities in loading, there are clear use cases where overriding a platform provided class is crucial. Can this be done as an all or nothing decision on module level? Don't think so, only in simple cases are these decisions module wide, and when was the last time you did something simple and got paid for it? Sometimes you want to provide a default implementation in case the platform, or any of the other modules, does not provide an implementation. Other times you want to be sure to get your own version. A simple rule as local first can not distinguish between these cases, nor can a rule like parent first satisfy you all the time.
Another problem with the JSR 277 and iJAM rules is that it treats classes as standalone entities, not part of a cohesive package. If your module overrides one class of a larger package you have something we call a split package. Split packages are nasty. First, classes in the same package have package private visibility (the default). However, this rule only works when those classes come from the same class loader. Obviously, you can get some very hard to understand errors when two classes in the same package get access errors for a field that is package private. Really, split packages are evil and it is quite surprising that JSR 277 allows them, as much as iJAM proposes this same behavior. Obviously, there are many other errors that can occur when half of your classes are loaded from your module and the other half from some other module. Each half could have quite interesting assumptions about the other half. Unless you enjoy debugging very strange errors, split packages are just not a recommended strategy. A package is not just part of the name of a class, it is a grouping concept that should be respected.
So how does OSGi address this issue? Well, our class loading rules are extensive because this is a highly complex area that we were unfortunate enough to have to learn over the last 9 years.
Our first rule is to load any java.* class from the parent class loader. As I explained, very few java.* classes can be overridden without wreaking havoc in some way (java.sql.* is the only example that comes to mind).
It then looks at the imported packages of a bundle. In OSGi, the manifest explicitly tells the framework which packages are expected to come from another bundle. These packages are listed in an Import-Package header with explicit constraints on the exporter.
If the package is not imported, the Framework will look in bundles linked with Required-Bundle. If that fails, the framework then looks in the jar and attached fragments. The developer can control searching the JAR in detail with the Bundle-Classpath header.
The class loading strategy is obviously an example that Einstein must have had in mind when he said: "Things should be as simple as possible, but not simpler". We learned the hard way that in the end, the developer must be able to control what he gets from where, but not at the expense of a potentially unworkable system. Though I like French laissez-faire, when it comes to class loading I prefer the more deterministic Anglo-Saxon approach.
To conclude, I really like the formal work that the iJAM paper shows, it is one of my secret desires to ever have a similar Z specification of the OSGi module layer. If the authors of the iJAM papers want to work on this, please let me help. However, I think that the class loading strategy in this paper is, just like the class loading strategy of JSR 277, unfortunately too simplistic for the complexity of the real world.
Peter Kriens
Thursday, October 4, 2007
Universal OSGi
There is an idea that has been simmering in the OSGi Alliance for quite some time: Universal OSGi. My first recollection of this idea is from 1999, when I was working in Linkoping at Ericsson Wireless Technology, a place in the middle of nowhere. I worked there a couple of days a week helping them to use the OSGi specifications on the Ericsson ebox. This was not an easy task because there were virtually no Java programmers and lots of native Linux device driver developers. These guys have it in their genes to see anything between them and the Linux API as a personal insult. The fact that the ebox was severely underpowered for Java, 25Mhz 486 - 32Mb RAM - 8Mb flash, did obviously not do anything for my popularity either. However, these guys are intelligent and once they understood the modularity model and the remote management model that this enabled, they decided that they wanted to have it too. well, not the Java but the modularity. They even had visions to replace the OSGi specifications with something more native.I objected to that model, and I still do. Though I am all too aware of the many flaws that the language was born with (I am a Smalltalker by heart, how could they miss closures!), I am profoundly impressed with what has been done on security and the abstraction of the the operating system. The OSGi technology provides some very crucial missing parts in Java, but a very large number of the features we provide are enabled by Java. I really do not believe that what is done in the modularity layer, the life cycle management layer, the service layer, and the security model can be done in any other existing environment. Not even .NET, though they get closest.
However, the need for integrating with native code does not go away because our model is better. The battlefield is littered with corpses that had a better model. It is a plain fact that in many of the markets that use OSGi technology, native code integration plays a major role. Java is good, but there is a lot of legacy code out there that is just not Java.
