The OSGi Extender Model

Installing and upgrading software is a complicated process that is very error prone in today’s operating system. In Windows, there is a whole industry for installers that take (some of) the pain out of installing a windows application. In Linux RPM package management is famous.

These installers are necessary because applications in operating systems touch many other subsystems. For example, to install an address book application in Linux, it is necessary to have at least the following things done:

• Testing if the dependencies are fulfilled
• Setup an SQL database and tables in MySQL
• Place the executables and files in an appropriate place
• Link the documentation and servlets to an Apache web server
• Link to the commands from the application from the appropriate bin directory
• Setup the permissions
• Etc.

An RPM script for the GNU Indent program looks like:

Name: indent
Version: 2.2.6
Release: 1
Source0: %{name}-%{version}.tar.gz
License: GPL
Group: Development/Tools
%description
The GNU indent program reformats C code to any of a
variety of formatting standards, or you can define your
own.
%setup
-q
%build
./configure
make install
%defattr(-,root,root)
/usr/local/bin/indent
%doc /usr/local/info/indent.info
%doc
%attr(0444,root,root) /usr/local/man/man1/indent.1
%doc COPYING AUTHORS README NEWS

Notice how many hard coded details are specified in this script. Scripts like these are highly sensitive to small changes in the deployment environment.

The model is that the application developer/deployer has to know it all. There is a good reason why installation scripts so often fail: coupling. Installation scripts require deep knowledge of the structure and setup of the target system. The installation script writer is one of the few persons that has to understand all aspects of the application to configure and initialize the environment. Uninstallation is even harder because it is not easy to know what files are created by the applications that should be removed. It is the unfortunate truth that after a year or two, my Windows operating system needs to be reformatted to get rid of the applications that are no longer removable because their installation script fails, a simple missing file can already cause such a script/installer to fail.

Installation scripts are so hard, that one day a friend of mine promised to eat the installation floppy (yes this was a long time ago) if it failed one more time. Alas, we served the floppy with a fresh salad to make it easier to digest.

Now once there was an operating system that did not require installation scripts at all: The Macintosh. When I moved form the Mac to the PC I was utterly confused with installers. On a Mac, installation means dragging a file to any folder on the Mac. That’s it. All OS related installation is performed automatically when an executable is copied, renamed, or deleted. The reason that this can magically (for PC users) work is because a Mac executable contained a simple database with installation information (the so called resources). When the executable is copied, the MacOS is called with a handle to the executable. Associations, icons, help, and many more resources are stored in this flexible database. Moving the file to another location? No problem, this is handled by the OS. Uninstalling? Simple, just delete the file.

With hindsight, it was obvious that my first week on the PC would be a disaster. After installing a number of applications, I decided to rearrange the folder structure, something I often did on the Mac without thinking about. Yes I got this pesky warning but what is a registry??? … A reformat was the only solution in the end.

Let us inspect the differences between the Mac and PC/Unix a bit deeper. An example of information that needs to be installed on both the Mac and the PC are the file associations. When a text file is double clicked, an editor needs to be opened, when an html file is clicked, we want to see the browser. In the PC this association is made by the file extension of the file. For example: .html, .txt, .doc. The association extension-to-application is made in the Windows Registry (Yes, I am now thoroughly familiar with it). On the Mac, the association is stored in the desktop database and copied from the resources of the executable.

It is interesting to compare the two strategies to set this information:

The Mac model has many advantages and few drawbacks. The biggest drawback is maybe psychological, the programmers feel less in control because so little installation work is left and delegated to other subsystems (job security!). Subsystems they do not have direct control over. However, this lack of control is maybe one of its greatest assets in my opinion. Over the years, the Macintosh was able to get more and more functionality that was immediately applicable to old applications. For example, the desktop database could be totally restructured because normal applications had no knowledge whatsoever about this database. Restructuring the Windows registry … eh, well that would be a major undertaking because any useful application on Windows has intricate knowledge of large parts of the registry.

Nowadays, many Mac applications also have installers because they have become dependent on many other components that are installed on the system. E.g. many applications use MySQL, a web server, and other components. In today’s operating systems, MySQL needs to be configured and initialized for a new application that uses it.

Looking at the advantages of the declarative model that was used in the Mac for file associations, would it not be nice if the other subsystems could work similarly?

For example, if the MySQL component could see that a new application was installed, it could look in the application resources and see if there was installation information for this MySQL component. If so, it could use this information to setup the tables and permissions. If the file was standardized (lets say standard SQL), it might even be picked up by another database like Progress, DB2 or Oracle, without the application being aware of this. Updates and installation could work in similar vein if the subsystem (in this case the database subsystem) could get notifications of updates and uninstallations.

An interesting aspect of this extendeer model is the robustness. Quite often, applications are uninstalled because they have become unreliable; their state is messed up. Relying on scripts that run in the context of this messed up state is not good, often creating a larger mess by not finishing and leaving expensive resources lingering on the system. If a messed up application gets uninstalled, the chance that the subsystems can clean up for the application is much higher because they are more likely to be in a healthy state.

If life could only be so good … But it can! This is exactly that the OSGi Alliance enables and promotes. Meet the Synchronous Bundle Listener.

Synchronous Bundle Listener

The Life Cycle layer in the OSGi Framework allows bundles to listen to installations, updates and uninstallations of other bundles. A normal Bundle Listener will handle those events asynchronous. That is, when you get the event, the cause has moved on and has not waited for you to look at it. The Synchronous Bundle Listener works differently, it will handle the same event, you guessed it, synchronously. With the Synchronous Bundle Listener you get a chance to do your thing before the cause of the event has passed. This allows the subsystem to perform its initialization before the application gets control.

In the Synchronous Bundle Listener event call back, the subsystem can look at the resources of the bundle and check if there is anything of its liking. If it is, it can use this resource to perform the necessary initialization for the new bundles.

This is all rather abstract so let us work out a simple example.

A Servlet Helper

If you write a servlet for an OSGi service platform than you must get the Http Service and register your servlet with it. This is boilerplate code, not very interesting. An extender could simplify this model by removing all the cruft for the servlet programmer. Whenever a bundle gets started, the servlet extender would inspect a header in the bundle about to be started. If this header was set, it would load the servlet class from the bundle and register it with the Http Service.

This model is easily programmed, you can find an example on the snippets site. This snippet discusses some of the implementation issues around extenders.

Issues

No model is perfect and obviously the extender model is no exception. There are a number of issues that may impact the suitability.

No Access to private area of bundle

Extenders do not have access to the private area of a bundle. This area is reserved for the bundle itself and should not be touched by other bundles. Any party that could access this area makes it impossible for a bundle to reason about its private data area. For example, locking is not necessary in the private area to prevent access from other parties.
If a bundle needs its private data area initialized, it can do this lazily in the start method. If it is necessary to initialize this data after install, the extender method is not suitable.

Permissions

Extenders run with their own permissions. An extender should therefore be careful not to perform operations on behalf of the bundle that the bundle would have no authority for. For example, a malicious bundle should not be able to wreak havoc because the extenders did things on its behalf without checking.

Then again, there is an advantage that an extender could do more privileged operations than the bundle itself is allowed. For example, an extender could create a database for a bundle without the bundle having permission to create it (obviously it requires permission to use it).

Extenders could scope the operations they have by using Permission Admin to get the permissions of the bundle and using a doPrivileged call with those permissions on the stack. For example, an extender could get the permissions of the signer and scope all actions with these permissions.

This issue needs to be analyzed in detail for each type of extender. In most cases a solution can be found but in certain cases extenders are not suitable.

Hanging Bundle Listeners

Synchronous Bundle Listeners are, well, synchronous. They are called in the middle of an installation process. If they do not return, the installation cannot be finished. Good frameworks will handle hanging synchronous bundle listeners, but it is not good when it happens.

Extenders are likely to be system components and should therefore have extra quality requirements. For this reason, a synchronous bundle listener must have AdminPermission[bundle,LISTENER]. The model is therefore less suitable when the initialization is done by untrusted components.

Conclusions

The life cycle events of the OSGi Framework enable a much more friendly installation model in comparison to traditional scripts and installers. By delegating application setup and initialization to the subsystem applications will require less, highly volatile, knowledge of the deployment platform. Further, subsystems have more possibilities to clean up and adapt to changes in the applications.

The single most important drawback is that the model is less familiar to Windows and Unix (but not Mac) programmers. In these environments, the lack of life cycle events and heavy reliance on the file system structure makes the described model impossible. This lack of familiarity by the programmer makes the OSGi model look like there is no control, a feeling many programmers distinctly dislike.

However, the OSGi is a component model where it is impossible to know all the life cycle events. By delegating the installation task to the different subsystems, the overall system becomes significantly more robust, reliable, future proof, and simpler to maintain.

Peter Kriens

OSGi Activities in the next Month

First some housekeeping. Next week 1 and 2 March 2007 we will have a meeting Cologne about end-2-end solutions related to OSGi technologies. This includes remote management and anything else that is necessary to run large scale OSGi installations. This meeting is open to members and non-members because we see this as a requirements gathering meeting. More information can be found on the meeting page.

As if this was not enough, we also will run a Vehicle requirements gathering meeting in Eindhoven at Siemens VDO. This is a successor to the successful meeting in Troy at Delphi. You can read more about this at the meeting page.

You can contact Kai Hackbarth if you need an invitation.

And last, but absolutely not least ... You are coming to the OSGi Developer Conference (sometimes also called EclipseCon 2007)? The program is fantastic! Really hoping to see many of the readers there in two weeks. We have a BOF at 19.30 Tuesday in the theater, do not miss it! The OSGi Alliance will provide drinks. There is still time to register, and OSGi members still get a discount.

Peter Kriens

OSGi Extenders

Tidbits

I wanted to blog about the OSGi Enterprise Expert Group but Eric Newcomer (IONA), one from the chairs beat me. I do not think I can improve his report, so please check it out. I fully agree with Eric that it was an exciting and energetic day with a lot of hope for the future (and work). If you want to participate in this work, you will have to become an OSGi member, but believe me, it is definitely going to be worth it!

I just finished working on the final details OSGi DevCon/EclipseCon . It really is becoming an amazing conference. We had many more submissions than we could select, and almost all submissions were of very high quality. You still get an early bird discount for the next two weeks, so sign up quickly. I sure hope to meet many of my blog readers.

Ok, one thing technical. Jon Bostrom showed me the N800, which is an improved version of the N770. This is an OSGi dream come true machine. For some reason, it does not include a GSM chip, but that could make the N800 an interesting companion with the E70 that I have, which contains also an OSGi framework. The devices could communicate over wifi or bluetooth to exchange data and code. Jon promised me an N800 once the channels are filled, once I get one I will report about my experiences. So many interesting areas, so little time ...

Peter Kriens

Can Someone Tell Steve Jobs About OSGi?

Steve Jobs in an interview with Jon Markoff: “Java’s not worth building in. Nobody uses Java anymore. It’s this big heavyweight ball and chain.”. Mr. Jobs is known to use the hyperbole, but I am afraid he has a point. I remember having a fiery discussion with Mr. Smalltalk, Ralph Johnson, in ’97 about how nice Java was. One of my key arguments was that Java was so lean and mean in comparison to Smalltalk. He shook his wise head and told me to wait and see. Oh boy, how right he was. Since then Java’s library growth has consistently beaten the growth of even the best hedge fund. Time to reflect and refactor are words one can not easily associate with the Java world. The JCP has given us a hodgepodge of inconsistent APIs split between worlds that seem to diverge farther every day.

Yes, I know. Four billion phones or more run Java, generating billions of dollars of revenue. However, everybody in family has a Java phone and non uses Java applications. When you look at what people run on their telephone then it is mostly games. Where are the killer apps? These apps would appear in the market when the operators and manufacturers would allow third parties to access all the resources. Of course, JSR 232 provides a very interesting architecture for interesting applications that will enable these types of applications, but phones are only just now appearing in the market.

Sadly, Java largely failed on the desktop. How many pleasant applications do you use that are written in Java? Eclipse is still the exception. Java seems promise so much for the embedded market but it is still a hard to sell Java in a consumer device due to the extra cost of the VM and increase in resource consumption. Java only shines in the server market, where resources are plenty and inefficiency is a minor inconvenience.

Why is the situation so bleak? The “Write Once, Run Anywhere” mantra (WORA) of Java is highly attractive. The software industry spends countless billions every year porting applications from one environment to the other. Application developers limit themselves to fewer markets because the cost to develop for additional platforms is just not worth it. There have been significant problems in the past with the WORA (write once, debug anywhere israther tired joked repeated ad nauseum by Microsoft) the joke), but today it works amazingly well across platforms, note again the Eclipse success story.

Personally, I think the biggest problem is that the Java architecture is not suited to what it desires to be: a ubiquitous computing environment. A key aspect of a ubiquitous computing environment is that it should be lean and mean at the core. Few want to carry the dead weight of someone else’s requirements. The key mistake Java/JCP made was splitting Java up in different domains: JavaCard, J2ME, JSE, and JEE. Then in different configurations: CLDC and CDC. Then further refined with profiles like Foundation, Personal, Basic, etc. The intentions were good; the result is a disastrous fragmentation.

Lean and mean means you need to be able to mix and match. One of the most interesting Java projects at this moment is therefore Apache Harmony. Harmony is an open source implementation of the Java SE VM and class library. Instead of taking a short-cut, cloning the Sun VM, they use a novel architecture of modularizing the Java class library. With Apache Harmony, you can start really small and add what is needed. Such an architecture permits the use of the same basic environment across industry and platforms. Developers no longer target a specific pre-defined profile but only express their requirements on the platform. When the application gets deployed and certain packages are missing, the platform can be dynamically extend the platform by adding the missing packages.

Modularization creates a market for solutions. It is no longer required to come up with a catch-all API in the JCP, which is too often designed under political pressure. For example, J2EE would never have been a success in a free market. It received its life because it was the “ordained” API for enterprise applications by virtue of its JCP origin. It was not until an external party like Interface21 showed with Spring that Enterprise applications can actually be lean and mean as well.

I think that Java is too big and too important to our industry to be shepherded by a single company like Sun. By trying to shield the environment from fragmenting they sadly have achieved the opposite. By exerting too much control they have enabled some pretty bad APIs. Why not set firm ground rules, that is a small VM and rules for modularization, then let different parties compete with solutions. The market will quickly pick the best practices and create de-facto as well as de-jura standards.

Time to reflect. What do Eclipse, Apache Harmony, and Spring, actually have in common? Hmm, maybe Steve Jobs should invite an OSGi evangelist to give a presentation?

Peter Kriens

P.S. You did register for EclipseCon I hope?

OSGi Vehicle Workshop in Detroit

Last Thursday (11/01/2006) we met for an automotive workshop at the Delphi headquarters in Troy with participants from Intel, Wind River Systems, Delphi, Renesas, Vetronix, Booz Allen, Ford, GM, Aonix, Siemens VDO, ProSyst, and Telcordia. This workshop was initiated by Kai Hackbarth (ProSyst). Kai is working with a number of companies in the VII project and noticed that there is a lot of interest in OSGi from the participants in this project.

The VII project has a very ambitious goal: it wants to reduce the number of deaths on the American roads and decrease congestion (this is similar to the CVIS project in Europe). To achieve this goal, the American Department of Transport (DOT) wants to build an extensive communication infrastructure that supports vehicle to vehicle communication as well as vehicle to centers communications based on the DSRC communications service. Vehicles will get an on board unit and the roads will get a roadside unit that manages the communication of nearby vehicles. This infrastructure can be used in many different ways. Some examples:

• Vehicles will transmit roadside information to central management centers which will be digested and dispatched as traffic information.


• When a vehicle breaks, it can send out this information to other vehicles


• Stop lights can send their state to oncoming vehicles

The number of scenarios that can be supported is of course only limited by your imagination. It is therefore that OSGi plays an important role in this project. The life cycle of a car is around 10 years while the life cycle of these types of services is significantly smaller. It is therefore obvious that some form of software downloading and management is required. This is of course the archetypical application area of the OSGi specifications. Maybe the most interesting aspect is that the on board equipment (OBE) can be used for other, commercial, applications as well. This is not a primary goal, but it enables a lot of possibilities!

We had modeled the meeting after the successful Enterprise meeting in August 2006. This meeting resulted in the OSGi Enterprise Expert Group, which will meet in Dublin at the end of this month. The format is simple; some of the participants present their ideas in a 5 minute presentation. This kicks off a discussion where we gather all the different areas that could be relevant for OSGi standardization. This list is then prioritized and further discussed.

We found the following areas of highest interest:

Resource management and hard real time issues
The traditional embedded concerns of resource management and hard real time issues came far ahead of all other items. There exists an OSGi RFC addressing some of these concerns but the focus on server side and desktop applications have moved these concerns to the background. It seems clear that this should be a number one concern for the vehicle group. Interestingly, Aonix presented a very interesting idea. They had create a VM where you could run hard real time applications written in a subset and annotated Java. The code would run in a special memory space that did not require garbage collection. This concept meshes obviously very nicely with the OSGi model of deployment. Special metadata could be used to indicate that specific classes should run on the hard real-time VM.

Vehicle Interface
A vehicle interface provides access to the vehicle configuration and status. There has been work in GST, AMI-C, and the OSGi VEG in this area. Both GST and OSGi use the OSGi Device Management based on OMA DM as the foundation. The complex aspect of the vehicle interface is to model the car. How many chairs, how many axis has each chair, how many windows, where are the seats located. Such an ontology requires vetting by a lot of knowledgeable parties.

HMI
Human-Machine Interface. We have been talking about the HMI since day one but we have never been able to standardize a single HMI. There are developments that seem to make the development of a generic HMI more feasible. Normally, the car manufacturers regard the look and feel of the vehicle as their prerogative but it is becoming clearer that the largest number of applications requires some form of a HMI. Therefore, an OSGi standard should address some way that applications can interact with a user while at the same time allowing as much as possible freedom to the car manufacturer.

Non-Java code integration
The demand to expand from Java to native code becomes larger and larger. In the embedded world it is rarely that there are green-field applications. Embedded systems must work with legacy code that is not easy ported to a Java environment. From the participants it was clear that we need a story how to integrate non-java environments with OSGi. As I discussed elsewhere, I strongly believe that our service registry is a very good mechanism to let different subsystems communicate, regardless of the language that is spoken.

Other
There were several other areas discussed, like: Integration with AutoSAR, handling of safety information, vehicle specifics around provisioning and management, a comprehensive driver model, diagnostics, provisioning of data, navigation model, security, a lightweight form of OSGi, privacy issues, reprogramming ECUs (other processors in the car), distribution, and some none technical areas.

Conclusion
A great deal of time was spent on discussing the OSGi vision of universal middleware and how to get large car companies to buy into the technology. Though the basic OSGi technology has proven itself in a large number of applications, it is not well known in higher echelons. Marketing the technology is hard because there is no product that needs to be sold, what we are selling is a promise. To fulfill this promise we need the participation of the key players in this market: the car manufacturers. Without strong backing from the car manufacturers, few suppliers will invest in pushing a technology like OSGi. We discussed how to get support from the car manufacturers (several were around the table) but that is a difficult issue due the state of the current car industry and lack of knowledge about OSGi in the higher echelons. I guess we have to continue as we do today and hope that the combined results of this work will convince the car manufacturer’s management to invest in our technology.

However, it was clear that OSGi plays an important role in the thinking of many participants of the VII project, just like the European GST and the CVIS project. For these architectures the only option is the OSGi service platform, or otherwise limit the standards to protocols only. Though protocols only can reduce the cost of the OBE, it adds cost in other places because the diversity is increased on the management level. It is therefore not unlikely that in the long run the OSGi service platform becomes a standard component in the car.

Overall, this was very good meeting where both technical and non-technical issues were openly discussed with representatives of the key player; there was a lot of energy in the room. We also intend to follow up this meeting with an OSGi Vehicle Expert Group meeting, further detailing the requirements. It looks like the OSGi technology will happen in our industry, now we only have the small detail of making the vehicle specific specifications left.

     Peter Kriens

Spring and OSGi: Jumping Beans

This week there was an interesting discussion going on in the Spring-OSGi mailing list. In this mailing list, a number of people discuss their fantastic work marrying Spring with OSGi. This project has created a lot of interest; Even JBoss seems to have been inspired by this work and is moving to OSGi, putting the majority of application servers on firm OSGi footing!

Anyway, the discussion was about the dynamics of services. Hal Hildebrand (Oracle) started a discussion about the cardinality of services. This opened a floodgate of mails that I tried to ignore due to after-vacation overload. However, Costin Leau (Interface21) asked me to jump in so I was forced to analyze the thread. I got myself a cup of strong coffee and wrestled through the mail flood.

The issue was revolving around the cardinality. The cardinality indicates how many services of a given type you require and how many of them you are interested in. The OSGi-Spring specification treats OSGi services similar to beans. This allows you to inject beans into other beans, following the concept of inversion of control and dependency injection. For unary services like an OSGi Log Service this works fairly well. However, the devil is usually in the details, so let us take a closer look.

The key differences between beans and OSGi services are:

• Beans are passive, beans are non-existent until someone creates them with Class.newInstance(). The whole idea of Inversion of Control (IoC) is that this creation is done by an outsider and not the POJO itself.


• You rarely require two beans of the same class. Why would you use two different database objects or log objects?

Most Java programmers will recognize this model. Many developers use the Class.forName to find a class and then use Class.newInstance to create an instance for extendibility. Virtually any (non-OSGi) Java application uses this model to extend or configure itself this way. Notice that there is a strong 1:1 relation between the Class and the instance in this model.

In contrast, OSGi services are more like jumping beans; they have a life of their own. When beans start jumping all over the place (spring, as they say in Dutch!) then most programmers will be shocked to the bones. Fortunately, once they grasp the power of a dynamic model they usually fall in love it. However, this model implies that the IoC is not limited to configuration and activation, it means that the IoC container must handle life cycle issues during the active life of the application.

Obviously, many Spring applications will not be able to handle these dynamics, or even want to handle these dynamics. The Spring-OSGi group therefore chose to hide these dynamics behind a proxy. Instead of directly injecting the service object, a proxy to this object is injected into the configured bean. This gives us a choice to control the behavior later when the underlying service arrives or goes away. However, it also gives us a choice of optionality and multiplicity. The proxy mechanism of Spring gives us full control, the tricky thing is to define what to do with this control.

These are exactly the issue we spent a lot of time discussing during the development of declarative services, inspired by Richard Hall’s service binder and designed by BJ Hargrave (IBM). Declarative services do not use proxies but instead bind and unbind services to the application using injection. After lots of deep discussions we came up with the following policies for binding and unbinding services to the application.

Mandatory/Optional
A mandatory service means that the service must be present before the application is activated and the application will be deactivated when there is no such service available anymore. An optional service does not influence the life cycle of the application. The declarative runtime can unbind a service and bind another service at will when the service is optional. Normal Spring beans are similar to mandatory services.

Mandatory is normally indicated in the cardinality as 1.., and optionality as 0.. .

Unary/Multiple
The application can be interested in a single service (unary) or it can track a set of services. The set may require some explanation. The dynamic service registry enables some very interesting software patterns. For example, one bundle manages a Bluetooth hardware interface. The whiteboard pattern shows that the simplest solution for this bundle is to register a service per discovered device, and unregister this service when the device is no longer reachable. Other bundles can then just track these services from the registry and use when required. This pattern nicely decouples the lifecycle of the bundles that participate, making clients easier to develop.

The idea that you can add and remove beans dynamically from a configured bean is new to Spring but it follows naturally from the OSGi model.

This cardinality is indicated with ..1 for unary and ..n for multiple.

Static/Dynamic
Once an application is activated because its dependencies are met, the lifecycle state of a referred service can change. The traditional choice is to deactivate the application and restart it when the conditions are right again; this is called the static policy. Some people are repelled by this idea but in an OSGi system this is quite natural, and well under the control of the operator. An update can cause a ripple effect of many bundles restarting but well designed systems should be robust enough to handle this. