Last week (2006/12/08) the OSGi Alliance had a webinar for their members. Last month, Susan Schwarze from marketing had asked me very sweetly to provide a demonstration for this webinar. After some soul searching I had proposed to make a demonstration using the OSGi Framework in a multi language setting. The scenario of the demo was simple: show how to start a framework, get some basic bundles installed, install some applications from the OSGi Bundle Repository (OBR) to show it all works, and then as grande finale, install a PHP interpreter and port a PHP Wiki to an OSGi bundle in real time. And this all must happen in 20 minutes.
The most interesting part of the demo was obviously the PHP part. I always thought that having a PHP interpreter on an OSGi framework would be really cool (However, hard this is to explain to my kids and wife)! With an OSGi based PHP interpreter you can deploy PHP applications as OSGi bundles. This is attractive to companies that today use PHP on remote servers as well as device vendors that want to design their web sites in PHP.
The demo was not the first time I had come up with PHP. I had searched the web for a PHP interpreter written in Java already several times but I had never found one until I discussed this demo with BJ Hargrave. He mentioned that there was an open source project/company doing a Java based PHP interpreter: Quercus. The interpreter is a part in the bigger Resin webserver. With my usual optimism, I though: “How hard can it be to port this interpreter to an OSGi bundle?”
Well, hard.
It started all very hopeful because there was a class Quercus Servlet and the OSGi HttpServer supports servlets very well. Unfortunately, the Quercus Servlet was assuming it ran inside the Resin server, making it impossible to use with another web server. Writing my own servlet was luckily only a few hours. Unfortunately, I could not resist simplifying the Quercus Servlet which added a few unnecessary hours to undo these simplifications when I understood why the complications were there in the first place.
The next problem I ran into was the file system. Normally PHP assumes that all the sources are in the file system and in our case the files had to come from a bundle. Quercus/Resin supported a virtual file system but it took some time to find out how you could provide a new source for files. It turned out you had to write your own Path sub-class and then set the root path to an instance of your type. All other files are then relative to that root. Reading was very easy to program, I just mapped all the read primitives to the resource access in the bundle. However, writing was a lot harder. At first, I decided to allow the bundle that carries the PHP files to specify its URI and a set of writable directories. That is, I architected the following headers:
PHP-Path: /mywiki
PHP-Data config, data, wiki.d
I got this to work but ran into the problem that some PHP programs actually re-write their own source files. Having special writable directories forced me to modify several PHP programs, obviously not a desirable architecture. Then it suddenly hit me (under the shower!) that I could just do the rather standard technique of copy-on-write. So I got rid of the PHP-Data header altogether. A read would now always first look on the file system (actually the bundle’s data directory) and then in the bundle’s resources. A bit tricky was the handling of directories; they should be automatically created when you wrote a file that also resided in the jar. After this (awfully late) insight, it became quite easy and I could use several PHP programs from the net without any modifications. This worked so well that I could show off this model with a real functioning Wiki that was wrapped in a bundle directly from the Internet!
However, I wanted one more splash and I found this splash in the Quercus modules. The Quercus interpreter was designed to be very modular; all its commands were written in separate modules which were loaded at startup. This model was too tantalizing to not use it to show off the dynamic capabilities of the OSGi Framework. Would it not be impressive if you could install a new Quercus Module on the fly as a bundle (Again, an excitement my family rarely shares)? So I decided to create a Service Tracker that listened to Quercus Module services. Those service objects were then added/removed as a module to the Quercus interpreter. I had to write a new remove function inside the interpreter because, like so many programs, it had no concept of modules that could go away.
Quercus Modules could now be written as OSGi bundles. This turned out very simple to do because bnd (a program/plugin that creates bundles from a recipe) supports declarative services as well. I could just create a small bnd file that builds the bundle and indicates what class should be registered under the QuercusModule interface. Like:
Private-Package: com.caucho.quercus.lib.zip
Import-Package: *
Service-Component: com.caucho.quercus.lib.zip.ZipModule;
provide:=com.caucho.quercus.module.QuercusModule
I had decided early on that I did not want to do the demo live. I guess I am becoming chicken at my old age; the chance that something fails is just a bit too high. I therefore used Instant Demo. This application can record everything that happens in a window. You can edit this recording and provide sound as well. Instant Demo then converts the total set of files to a flash file so you can play it in any browser. I must say, using such a program is a humbling experience because every little error requires a restart. It was painful to have to repeat the same actions over and over again just to get it right.
Anyway, after a lot of hard work I finally succeeded in making four short movies about this demo. If you are interested, download this presentation and play it. Let me know if you think we should have more of such demos. Should we also do this for JRuby and run Ruby-On-Rails applications on OSGi Frameworks? Hmm, sounds cool!
Peter Kriens
P.S. The Quercus interpreter is not available because the code is not industrialized. It really worked for a demo (nothing was faked) but it requires more work to harden it for real use. However, I will contact the resin people to see if they like to modularize their webserver. Their web server screams for OSGi technology!
OSGi Blog
Monday, December 11, 2006
Monday, November 27, 2006
Unfaithful thoughts
Say “OSGi” and Java is not very far behind, they have been married since the early days. When we started working on the specifications Java was our obvious partner, and OSGi’s parents were well acquainted which made the marriage quite convenient. We needed a portable and security sensitive environment and we were very smitten with Java. However, the world has moved on since then and many other interesting partners are popping up all the times. People tell me that the adoption of OSGi technology is often hindered by the overhead that is introduced by a Java Virtual Machine and its sometimes obese library (it was very slim when we started!). We should also not forget the enormous amount of legacy code available in a diverse range of languages that many companies are unable to port. So far we had to deny them the wonderful features that OSGi offers like modularization and (remote) management. Why not let our minds wander off a bit and see what options there are?
A very interesting trend at this moment is the efforts of many to port native environments to the Java VM. Last week at Oredev a Ruby expert told me that JRuby (the Java implementation of the interpreter) had a significant better thread model than Ruby itself. JRuby is getting awfully close to implement the full Ruby language, which could create a drive to make JRuby a preferred environment because it can painlessly integrate any efforts written in Java. A similar story can be told about Jython, a Java based implementation of Python.
Even PHP is possible; I am currently preparing a demonstration for the 7 December OSGi members only webinar. We will show standard PHP applications running on an OSGi framework. The PHP interpreter (Quercus, from the Resin web server) is completely written in Java and it was not hard to turn it into a bundle. I just needed an activator that tracks bundles with PHP pages and registers a servlet under the right namespace. It is really cool to see how you can now write a web site in PHP and deploy it to an OSGi framework as a bundle!
The Java VM misses a few instructions to implement dynamically typed languages efficiently, but there are discussions going on to add these instructions to the VM. VM based solutions remain almost all of the advantages of using Java because they use the same programming and security model. Using the Java VM as the common denominator is therefore a very nice model and I expect to see many examples of this model in the coming years.
The VM model still leaves out native applications, and there are some really cool native applications out there. The standard Java solution is JNI, which, well, ehh, sucks; JNI is extremely intrusive and pedantic. Porting a C application to run under a Java VM is a painful exercise. The alternative is to use some communication mechanism, like for example sockets, and communicate over this medium. Doable, but it is hard because it requires addressing mechanisms, marshalling (serializing) to communicate any parameters, and many more chores: see CORBA if you want to know how painful this can be.
One of the best features of the OSGi Framework is the service registry. The service registry really decouples bundles from each other. A bundle that needs to communicate with another bundle never directly addresses that other bundle but uses a shared service to communicate. A service is defined by the API it implements and a set of properties. Services are detected or found dynamically and then bound. The service registry arbitrates between providers and consumers and provides appropriate security and life cycle control.
The most elegant solution to the legacy and native problem is an implementation of the service registry in C. Most languages besides Java support a very easy integration with C libraries. On the PC and on Linux it is quite easy to directly call DLLs or shared libraries from most dynamic languages. Writing a service registry in C that has the same semantics as the OSGi service registry should not be rocket science. Using a fast communication medium like shared memory would minimize any overhead. Key problems are the garbage collection, leasing issues, and marshalling. You could optimize the registry for tightly coupled clusters, though the technology obviously enables more loosely coupled solutions.
There are of course many interesting questions remaining. Should the management of the bundles remain in Java, or should these aspects be moved to a C library as well, allowing full Java free implementations? Obviously Java will be the best supported environment for a long time to come, however, living apart together might not be a bad solution to satisfy the requirements that I am hearing more and more.
Peter Kriens
A very interesting trend at this moment is the efforts of many to port native environments to the Java VM. Last week at Oredev a Ruby expert told me that JRuby (the Java implementation of the interpreter) had a significant better thread model than Ruby itself. JRuby is getting awfully close to implement the full Ruby language, which could create a drive to make JRuby a preferred environment because it can painlessly integrate any efforts written in Java. A similar story can be told about Jython, a Java based implementation of Python.
Even PHP is possible; I am currently preparing a demonstration for the 7 December OSGi members only webinar. We will show standard PHP applications running on an OSGi framework. The PHP interpreter (Quercus, from the Resin web server) is completely written in Java and it was not hard to turn it into a bundle. I just needed an activator that tracks bundles with PHP pages and registers a servlet under the right namespace. It is really cool to see how you can now write a web site in PHP and deploy it to an OSGi framework as a bundle!
The Java VM misses a few instructions to implement dynamically typed languages efficiently, but there are discussions going on to add these instructions to the VM. VM based solutions remain almost all of the advantages of using Java because they use the same programming and security model. Using the Java VM as the common denominator is therefore a very nice model and I expect to see many examples of this model in the coming years.
The VM model still leaves out native applications, and there are some really cool native applications out there. The standard Java solution is JNI, which, well, ehh, sucks; JNI is extremely intrusive and pedantic. Porting a C application to run under a Java VM is a painful exercise. The alternative is to use some communication mechanism, like for example sockets, and communicate over this medium. Doable, but it is hard because it requires addressing mechanisms, marshalling (serializing) to communicate any parameters, and many more chores: see CORBA if you want to know how painful this can be.
One of the best features of the OSGi Framework is the service registry. The service registry really decouples bundles from each other. A bundle that needs to communicate with another bundle never directly addresses that other bundle but uses a shared service to communicate. A service is defined by the API it implements and a set of properties. Services are detected or found dynamically and then bound. The service registry arbitrates between providers and consumers and provides appropriate security and life cycle control.
The most elegant solution to the legacy and native problem is an implementation of the service registry in C. Most languages besides Java support a very easy integration with C libraries. On the PC and on Linux it is quite easy to directly call DLLs or shared libraries from most dynamic languages. Writing a service registry in C that has the same semantics as the OSGi service registry should not be rocket science. Using a fast communication medium like shared memory would minimize any overhead. Key problems are the garbage collection, leasing issues, and marshalling. You could optimize the registry for tightly coupled clusters, though the technology obviously enables more loosely coupled solutions.
There are of course many interesting questions remaining. Should the management of the bundles remain in Java, or should these aspects be moved to a C library as well, allowing full Java free implementations? Obviously Java will be the best supported environment for a long time to come, however, living apart together might not be a bad solution to satisfy the requirements that I am hearing more and more.
Peter Kriens
Friday, November 3, 2006
Domain Specific Languages
Last week I visited the OOPSLA in Portland to promote the OSGi. During that time I also visited many different sessions. Interestingly, only this week I started to see a trend in the conference, strangely enough not while I was there. Trends on OOPSLA are important; this is the conference where objects, patterns, aspects, and many other innovations were first discussed.
The trend is the focus on domain specific languages. We seem to be entering the time when Java is not the one size that fits all we something think it is. Interestingly, one of the key persons in the Java world (Guy Steele) had a presentation about Fortress, a language that is intended to replace Fortran. The syntax is highly optimized for mathematics. A key aspect is that the language should be readable by the domain experts. This means they have operator overloading because there are many mathematical operators that have formalized definitions. They allow the overloading of many Unicode characters, solving one of the problems with C++ where there were too few operators to work with.
However, there were many other presentations that address the language issue. A company called Intentional Software presented an editor that could display and edit the same abstract syntax tree in many different forms. This ranged from a mathematical expressions with all the nice looking formatting, to Java code, and all the way to the graphically display menu structure of a mobile phone. Again, the interesting aspect is that syntax does matter. But there was more. Another presentation told the story of a company that had 400 pages of requirements and domain knowledge. The estimate was that the system was roughly going to be 20.000 pages: a 50x increase in size. The presenter’s solution was to model the domain in a domain specific language and generate the system from the program written in that language. The expectation was that the system could be written in around 4000 pages of code. During the rest of the OOPSLA there were many more Domain Specific Language (DSL) presentations.
You see the shimmers of Domain Specific Languages in things like Ant and Maven. The XML files that parametrize Ant and Maven define a language. Unfortunately, they picked XML, which caused much of the clarity of a domain specific language to be lost in the sea of tags. Many developers have attempted to design domain specific languages using XML; the art of real language design seems lacking in today’s computer science classes. And it is so easy today with javacc, antlr, and other compiler compilers.
What does this mean for the OSGi service platform? The OSGi Service Platform is currently highly Java oriented, do we need other languages?
First there is already a domain specific language for Eclipse plugins called eScript, written as an example for the Eclipse FAQ book. Unfortunately it does not run as is on Eclipse 3.3, but it is an interesting approach. Instead of mucking around with MANIFEST.MF, plugin.xml, resources and Java code, it is all specified in one concise file. I think this approach is promising. An environment like Eclipse contains so much functionality, it would be very nice if one could tap into that functionality with a short script instead of all the work involved today.
I think the OSGi service platform is a great platform to provide the runtime for Domain Specific Languages. The OSGi provides a wonderful plumbing infrastructure for the generators that map the DSL programs. The nice thing about DSLs is that they only specify domain related issues and leave the plumbing and other chores to the environment. This implies that any overhead that the OSGi service platform requires can easily be created by the generators. This will make it very easy for people to write highly productive and specialized code while not being bothers with plumbing.
Look for DSL and you will find more than just modems. After a long time where very few languages were developed, I expect to see many domain languages to see the light in the coming years. I hope the Java community can embrace this opportunity by allowing these languages to run on the Java VMs and not oppose these innovations. Domain Specific Languages could be a great boon for our productivity, and could make the OSGi Service Platform more attractive to a larger audience.
Peter Kriens
The trend is the focus on domain specific languages. We seem to be entering the time when Java is not the one size that fits all we something think it is. Interestingly, one of the key persons in the Java world (Guy Steele) had a presentation about Fortress, a language that is intended to replace Fortran. The syntax is highly optimized for mathematics. A key aspect is that the language should be readable by the domain experts. This means they have operator overloading because there are many mathematical operators that have formalized definitions. They allow the overloading of many Unicode characters, solving one of the problems with C++ where there were too few operators to work with.
However, there were many other presentations that address the language issue. A company called Intentional Software presented an editor that could display and edit the same abstract syntax tree in many different forms. This ranged from a mathematical expressions with all the nice looking formatting, to Java code, and all the way to the graphically display menu structure of a mobile phone. Again, the interesting aspect is that syntax does matter. But there was more. Another presentation told the story of a company that had 400 pages of requirements and domain knowledge. The estimate was that the system was roughly going to be 20.000 pages: a 50x increase in size. The presenter’s solution was to model the domain in a domain specific language and generate the system from the program written in that language. The expectation was that the system could be written in around 4000 pages of code. During the rest of the OOPSLA there were many more Domain Specific Language (DSL) presentations.
You see the shimmers of Domain Specific Languages in things like Ant and Maven. The XML files that parametrize Ant and Maven define a language. Unfortunately, they picked XML, which caused much of the clarity of a domain specific language to be lost in the sea of tags. Many developers have attempted to design domain specific languages using XML; the art of real language design seems lacking in today’s computer science classes. And it is so easy today with javacc, antlr, and other compiler compilers.
What does this mean for the OSGi service platform? The OSGi Service Platform is currently highly Java oriented, do we need other languages?
First there is already a domain specific language for Eclipse plugins called eScript, written as an example for the Eclipse FAQ book. Unfortunately it does not run as is on Eclipse 3.3, but it is an interesting approach. Instead of mucking around with MANIFEST.MF, plugin.xml, resources and Java code, it is all specified in one concise file. I think this approach is promising. An environment like Eclipse contains so much functionality, it would be very nice if one could tap into that functionality with a short script instead of all the work involved today.
I think the OSGi service platform is a great platform to provide the runtime for Domain Specific Languages. The OSGi provides a wonderful plumbing infrastructure for the generators that map the DSL programs. The nice thing about DSLs is that they only specify domain related issues and leave the plumbing and other chores to the environment. This implies that any overhead that the OSGi service platform requires can easily be created by the generators. This will make it very easy for people to write highly productive and specialized code while not being bothers with plumbing.
Look for DSL and you will find more than just modems. After a long time where very few languages were developed, I expect to see many domain languages to see the light in the coming years. I hope the Java community can embrace this opportunity by allowing these languages to run on the Java VMs and not oppose these innovations. Domain Specific Languages could be a great boon for our productivity, and could make the OSGi Service Platform more attractive to a larger audience.
Peter Kriens
Friday, October 20, 2006
OSGi Developer Conference
The OSGi has teamed up with EclipseCon 2007, 5-8 March, Santa Clara California to organize the 2007 OSGi developer conference. This will be the premier conference for OSGi developers to attend in 2007.
We are currently looking for submissions! Long talks, short talks, tutorials, demos, and panels. Please submit a proposal to the Eclipse submissions site in the OSGi track. Proposals will be reviewed by the program committee, where we have a seat.
Some of the submission deadlines are near, please submit your proposal as soon as possible. The submissions must be registered at the EclipseCon Submissions site. Do not forget to register at the OSGi track.
I know there are a large number of interesting projects under way that are heavily using OSGi technologies. If you are working on one of those projects, do not hesitate to submit a proposal, we have quite a few slots.
We have a proposal on the table to add an OSGi user groups meeting at the
end of the conference. We will let you know more soon.
Peter Kriens
We are currently looking for submissions! Long talks, short talks, tutorials, demos, and panels. Please submit a proposal to the Eclipse submissions site in the OSGi track. Proposals will be reviewed by the program committee, where we have a seat.
Some of the submission deadlines are near, please submit your proposal as soon as possible. The submissions must be registered at the EclipseCon Submissions site. Do not forget to register at the OSGi track.
I know there are a large number of interesting projects under way that are heavily using OSGi technologies. If you are working on one of those projects, do not hesitate to submit a proposal, we have quite a few slots.
We have a proposal on the table to add an OSGi user groups meeting at the
end of the conference. We will let you know more soon.
Peter Kriens
Thursday, October 19, 2006
JSR 277 Review
The JSR 277 Java Module System is an important JSR for the OSGi Alliance and me. Though we have been working on Java modularization since 1998 I did not get a chair on the table; the table was already full with 14 seats. Last week, the current 20 man Expert Group has recently dispatched their Early Draft. So this is a personal account of my reading of the Early Draft.
JSR 277 defines a static module system that is similar to the OSGi Require-Bundle approach. Modules define their metadata in the
Module definitions can be obtained from repositories. Repositories contain module definitions that provide metadata to guide the resolving process. Name and version ranges are supported from the metadata. However, an import policy class can be used to do more complex resolving strategies. Repositories can be file or URL based, allowing for remote repositories. Repositories are hierarchically linked; if a repository can not find a module definition, it will delegate to its parent. At the top of the chain is a system repository and above that a boot repository. Module definitions are instantiated before they can be used; this associates a class loader with a module. When all the modules are resolved and instantiated, the main class is called and the program can execute. There is no API for unloading a module.
The ambition level of JSR 277 is significantly lower than where the OSGi specifications are today, even way lower than we set out in 1998. In a way, for me the JSR 277 proposal feels rather toyish. The Expert Group took a simplistic module loading model and ignored many of the lessons that we learned over the past 8 years. No built in consistency, no unloading, no package based sharing. Maybe we wasted a lot of time or solved unimportant problems, but I really do not think so. It would be ok if they had found a much cleaner model, well supported by the VM, but it is clearly not. Their model is based on delegated class loaders, just like the OSGi framework minus 8 years of experience. Ok, I am biased, so let us take a closer look. Though just a warning, the following text is utterly boring unless you are handicapped with a deep interested in Java class loaders. Much of the work over the past 8 years was making bundle developers worry about application functionality and let the OSGi framework worry about class loaders. This stuff should be under the covers. Alas.
The key disappointment in JSR 277 is the lack of dynamics. There is no dynamic loading and unloading of modules/bundles, any change will require a reboot of the VM. The model is geared to the traditional application model of starting an application, running, and then killing the VM. Fortunately, JSR 291 Dynamic Component Support addresses dynamics and more comprehensive module loading, so let’s hope that will become the Java standard.
The most surprising feature of JSR 277 is a total lack of consistency checking. Modules are loaded by name and version only; there is no verification that the graph of class loaders form a consistent class space. Take the example in paragraph 8.3.3.3. There are four modules:
Modules resolve independently, implying that module
A key problem with the Require-Bundle/Module approach is ordering. Though the EDR implies that classes are searched in a specific order, this still depends on what other modules do. Assume we have:
The order for module
Split packages are packages that come partly from one module and partly from another module. Split packages are nasty because the package security access does not work and it is also easy to unexpectedly shadow classes and resources.
Security problems occur with split packages when you load class
Worst of all, none of this is under the control of the importer. The importer just imports a name and version, and then receives whatever the exporter decides to export. Obviously, this is a brittle connection. For example, module
Another problem with modules is their fan out. Typical projects create JARs that are useful in a number of situations. For example, a program like bnd (creates bundle manifest headers from the class files) can be used as an Ant task, an Eclipse plugin, from the command line, and a Maven plugin. It will therefore have to import packages from Ant, Eclipse, and Maven. Those dependencies are needed and are therefore more or less ok (though it is nice to make them optional, a feature missing from JSR 277). However, Ant will import another (large) set of modules. Similarly for maven and Eclipse, ad nauseum. The bnd program is a terrifying example, but the problem is that the usual fan out of a module is large and the fact that dependencies are transitive can worsen this effect. If you want to see the effect of module like imports, check out maven. A simple hello world program can drag in hundreds of modules due to the transitive dependencies. Modules (and Require Bundle) as well are a typical example of creating a constructs to solve a problem but simultaneously creating problems on the next level, problems which are usually ignored in the first version only to bite the early adopters.
A puzzling aspect of 277 is the dependency on super packages from JSR 294. Super packages are shrouded in a veil of mystery. The only public information so far came from Gilad Bracha’s blog. JSR 277 unveils a bit more but many questions are left unanswered. Super packages list all their member classes and a class can only be member of a single module. In 277, it is implied that this module file maps closely to the module metadata. If this is true, than this is a huge constraint. It means that the deployment format is rigidly bound to the development time modules. Each development module must be deployed separately.
I’ve found that managing the packaging is a powerful tool for the deployer. I have written many bundles that mix and match packages from different projects. This flexibility is needed because there are two counter acting forces. On one side you want to simplify deployment by deploying the minimum number of bundles. Anybody that had to chase dependencies knows how annoying the need for more and more bundles. On the other hand, bigger bundles are likely to create more dependencies because they contain more (sometimes unnecessary) code. So you like to minimize the number of external dependencies. Sometimes the best solution is to include the code of other JARs in a deployment JAR. The proposed super packages will put a stop to that model: it will be impossible to manage the deployment packaging because the programmer has decided the packaging a priori; bad idea.
The biggest regret the EG members will have within 2 years is the import policy. Why? Isn’t it a nice idea that the programmer can participate in the resolving? Well, in Java the solution to those problems looks deceptively simple: use a class to abstract the required functionality. An import policy is a class in your module that gets called during the resolving of the module. It can inspect the module metadata, query the repositories and bind other modules.
In the OSGi Alliance, we inherited a similar idea from Java Embedded Server (JES), the archetypical OSGi framework developed by SUN. After countless hours talking and testing we decided that it was a bad idea because of 2 key reasons:
Wasn’t there anything I liked? Well, I like the idea of the repositories. The OSGi framework maintains the repository internally with its installed bundles, it left external repositories outside the specification by standardizing the installation API. Repositories allow the framework to download or find modules just before activation. This is an interesting model, pursued by Maven and the OSGi Alliance with OBR.
However, I think the JSR 277 repository model is too simplistic. It codifies the current maven repository model, which is still immature and will likely change over time. For example, the current model takes only two extra constraints into account: OS, and platform. Unfortunately, life is seriously more complicated. OS’s have their own versioning scheme, processors have compatibility matrices (i.e. an x86 runs on a i586, win32 is compatible with WinXP but not vice versa), the OS is often a variation of OS and window system. Encoding these constraints in the file name is obviously bound to collide with reality one day. In contrast, the OSGi OBR uses a generic requirement-capability model derived from JSR 124 that is much better suited for finding the right module.
Well, the paper document I have reviewed is heavily covered with my marker and I could continue for more pages. However, it is too much for this blog. Well, ok, last complaint: the syntax for version ranges, it is too tempting to leave it alone (though it is not that important). The industry more or less has standardized on versions with a major, micro, minor number and a string qualifier. JSR 277 adds an update number that is between minor and qualifier (Maybe one day someone can explain to me why we need all those number while the only thing that is signaled is a change that is backward compatible or not, but developers seem to like to have all those numbers). However, I have no problem adding another number beneath minor. I do have a problem with a version range syntax that is obtuse and non-standard.
Intervals have a mathematical notation that is easy to understand. Parentheses are non-inclusive and brackets are inclusive. The interval 1 < x <= 5 is therefore represented as (1,5]. [2.3,5.1) indicates any version that is more or equal to 2.0 and less than 5.1. Simple, elegant, has been around since Pythagoras. Choosing this notation for the OSGi specifications was a no-brainer.
Now, let us take a look at what JSR 277 brewed. They use the same terminals as regular expressions, but they have a very different meaning. A partial version can be suffixed with a + (anything later) or a * (anything with the same prefix). So if you say 1+ you mean [1,∞) or 1. 1* is [1,2). The JSR uses square brackets to fix the first positions while floating the last. For example, 1.[2.3+] is [1.2.3, ∞) and 1.[2.3*] is [1.2.3,1.2.4). The JSR 277 syntax has no easy formulation for the not uncommon case [1,5). The only way to express this is with concatenation of version ranges: 1*;2*;3*;4*. Ok, enough about this strange syntax.
Conclusion. JSR 277 takes a simplistic view of the world, ignoring many real life problems: consistency, optionality, split packages, etc. The only way this EG can pass its final review is lack of attention for detail of the Executive Committee or alternatively, serious muscle power of SUN. JSR 277 is a missed opportunity for Java. I do not doubt that this specification will end up in Java 7, but it will further fragment the Java world for no technical reason. Not only is this specification impossible to implement on J2ME any time soon, it will also leave the many OSGi adopters out in the cold. Why?
Peter Kriens
JSR 277 defines a static module system that is similar to the OSGi Require-Bundle approach. Modules define their metadata in the
MODULE-INF/METADATA.module resource. Dependencies are pecified in the import clause with the module’s symbolic name and version range. Other clauses in the module’s metadata list the resources and classes that are visible to other modules. When a JSR 277 application gets started, it should have a main class with a static main method. However, a module must be resolved, as well as its required modules, before the main method is called.Module definitions can be obtained from repositories. Repositories contain module definitions that provide metadata to guide the resolving process. Name and version ranges are supported from the metadata. However, an import policy class can be used to do more complex resolving strategies. Repositories can be file or URL based, allowing for remote repositories. Repositories are hierarchically linked; if a repository can not find a module definition, it will delegate to its parent. At the top of the chain is a system repository and above that a boot repository. Module definitions are instantiated before they can be used; this associates a class loader with a module. When all the modules are resolved and instantiated, the main class is called and the program can execute. There is no API for unloading a module.
The ambition level of JSR 277 is significantly lower than where the OSGi specifications are today, even way lower than we set out in 1998. In a way, for me the JSR 277 proposal feels rather toyish. The Expert Group took a simplistic module loading model and ignored many of the lessons that we learned over the past 8 years. No built in consistency, no unloading, no package based sharing. Maybe we wasted a lot of time or solved unimportant problems, but I really do not think so. It would be ok if they had found a much cleaner model, well supported by the VM, but it is clearly not. Their model is based on delegated class loaders, just like the OSGi framework minus 8 years of experience. Ok, I am biased, so let us take a closer look. Though just a warning, the following text is utterly boring unless you are handicapped with a deep interested in Java class loaders. Much of the work over the past 8 years was making bundle developers worry about application functionality and let the OSGi framework worry about class loaders. This stuff should be under the covers. Alas.
The key disappointment in JSR 277 is the lack of dynamics. There is no dynamic loading and unloading of modules/bundles, any change will require a reboot of the VM. The model is geared to the traditional application model of starting an application, running, and then killing the VM. Fortunately, JSR 291 Dynamic Component Support addresses dynamics and more comprehensive module loading, so let’s hope that will become the Java standard.
The most surprising feature of JSR 277 is a total lack of consistency checking. Modules are loaded by name and version only; there is no verification that the graph of class loaders form a consistent class space. Take the example in paragraph 8.3.3.3. There are four modules:
A, B, C, and D (-> means import).
A -> B v1.0, C v1.0
B -> D v1.1
C -> D v1.0
Modules resolve independently, implying that module
B will resolve to module D v1.1 and module C will resolve to module D v1.0. The result is that module A can see the same class coming from module D v1.0 and module D v1.1, likely resulting in class cast exceptions. Or more concrete, assume module A is your application and module B and module C implement some web framework and module D is javax.servlet 2.1 and 2.4. If you use a Servlet, do you get it through module B or C? Errors resulting from this inconsistency can be very hard to trace. There exists a method in the Module class called deepValidate() that uses a brute force method by loading all the classes in all modules to see if class cast pr class not found exceptions occur. Obviously this is very expensive and not even bound to find most consistency problems, just loading problems!A key problem with the Require-Bundle/Module approach is ordering. Though the EDR implies that classes are searched in a specific order, this still depends on what other modules do. Assume we have:
A -> B, C, D
B -> D
The order for module
A is not B, C, D but instead B, D, C because module B also imports module <D (assuming classes we re-exported). This is one of the reasons I do not like Require-Bundle, though practically we have much less of a problem than JSR 277 will have. The issue is split packages.Split packages are packages that come partly from one module and partly from another module. Split packages are nasty because the package security access does not work and it is also easy to unexpectedly shadow classes and resources.
Security problems occur with split packages when you load class
com.p.A from module A, and com.p.B from module B. The different class loaders will make it impossible for A to see package private methods in B despite the fact they reside in the same package. This is not a theoretical problem especially if multiple versions of the same module are supported. It is easy to pickup a new class from a new version but then load the auxiliary classes from a module that is listed earlier. Worst of all, none of this is under the control of the importer. The importer just imports a name and version, and then receives whatever the exporter decides to export. Obviously, this is a brittle connection. For example, module
A contains packages a and b. One day module A is refactored and package b is moved to module B. This is not uncommon because big modules are not easy to work with, more about that later. Unfortunately, all clients of module A must now be changed to add module B to their import lists even though nothing has changed except the packaging. Why is this not a problem with OSGi imports? Well, OSGi imports packages, in the previous example package b will just be obtained from module B without any change. It might not seem like a big deal, but for large systems this can amount to major work.Another problem with modules is their fan out. Typical projects create JARs that are useful in a number of situations. For example, a program like bnd (creates bundle manifest headers from the class files) can be used as an Ant task, an Eclipse plugin, from the command line, and a Maven plugin. It will therefore have to import packages from Ant, Eclipse, and Maven. Those dependencies are needed and are therefore more or less ok (though it is nice to make them optional, a feature missing from JSR 277). However, Ant will import another (large) set of modules. Similarly for maven and Eclipse, ad nauseum. The bnd program is a terrifying example, but the problem is that the usual fan out of a module is large and the fact that dependencies are transitive can worsen this effect. If you want to see the effect of module like imports, check out maven. A simple hello world program can drag in hundreds of modules due to the transitive dependencies. Modules (and Require Bundle) as well are a typical example of creating a constructs to solve a problem but simultaneously creating problems on the next level, problems which are usually ignored in the first version only to bite the early adopters.
A puzzling aspect of 277 is the dependency on super packages from JSR 294. Super packages are shrouded in a veil of mystery. The only public information so far came from Gilad Bracha’s blog. JSR 277 unveils a bit more but many questions are left unanswered. Super packages list all their member classes and a class can only be member of a single module. In 277, it is implied that this module file maps closely to the module metadata. If this is true, than this is a huge constraint. It means that the deployment format is rigidly bound to the development time modules. Each development module must be deployed separately.
I’ve found that managing the packaging is a powerful tool for the deployer. I have written many bundles that mix and match packages from different projects. This flexibility is needed because there are two counter acting forces. On one side you want to simplify deployment by deploying the minimum number of bundles. Anybody that had to chase dependencies knows how annoying the need for more and more bundles. On the other hand, bigger bundles are likely to create more dependencies because they contain more (sometimes unnecessary) code. So you like to minimize the number of external dependencies. Sometimes the best solution is to include the code of other JARs in a deployment JAR. The proposed super packages will put a stop to that model: it will be impossible to manage the deployment packaging because the programmer has decided the packaging a priori; bad idea.
The biggest regret the EG members will have within 2 years is the import policy. Why? Isn’t it a nice idea that the programmer can participate in the resolving? Well, in Java the solution to those problems looks deceptively simple: use a class to abstract the required functionality. An import policy is a class in your module that gets called during the resolving of the module. It can inspect the module metadata, query the repositories and bind other modules.
In the OSGi Alliance, we inherited a similar idea from Java Embedded Server (JES), the archetypical OSGi framework developed by SUN. After countless hours talking and testing we decided that it was a bad idea because of 2 key reasons:
- By definition, you do not have a valid Java environment before you resolved all the required modules. Code executed in a module that is not resolved is in limbo and is bound to run into problems. There are also several security related issues.
- Having a procedural solution will prevent management systems from predicting what module will resolve to what module. Within the OSGi specifications, we spent countless hours to make the systems predictable for this reason. This is necessary because in the future more and more systems will be managed (I can’t wait for that day!). Inserting a user class in to the resolution process leaves the management system in the blind. Interestingly, Java made the same mistake with security permissions. Abstracting the permissions as a class is good OO practice, but it kills any possible optimization. A declarative approach could have significantly reduced the 20%-30% overhead of Java security while the flexibility that the model offers is rarely used.
Wasn’t there anything I liked? Well, I like the idea of the repositories. The OSGi framework maintains the repository internally with its installed bundles, it left external repositories outside the specification by standardizing the installation API. Repositories allow the framework to download or find modules just before activation. This is an interesting model, pursued by Maven and the OSGi Alliance with OBR.
However, I think the JSR 277 repository model is too simplistic. It codifies the current maven repository model, which is still immature and will likely change over time. For example, the current model takes only two extra constraints into account: OS, and platform. Unfortunately, life is seriously more complicated. OS’s have their own versioning scheme, processors have compatibility matrices (i.e. an x86 runs on a i586, win32 is compatible with WinXP but not vice versa), the OS is often a variation of OS and window system. Encoding these constraints in the file name is obviously bound to collide with reality one day. In contrast, the OSGi OBR uses a generic requirement-capability model derived from JSR 124 that is much better suited for finding the right module.
Well, the paper document I have reviewed is heavily covered with my marker and I could continue for more pages. However, it is too much for this blog. Well, ok, last complaint: the syntax for version ranges, it is too tempting to leave it alone (though it is not that important). The industry more or less has standardized on versions with a major, micro, minor number and a string qualifier. JSR 277 adds an update number that is between minor and qualifier (Maybe one day someone can explain to me why we need all those number while the only thing that is signaled is a change that is backward compatible or not, but developers seem to like to have all those numbers). However, I have no problem adding another number beneath minor. I do have a problem with a version range syntax that is obtuse and non-standard.
Intervals have a mathematical notation that is easy to understand. Parentheses are non-inclusive and brackets are inclusive. The interval 1 < x <= 5 is therefore represented as (1,5]. [2.3,5.1) indicates any version that is more or equal to 2.0 and less than 5.1. Simple, elegant, has been around since Pythagoras. Choosing this notation for the OSGi specifications was a no-brainer.
Now, let us take a look at what JSR 277 brewed. They use the same terminals as regular expressions, but they have a very different meaning. A partial version can be suffixed with a + (anything later) or a * (anything with the same prefix). So if you say 1+ you mean [1,∞) or 1. 1* is [1,2). The JSR uses square brackets to fix the first positions while floating the last. For example, 1.[2.3+] is [1.2.3, ∞) and 1.[2.3*] is [1.2.3,1.2.4). The JSR 277 syntax has no easy formulation for the not uncommon case [1,5). The only way to express this is with concatenation of version ranges: 1*;2*;3*;4*. Ok, enough about this strange syntax.
Conclusion. JSR 277 takes a simplistic view of the world, ignoring many real life problems: consistency, optionality, split packages, etc. The only way this EG can pass its final review is lack of attention for detail of the Executive Committee or alternatively, serious muscle power of SUN. JSR 277 is a missed opportunity for Java. I do not doubt that this specification will end up in Java 7, but it will further fragment the Java world for no technical reason. Not only is this specification impossible to implement on J2ME any time soon, it will also leave the many OSGi adopters out in the cold. Why?
Peter Kriens
Friday, September 29, 2006
Alternative OSGi Styles
When you spent most of your daily life worrying about extremely detailed Java class loader peculiarities then you tend to start thinking that what you’re doing is really important. This feeling is amplified when you see all those important people taking it really serious. And then you suddenly get confronted by a group of people that completely violate our specification but still do something very interesting.
I am talking about osxa. They looked at our specification and decided that the goal was lofty but that it contained too much unnecessary cruft for them. They just took the interesting 80% (well for them interesting) and ignored the rest. I almost took it personal, but they just put all bundles on the class path! Ok, it meant that they did not need class loaders anymore and that opened a larger range of deployment scenarios. For example, they can deploy their applications in a JNLP web start without the need for signing.
Obviously this implies they have no proper isolation between bundles; bundles can conflict in their packages. However, this is isolation is not always needed, especially when the set of bundles that gets deployed together is fixed. These conflicts can then be resolved ahead of time.
And this is the different, surprising, perspective that they took. They love the service registry and do not care about the module layer. They needed a model where they could use many different components, bunch them up together, and deploy them as a whole in a JNLP deployment or inside web app server. What me worry about bundle life cycles?
Actually, when I realized how much they liked the service registry, I could not stop myself from warming up to them. I am often told that the real important stuff of the OSGi is the module layer. The service registry is just a hanger on. In JSR 291 we actually tried to remove the service registry to simplify the specification. We did not because the module layer and the service registry were very hard to separate. However, I always liked the services most; for me, the module layer was the necessity but the services were interesting.
Obviously the osxa people trample over our carefully crafted specifications. However, I think their perspective is interesting. I think it demonstrates the power of the specification that you can write bundles that will run in a compliant framework but also in something that is wildly different. I have always felt that bundles are the important parts; as long as bundles can write against a good specification, then framework developers should be allowed a lot of leeway. It is interesting to see how they use quite a few equinox bundles.
Anyway, the osxa will not receive compliance anytime soon because they completely discard the module layer and take a lot of liberty with other aspects. They can therefore obviously not call themselves an OSGi framework implementation and developers should be very careful not to target any of the peculiarities of this framework. Still it is interesting to see that the success of the OSGi specifications is enabling these initiatives. Initiatives that can have value in certain circumstances and clearly prove the viability of the OSGi model.
Now, if they only could get their demo working again …
Peter Kriens
I am talking about osxa. They looked at our specification and decided that the goal was lofty but that it contained too much unnecessary cruft for them. They just took the interesting 80% (well for them interesting) and ignored the rest. I almost took it personal, but they just put all bundles on the class path! Ok, it meant that they did not need class loaders anymore and that opened a larger range of deployment scenarios. For example, they can deploy their applications in a JNLP web start without the need for signing.
Obviously this implies they have no proper isolation between bundles; bundles can conflict in their packages. However, this is isolation is not always needed, especially when the set of bundles that gets deployed together is fixed. These conflicts can then be resolved ahead of time.
And this is the different, surprising, perspective that they took. They love the service registry and do not care about the module layer. They needed a model where they could use many different components, bunch them up together, and deploy them as a whole in a JNLP deployment or inside web app server. What me worry about bundle life cycles?
Actually, when I realized how much they liked the service registry, I could not stop myself from warming up to them. I am often told that the real important stuff of the OSGi is the module layer. The service registry is just a hanger on. In JSR 291 we actually tried to remove the service registry to simplify the specification. We did not because the module layer and the service registry were very hard to separate. However, I always liked the services most; for me, the module layer was the necessity but the services were interesting.
Obviously the osxa people trample over our carefully crafted specifications. However, I think their perspective is interesting. I think it demonstrates the power of the specification that you can write bundles that will run in a compliant framework but also in something that is wildly different. I have always felt that bundles are the important parts; as long as bundles can write against a good specification, then framework developers should be allowed a lot of leeway. It is interesting to see how they use quite a few equinox bundles.
Anyway, the osxa will not receive compliance anytime soon because they completely discard the module layer and take a lot of liberty with other aspects. They can therefore obviously not call themselves an OSGi framework implementation and developers should be very careful not to target any of the peculiarities of this framework. Still it is interesting to see that the success of the OSGi specifications is enabling these initiatives. Initiatives that can have value in certain circumstances and clearly prove the viability of the OSGi model.
Now, if they only could get their demo working again …
Peter Kriens
Tuesday, September 26, 2006
OSGi UIs and the Web
The OSGi user interface has been a problematic aspect from the beginning, despite the fact that there are so many to chose from: awt, swt, lcdui, swing, thinlet, flash, svg, p3ml, proprietary, and html based. We tried many, many times to come up with a model that all could live with, so far to no avail.
However, one model that seems to emerge is web based UIs. The advent of DHTML, Javascript, and SVG provide an extremely rich environment for application development. Unfortunately, Javascript is not a nice language to handle large and complex problems. Many companies solve this by using webservices to distribute the work between the graphic user interface (GUI) and a server. Web services are remote procedure calls done over the HTTP(S) protocol. This model is reasonably well supported by Javascript using the XMLHttpRequest object.
Th web services model maps very well to an OSGi service platform. In a way, you just need a way to easily call the services in the OSGi service registry from Javascript. So how would such an application look like?
Let us first layout the bundles I like to think in bundles because it is nicely concrete and down to earth. We obviously need a web server, which is easy to fulfill with the OSGi Http Service. We then require a bundle to handle the grunts of the remote procedure calling and another bundle to prove that it works.
The rpc calling bundle has the responsibility to handle the remote procedure calls from Javascript and map them to calls on an OSGi service. I’ve called this bundle webrpc. The second bundle is the demo application. This summer I emotionally blackmailed my son Thomas to learn how to write a program; he caved in and wrote a working Sudoku game in Java after a false start in C++. This exercise gave me more experience with the Sudoku domain than I ever wanted to have so the demo will be a sudoku bundle.
The webrpc bundle should not indiscriminately export services. There are obviously security issues but also not all services are well suited to be exported. However, we’d like to keep the service as simple as possible. The choice was made to look for the public property on the service registration. This is easy to set with declarative services and it means we can use a POJO as our service; the service itself is not involved with the remote procedure calling. The value of the property is the name under which the service should be known to the outside world.
I could have used a complete web services stack, there are an amazing amount of those applications available, almost all written in Java. However, I wanted to run the demo on my Nokia E70 phone that runs an OSGi service platform. Memory is only abundant on PCs and services, so a complete web services stack can add up. It also adds up on the Javascript side because a compliant web services stack is non-trivial. I therefore decided to optimize space and implement a very lite model of RPC over HTTP. Requests are done using the HTTP protocol. The path provides the service and its method, while the query parameters provide the arguments. I used 0 for the first argument, 1 for the next, and so on. For example:
http://localhost/rpc/sudoku/newGame?0=simple
This request will invoke the newGame method on the service that is registered with the property public set to “sudoku”.
The webrpc bundle depends on the Http Service. When there is an Http Service present, it registers itself under the /rpc alias. The servlet will get called for all requests that start with /rpc. For such a request, the webrpc bundle looks up the service name (in the example sudoku) in the service registry using a simple query string with the OSGi filter language to do this. The next part is calling the right method in this service; this is non-trivial because it is necessary to coerce all the arguments from strings into the method argument types. I wrote a separate Invoker class to handle this coercion. Read it and weep. Most of the work is handling Java’s silly primitives.
When the method has been called it returns an Object. This object must be shipped back to the Javascript in the browser. Now the gut reaction of most programmers is to think XML. Not me this time, there is another standard called JSON that is so much easier for Javascript. It is a representation of strings, numbers, arrays, and maps that is compatible with the Javascript syntax. Converting an object into JSON is quite straightforward for values like numbers, strings, maps, and arrays; references to objects are a bit harder but that is a nice subject for another article. The JSON value is then returned as the body of the HTTP response. The Javascript will receive this text body and compile it to Javascript arrays, values, and dictionaries. From the Javascript point of view it can not be simpler.
The webrpc bundle maps requests that do not look like a method name to a getResource() call on the service object’s class. A service implementer can place all its web resources in the www directory and they are automatically mapped. That is, if someone requests /rpc/sudoku/prototype.js, it will load the resource from aQute/sudoko/www/prototype.js. Assuming the implementation class is in the aQute.sudoku package.
All this effort in the webrpc bundle has makes the sudoku bundle almost trivial. Well, I guess it was trivial in the first place because the server only calculates the board and this is not rocket science. The Soduko service only implements a newGame method. This method returns an array of 81 integers (remember how 9x9 was?). Negative integers mark positions that can be shown from the beginning; positive integers are to be guessed by the player. For the HTML side I decided to make a splash screen in SVG. Hey! We now have those fantastic tools, so let us use them. Though splash screen sounds more dramatic than it turned out, I admit, I am a lousy artist.
For the handling of the HTML and http requests calls I am using prototype.js. The Javascript and HTML is actually quite small. The Sudoku board is completely drawn in HTML and formatted using XML. When the Javascript starts up, it requests a new game from the Sodoku service. When this information (asynchronously) arrives, it formats the board with the received information. Rather cool in this model is the use of Cascading Style Sheets (CSS). CSS makes the mundane detail of formatting all the little details nicely manageable; however, remember I am not an artist so the looks can be improved.
Both the webrpc and sudoku bundles used declarative services. The webrpc bundle was hard to test standalone because it relies heavily on the OSGi service registry. The Sudoku service was however a POJO and could therefore easily be tested.
Try it out! If you think it looks too big, just make the window smaller. Notice how nicely it all scales because CSS and SVG are used to adjust all parameters to the window size.
You can download the bundles from the OSGi Bundle Repository (OBR). The JAR’s contain the source code of the bundles in the OPT-INF/src directory. See how extremely little code is required to achieve the goals. Enjoy!
Peter Kriens
However, one model that seems to emerge is web based UIs. The advent of DHTML, Javascript, and SVG provide an extremely rich environment for application development. Unfortunately, Javascript is not a nice language to handle large and complex problems. Many companies solve this by using webservices to distribute the work between the graphic user interface (GUI) and a server. Web services are remote procedure calls done over the HTTP(S) protocol. This model is reasonably well supported by Javascript using the XMLHttpRequest object.
Th web services model maps very well to an OSGi service platform. In a way, you just need a way to easily call the services in the OSGi service registry from Javascript. So how would such an application look like?
Let us first layout the bundles I like to think in bundles because it is nicely concrete and down to earth. We obviously need a web server, which is easy to fulfill with the OSGi Http Service. We then require a bundle to handle the grunts of the remote procedure calling and another bundle to prove that it works.
The rpc calling bundle has the responsibility to handle the remote procedure calls from Javascript and map them to calls on an OSGi service. I’ve called this bundle webrpc. The second bundle is the demo application. This summer I emotionally blackmailed my son Thomas to learn how to write a program; he caved in and wrote a working Sudoku game in Java after a false start in C++. This exercise gave me more experience with the Sudoku domain than I ever wanted to have so the demo will be a sudoku bundle.
The webrpc bundle should not indiscriminately export services. There are obviously security issues but also not all services are well suited to be exported. However, we’d like to keep the service as simple as possible. The choice was made to look for the public property on the service registration. This is easy to set with declarative services and it means we can use a POJO as our service; the service itself is not involved with the remote procedure calling. The value of the property is the name under which the service should be known to the outside world.
I could have used a complete web services stack, there are an amazing amount of those applications available, almost all written in Java. However, I wanted to run the demo on my Nokia E70 phone that runs an OSGi service platform. Memory is only abundant on PCs and services, so a complete web services stack can add up. It also adds up on the Javascript side because a compliant web services stack is non-trivial. I therefore decided to optimize space and implement a very lite model of RPC over HTTP. Requests are done using the HTTP protocol. The path provides the service and its method, while the query parameters provide the arguments. I used 0 for the first argument, 1 for the next, and so on. For example:
http://localhost/rpc/sudoku/newGame?0=simple
This request will invoke the newGame method on the service that is registered with the property public set to “sudoku”.
The webrpc bundle depends on the Http Service. When there is an Http Service present, it registers itself under the /rpc alias. The servlet will get called for all requests that start with /rpc. For such a request, the webrpc bundle looks up the service name (in the example sudoku) in the service registry using a simple query string with the OSGi filter language to do this. The next part is calling the right method in this service; this is non-trivial because it is necessary to coerce all the arguments from strings into the method argument types. I wrote a separate Invoker class to handle this coercion. Read it and weep. Most of the work is handling Java’s silly primitives.
When the method has been called it returns an Object. This object must be shipped back to the Javascript in the browser. Now the gut reaction of most programmers is to think XML. Not me this time, there is another standard called JSON that is so much easier for Javascript. It is a representation of strings, numbers, arrays, and maps that is compatible with the Javascript syntax. Converting an object into JSON is quite straightforward for values like numbers, strings, maps, and arrays; references to objects are a bit harder but that is a nice subject for another article. The JSON value is then returned as the body of the HTTP response. The Javascript will receive this text body and compile it to Javascript arrays, values, and dictionaries. From the Javascript point of view it can not be simpler.
The webrpc bundle maps requests that do not look like a method name to a getResource() call on the service object’s class. A service implementer can place all its web resources in the www directory and they are automatically mapped. That is, if someone requests /rpc/sudoku/prototype.js, it will load the resource from aQute/sudoko/www/prototype.js. Assuming the implementation class is in the aQute.sudoku package.
All this effort in the webrpc bundle has makes the sudoku bundle almost trivial. Well, I guess it was trivial in the first place because the server only calculates the board and this is not rocket science. The Soduko service only implements a newGame method. This method returns an array of 81 integers (remember how 9x9 was?). Negative integers mark positions that can be shown from the beginning; positive integers are to be guessed by the player. For the HTML side I decided to make a splash screen in SVG. Hey! We now have those fantastic tools, so let us use them. Though splash screen sounds more dramatic than it turned out, I admit, I am a lousy artist.
For the handling of the HTML and http requests calls I am using prototype.js. The Javascript and HTML is actually quite small. The Sudoku board is completely drawn in HTML and formatted using XML. When the Javascript starts up, it requests a new game from the Sodoku service. When this information (asynchronously) arrives, it formats the board with the received information. Rather cool in this model is the use of Cascading Style Sheets (CSS). CSS makes the mundane detail of formatting all the little details nicely manageable; however, remember I am not an artist so the looks can be improved.
Both the webrpc and sudoku bundles used declarative services. The webrpc bundle was hard to test standalone because it relies heavily on the OSGi service registry. The Sudoku service was however a POJO and could therefore easily be tested.
Try it out! If you think it looks too big, just make the window smaller. Notice how nicely it all scales because CSS and SVG are used to adjust all parameters to the window size.
You can download the bundles from the OSGi Bundle Repository (OBR). The JAR’s contain the source code of the bundles in the OPT-INF/src directory. See how extremely little code is required to achieve the goals. Enjoy!
Peter Kriens
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