The only thing we have on offer for native code integration is through the Java Native Interface (JNI) and remote procedure calls like with web services, CORBA, etc. Anybody that tried to program with JNI knows how painful and intrusive it can be. I do not think I would have survived Linkoping proposing JNI. Remote procedure calls are better, and well known in the industry. However, remote procedure calls provide interoperation but no modularity, life cycle, or security. The interoperation through remote procedure calls works well as is proven many times, it lacks the tight integration of all important computing aspects that the OSGi technology provides.
Meet Universal OSGi. This is not a worked out concept but work in progress. Universal OSGi is an attempt to use the normal run of the mill Java based OSGi service platform but provide a model where native code can be integrated. This means that you should be able to download bundles with a native executable, like a DLL or shared library. Lets call these native bundles.
You should be able to express the dependencies of these native bundles in the manifest on other native bundles so that a management system could calculate the transitive dependencies, just like a Java bundle. Then you should be able to start and stop these bundles, which should result in loading the native code in another process and providing it with its dependencies, and starting it. The native code should be able to get and register services which are used by the native code to communicate with the rest of the system.
Handling the native code dependencies and the life cycle issues is not trivial but definitely doable. A native handler bundle can use the extender model to detect native bundles of a type it recognizes. This bundle would somehow have to interact with the resolver because the Java resolver needs to know when a native bundle can be resolved (or maybe this could be expressed with Require-Bundle). If the native bundle is started, the native handler will load the native code in another process. When the native bundle is stopped, the native handler cleans up by killing the process. Not trivial, but doable.
The hard part is the communication aspect. Obviously, the service layer is the crucial middle man, requiring that the native code can communicate with the OSGi service registry, hosted in the Java VM process. This requires a native API that maps the primitives of the OSGi service registry to C, C++, .NET, PHP, Haskell, etc. Primitives like registering a service, getting a service, and listening for services. And of course coupled to the life cycle layer. If a bundle is stopped, all its services must be unregistered. This registry is still doable, albeit a bit less trivial. The hardest part is how the services are mapped in the remote procedure calls. This is the problem that many have tried and few really succeeded because it somehow always remains messy. CORBA has the Interface Definition Language (IDL) which was supposed to be the mother of all languages but largely failed in the Java world because its C++ orientation made mapping it to Java painful. I remember a long ago project where we had two class for every parameter because that was the way output parameters could be modeled, a concept well known to C++ but unkown to Java.
For Universal OSGi, it is likely that the best solution is the Java interface as an "IDL". Not only do we already have a lot of experience with Java interfaces, they are also conceptually very clean, not associated with an implementation. In Java it is already trivial to proxy interfaces. it will therefore be necessary to map Java interfaces in a mechanical way to concepts known in the native environment. For example, in C++ a Java interface can be mapped to an abstract base class that can be used as mixin. Most OSGi service specifications are very suitable for this mapping.
A key problem in designing such a communication system is how the remote procedure calls are handled. A remote procedure call crosses the process boundary and pointers to memory locations are therefore no longer valid. Each process has its own memory. There are two solutions to this problem. One can pass the value to which the pointer is pointing, or one can pass a symbolic reference of the object. Passing a value can be done with immutable objects like int, String, etc. but it can not be done for complex objects like java.lang.Class. If a mutable object is passed by value, changes on the remote side are not reflected on the caller's side, changing the behavior for remote and local calling. However, one can proxy any complex object by passing a symbolic reference and doing the same for any objects that are exchanged in method calls. The other side must recognize this reference and do a remote procedure call back into the caller's process for all methods. This model is called proxying. It is too expensive for real life communications due to the latency and bandwidth constraints. For Universal OSGi it might be viable because it runs all the participants on the same device. That allows all communications tobe done with very fast techniques like shared memory.
These are intriguing and exciting ideas that could truly make OSGi technology more universally applicable. However, there are a lot of technical details to iron out and even when that has been done, there is a lot of spec work for different native languages. We need members that are willing to make this work. Interested?
Peter Kriens
Friday, September 14, 2007
Simplicity, Size, and Seniority
It must have been 1996 when I explained to Ralph Johnson that I liked Java over Smalltalk because it was so small and simple. He looked at me warily and said: "wait a couple of years, it will grow". Ralph is a wise man and after a few years when Java grew from a handful of packages to hundreds of packages I understood what he had meant.
I was reminded of this conversation when I read a blog that compared HK2 with OSGi. HK2 was simpler because it had only 50 classes (!) and only a few pages of documentation. To be honest, I had not yet heard of HK2 so I investigated. This turns out to be the Glassfish module system developed by Sun. From the blog you get the false impression that OSGi is the Goliath spec of thousands of classes and HK2 is this new little David that has a more human size.
Interestingly, it is more the other way around, despite their immaturity. Their module/life cycle layer is comprised of 30 classes, their service/component layer is 20 classes. Security is absent. The only mandatory package in OSGi is org.osgi.framework, which is comprised of only 25 classes and contains the security, module, and service layer. Realistically one should add Package Admin, which adds another 3 classes. Still significantly smaller than the 50 classes of HK2, and they have only just started while we have been around for 9 years and had to address all the hard problems that happen when you build real life applications for constrained environments. All the other classes in the OSGi specifications are optional, when you have a specific problem we might have a specification that addresses that problem, when you do not care, just stick to the framework.
HK2 stands for Hundred Kilobytes Kernel. I guess this is something similar as the 1 dollar stores that over time become extremely creative or outright deceit because this (artificial) limit is not maintainable. I am not sure where they are today, but I am pretty sure that HK2 will require more than 100K within 2 years if they want to do it right, the real world tends to be quite messy.
Anyway, for a specification the code size of the implementations is a bit different. OSGi R3 can be implemented in less than 100k, look at Concierge. However, R4 likely requires a larger implementation because it addresses so many more use cases. Apache Felix and Knopflerfish are around 250K and Eclipse Equinox is around 900K. Why is Equinox so big in comparison? Well, their non-functional requirements state that they must be able to handle 5000 bundles. To make such a gigantic system work, they need to revert to many optimizations and caching techniques which add complexity. This difference is one of the best illustrations why the OSGi specifications are important. Depending on your needs you can choose to use Felix, Knopflerfish, Equinox, or a commercial implementation. Each implementation has made trade offs that you can match against your requirements. However, they all over the same basic OSGi environments and most bundles run on all systems.
Comparing the documentation of HK2 and OSGi it is the other way around: OSGi is bigger. The core OSGi framework specification with the optional system services is 288 pages. I agree it is a lot easier to read the 3 web pages and the Javadoc of HK2, but does it help you when there is a problem? It is interesting how the size of the documentation can be used against you. Yesterday, on the Python list someone asked if there could be an OSGi for Python and the reply was a joke about how "impressed" he was with the 1000 pages of the specification. To set the record straight, all the OSGi specifications combined comprise a 1000 pages but the core is less than 300 pages including the Javadoc. This is still a lot of text but we really tried very hard to make them as readable and usable as possible. So what do you prefer? A handful of web pages or a detailed specification. I can only defer to another wise man: "Things should be as simple as possible, but not simpler".
Peter Kriens
I was reminded of this conversation when I read a blog that compared HK2 with OSGi. HK2 was simpler because it had only 50 classes (!) and only a few pages of documentation. To be honest, I had not yet heard of HK2 so I investigated. This turns out to be the Glassfish module system developed by Sun. From the blog you get the false impression that OSGi is the Goliath spec of thousands of classes and HK2 is this new little David that has a more human size.
Interestingly, it is more the other way around, despite their immaturity. Their module/life cycle layer is comprised of 30 classes, their service/component layer is 20 classes. Security is absent. The only mandatory package in OSGi is org.osgi.framework, which is comprised of only 25 classes and contains the security, module, and service layer. Realistically one should add Package Admin, which adds another 3 classes. Still significantly smaller than the 50 classes of HK2, and they have only just started while we have been around for 9 years and had to address all the hard problems that happen when you build real life applications for constrained environments. All the other classes in the OSGi specifications are optional, when you have a specific problem we might have a specification that addresses that problem, when you do not care, just stick to the framework.
HK2 stands for Hundred Kilobytes Kernel. I guess this is something similar as the 1 dollar stores that over time become extremely creative or outright deceit because this (artificial) limit is not maintainable. I am not sure where they are today, but I am pretty sure that HK2 will require more than 100K within 2 years if they want to do it right, the real world tends to be quite messy.
Anyway, for a specification the code size of the implementations is a bit different. OSGi R3 can be implemented in less than 100k, look at Concierge. However, R4 likely requires a larger implementation because it addresses so many more use cases. Apache Felix and Knopflerfish are around 250K and Eclipse Equinox is around 900K. Why is Equinox so big in comparison? Well, their non-functional requirements state that they must be able to handle 5000 bundles. To make such a gigantic system work, they need to revert to many optimizations and caching techniques which add complexity. This difference is one of the best illustrations why the OSGi specifications are important. Depending on your needs you can choose to use Felix, Knopflerfish, Equinox, or a commercial implementation. Each implementation has made trade offs that you can match against your requirements. However, they all over the same basic OSGi environments and most bundles run on all systems.
Comparing the documentation of HK2 and OSGi it is the other way around: OSGi is bigger. The core OSGi framework specification with the optional system services is 288 pages. I agree it is a lot easier to read the 3 web pages and the Javadoc of HK2, but does it help you when there is a problem? It is interesting how the size of the documentation can be used against you. Yesterday, on the Python list someone asked if there could be an OSGi for Python and the reply was a joke about how "impressed" he was with the 1000 pages of the specification. To set the record straight, all the OSGi specifications combined comprise a 1000 pages but the core is less than 300 pages including the Javadoc. This is still a lot of text but we really tried very hard to make them as readable and usable as possible. So what do you prefer? A handful of web pages or a detailed specification. I can only defer to another wise man: "Things should be as simple as possible, but not simpler".
Peter Kriens
Thursday, September 6, 2007
OSGi Summer School
Sometimes being an evangelist has its perks. Last week I was a guest at a very beautiful spot in Brittany at the École été OSGi. The only thing I have to do is talk about OSGi! Ok, the travel was a tad long for the distance, the hotel where we are staying took the car, a plane, a bus, a train, and another bus to get to the path that floods twice a day. At times of flood, one has to use a simple ferry. However, the location definitely has charm.
To my surprise, there were 55 students registered, to my even bigger surprise they had to deny many people that registered late because there was only limited place. It really looks like the OSGi technology is starting to make inroads. Especially the fact that the business people outnumber the academics. For a long time, the academics (or at least research people) outnumbered the business people. I love academics, but of course we need people from the industry to move OSGi into the next phase of universal middleware.
This morning I gave the opening speech. Instead of trying to explain the technology, I gave a more emotional story about the way the work on the OSGi got started, how we struggled, what failed, and what finally worked. I also spent a good amount of time on lingering about why service oriented programming is so important. As I wrote last Monday, they key is that service oriented programming allows us to reason about systems that have a much coarser granularity than objects. I tried to hard to reach the French audience, however I betted that their English was better than my lousy French. Hope they got the message!
This afternoon everybody worked hard on a prepared course. It was really cool to see all those people struggling to sell and buy service on an Apache Felix framework. well, it is time for dinner so the only thing I can say, wish you were here!
Peter Kriens
Tuesday, September 4, 2007
SOA & OSGi
Nine years ago, when we started working on the OSGi specifications, we used the word service to describe the object that acted as a conduit between bundles. Today, Service Oriented Architectures (SOA) are hot and every software vendor seems intent on confusing a muddy picture even further by bringing their products under this wide umbrella. The result is that many people file OSGi under webservices, as the most popular exponent of SOA, and then conveniently ignore any further documentation because they already know what web services are. As an example, a friend of mine consistently gets his papers rejected because many of the reviewers make this very false assumption.
A bit of history. In 1985, I was developing systems for the newspaper market and we just started our Magnus Opus: a newspaper page makeup terminal (this was the time that a 25 Mhz 286 was state of the art). This product was of strategic importance for our company and I did due diligence on a lof of languages to select the best one for this project. After discovering Smalltalk/V 286, I fell head over heels in love. I thought object oriented (OO) software was the best thing since sliced bread! I never had been comfortable with the common design methodologies at that time called Structured Design. Discovering OO felt like coming home after a long journey. After grasping the technology I made my second worst advice ever (the worst was when I told a programmer friend in 1994 that building an e-commerce shopping cart application was a silly idea, sorry Matts). I told my manager that if we started to build our systems using objects we would be able to get fantastic reuse after a year or so. Obviously under the condition that he allowed me to go to all these cool conferences like the OOPSLA. I assured him, new applications could be build in days instead of years, just using objects ...
Well, as most of us know it did not turn out this way. Yes, we got a lot of advantages using OO technology but reuse turned out a lot harder than we thought. Sadly, I think the key cause of the lack of reuse was the violation of the two sacred rules of Structured Design: low coupling, high cohesion. It was very much like a couple of girls on the beach trying desperately not to expose themselves to the boys in front of them while changing into their swimming gear; not realizing that there were also some boys sitting behind them. We were so busy hiding our data in instance variables writing get and set methods that we never realized that we were exposing the object structure to anybody using our classes. That is, if you do
At the end of the previous millennium our industry learned many hacks to reduce this structural coupling; the most (in)famous is the
Also, if you look at one of the great insights of the last decade, Inversion of Control (IoC), you see that this is really about decoupling. IoC (or the Hollywood principle of "do not call us, we will call you") takes an object and provides it with its dependencies instead of this object trying to create its dependencies. This simple idea allows for POJOs, objects that are minimally coupled.
Services are trying to solve the same problem: minimize complexity by decoupling. The key insight that lead to the service model is that the OO has an important role in modularization on the small scale but we need something of a bigger granularity on the larger scale. A modularity that minimizes coupling to well defined conduits between coarse components. This was exactly the problem we tried to solve since that 1998 meeting in the Research Triangle Park in Raleigh where the technical OSGi work got started. Our key assignment was to let software from many different service providers collaborate in a single residential gateway. Trying to standardize large class libraries with highly coupled frameworks would have taken forever and it would have done a terrible job. By focusing on the coarse components (the bundles) that would collaborate through objects named by their interface that were found in a registry (the services) we created a model that had minimal, well defined, and documented coupling between bundles.
The key advantage of SOA is therefore the reduced coupling between the blocks of a system. The model became wide known with Webservices because Microsoft and IBM decided to put a lot of marketing dollars in that direction, making SOA almost synonymous with XML, SOAP, HTTP, some other acronyms, and associated distributed computing problems. It is not. Service Oriented Architectures reduces the complexity caused by not managing structural coupling by providing well defined conduits between components. Webservices achieve this using a myriad of acronyms, OSGi technology achieves this by using an extremely low overhead in-VM service publish/find/bind model.
So the key question is if the confusion between Webservices and OSGi is doing us harm? I think it does, many people fail to look further because they assume they understand. For example, I am flabbergasted that leading software magazines like ACM Communications and IEEE Software never had an in-depth article about OSGi technology because I strongly believe that the ramifications of this technology for our industry are profound.
Peter Kriens
A bit of history. In 1985, I was developing systems for the newspaper market and we just started our Magnus Opus: a newspaper page makeup terminal (this was the time that a 25 Mhz 286 was state of the art). This product was of strategic importance for our company and I did due diligence on a lof of languages to select the best one for this project. After discovering Smalltalk/V 286, I fell head over heels in love. I thought object oriented (OO) software was the best thing since sliced bread! I never had been comfortable with the common design methodologies at that time called Structured Design. Discovering OO felt like coming home after a long journey. After grasping the technology I made my second worst advice ever (the worst was when I told a programmer friend in 1994 that building an e-commerce shopping cart application was a silly idea, sorry Matts). I told my manager that if we started to build our systems using objects we would be able to get fantastic reuse after a year or so. Obviously under the condition that he allowed me to go to all these cool conferences like the OOPSLA. I assured him, new applications could be build in days instead of years, just using objects ...
Well, as most of us know it did not turn out this way. Yes, we got a lot of advantages using OO technology but reuse turned out a lot harder than we thought. Sadly, I think the key cause of the lack of reuse was the violation of the two sacred rules of Structured Design: low coupling, high cohesion. It was very much like a couple of girls on the beach trying desperately not to expose themselves to the boys in front of them while changing into their swimming gear; not realizing that there were also some boys sitting behind them. We were so busy hiding our data in instance variables writing get and set methods that we never realized that we were exposing the object structure to anybody using our classes. That is, if you do
new Foo() from class Bar, you just coupled the classes Foo and Bar for eternity. Trying to reuse Bar immediately drags in Foo. Now if Foo and Bar had been cohesive (meaning they are really parts of the same function) this would not be such a problem. However, because we were so busy writing get/set methods we mostly ended up with systems that had transitive dependencies that basically included all classes in your company's repository. If you want to see an example, write a "Hello world" program with an empty local Maven repository. Not only made this coupling reuse very hard, the coupling made making changes also very complicated because many changes were far reaching.At the end of the previous millennium our industry learned many hacks to reduce this structural coupling; the most (in)famous is the
Factory pattern. This pattern allowed you to manage the coupling outside the original program by moving the coupling to a runtime decision. However, it is a hack. OO languages have no built-in answer to this problem, just look at the large number of attempts one can find in Java to solve this coupling problem.Also, if you look at one of the great insights of the last decade, Inversion of Control (IoC), you see that this is really about decoupling. IoC (or the Hollywood principle of "do not call us, we will call you") takes an object and provides it with its dependencies instead of this object trying to create its dependencies. This simple idea allows for POJOs, objects that are minimally coupled.
Services are trying to solve the same problem: minimize complexity by decoupling. The key insight that lead to the service model is that the OO has an important role in modularization on the small scale but we need something of a bigger granularity on the larger scale. A modularity that minimizes coupling to well defined conduits between coarse components. This was exactly the problem we tried to solve since that 1998 meeting in the Research Triangle Park in Raleigh where the technical OSGi work got started. Our key assignment was to let software from many different service providers collaborate in a single residential gateway. Trying to standardize large class libraries with highly coupled frameworks would have taken forever and it would have done a terrible job. By focusing on the coarse components (the bundles) that would collaborate through objects named by their interface that were found in a registry (the services) we created a model that had minimal, well defined, and documented coupling between bundles.
The key advantage of SOA is therefore the reduced coupling between the blocks of a system. The model became wide known with Webservices because Microsoft and IBM decided to put a lot of marketing dollars in that direction, making SOA almost synonymous with XML, SOAP, HTTP, some other acronyms, and associated distributed computing problems. It is not. Service Oriented Architectures reduces the complexity caused by not managing structural coupling by providing well defined conduits between components. Webservices achieve this using a myriad of acronyms, OSGi technology achieves this by using an extremely low overhead in-VM service publish/find/bind model.
So the key question is if the confusion between Webservices and OSGi is doing us harm? I think it does, many people fail to look further because they assume they understand. For example, I am flabbergasted that leading software magazines like ACM Communications and IEEE Software never had an in-depth article about OSGi technology because I strongly believe that the ramifications of this technology for our industry are profound.
Peter Kriens
Thursday, August 23, 2007
Diversity is Good
A couple of weeks ago there was a discussion on the OSGi developer list about a support library for OSGi programmers that is intended to make the life of an OSGi programmer easier. This library was developed on, well lets say, framework E. A developer in Asia tried this library on framework K and found out that the library did not work on this framework. After a couple of messages back and forth the library developer basically said that it was probably a bug in framework K and that the best solution was to let the framework K developers figure out where the bug was.
I made a lot of noise on the list because I think we should develop against the OSGi specifications, not against a particular framework implementation. The fact that it runs on framework E does not mean by definition that it is good, even though I know that the developers of framework E are really good.
Now I have been in this industry for 30 years and I still have not learned to sit still and wait until everybody has forgotten the issue. This stupidity obviously resulted the problem ending up in my lap. Today I decided to run the faulty code on frameworks K, E, and F. This is a lot easier to write down than to do it, actually. Each framework has its own peculiar way of starting and it took me quite some time to get the whole setup running. To be able to switch quickly, I used the aQute File Install bundle. This is a simple bundle that watches a directory and installs/uninstall bundles depending on their presence in this folder. As a bonus, it can also manage configuration data in a similar vein. I then created Eclipse launchers for each of the frameworks.
After this hard labor, finding the bug was not too hard. It turned out there was a misinterpretation of the spec that should be easy to fix. However, it was almost a shame to see how much preparation I had to do to be able to run bundles on multiple frameworks. The setup I had created I thought was quite fancy, it was easy to test all kinds of bundles this way.
I therefore decided to use the altruism of Google and use some of their abundant disk space as a Google project. Wonder if all those Google investors know how their money is spent to support open source? Anyway, I created the multi-osgi project. Within Eclipse you can check this out with SVN (you need Subclipse, Subversive, or some other SVN plugin. The URL to the project is http://code.google.com/p/multi-osgi/source.
The project works quite simple. In Eclipse, you can just find an Equinox, Knopflerfish, and Felix launch in the Run/Debug Dialog. From a shell, you can just switch to the Felix, Knopflerfish, or Eclipse directory and do run (in DOS) or ./run.sh in Unix.
Once you got one or more frameworks running, you can easily install the bundles by just dropping them in the
I will keep maintaining this project for the foreseeable future. However, I would welcome anybody that is willing to add other frameworks into the mix, or write scripts for special environments. Obviously, the committers of the different frameworks or commercial vendors are more then welcome.
Peter Kriens
I made a lot of noise on the list because I think we should develop against the OSGi specifications, not against a particular framework implementation. The fact that it runs on framework E does not mean by definition that it is good, even though I know that the developers of framework E are really good.
Now I have been in this industry for 30 years and I still have not learned to sit still and wait until everybody has forgotten the issue. This stupidity obviously resulted the problem ending up in my lap. Today I decided to run the faulty code on frameworks K, E, and F. This is a lot easier to write down than to do it, actually. Each framework has its own peculiar way of starting and it took me quite some time to get the whole setup running. To be able to switch quickly, I used the aQute File Install bundle. This is a simple bundle that watches a directory and installs/uninstall bundles depending on their presence in this folder. As a bonus, it can also manage configuration data in a similar vein. I then created Eclipse launchers for each of the frameworks.
After this hard labor, finding the bug was not too hard. It turned out there was a misinterpretation of the spec that should be easy to fix. However, it was almost a shame to see how much preparation I had to do to be able to run bundles on multiple frameworks. The setup I had created I thought was quite fancy, it was easy to test all kinds of bundles this way.
I therefore decided to use the altruism of Google and use some of their abundant disk space as a Google project. Wonder if all those Google investors know how their money is spent to support open source? Anyway, I created the multi-osgi project. Within Eclipse you can check this out with SVN (you need Subclipse, Subversive, or some other SVN plugin. The URL to the project is http://code.google.com/p/multi-osgi/source.
The project works quite simple. In Eclipse, you can just find an Equinox, Knopflerfish, and Felix launch in the Run/Debug Dialog. From a shell, you can just switch to the Felix, Knopflerfish, or Eclipse directory and do run (in DOS) or ./run.sh in Unix.
Once you got one or more frameworks running, you can easily install the bundles by just dropping them in the
load directory. A good way to find bundles is to go to the OBR site. The input box allows you to type search criteria. For example, you want to install a shell. Just look for "Shell". This gives two bundles from the Apache Felix project. Just click on the download icon and save the file in the multi-osgi/load directory. You should be able to use this shell from the console. Equinox already starts up with a console, so you will get some interaction. Try it!I will keep maintaining this project for the foreseeable future. However, I would welcome anybody that is willing to add other frameworks into the mix, or write scripts for special environments. Obviously, the committers of the different frameworks or commercial vendors are more then welcome.
Peter Kriens
Subscribe to:
Posts (Atom)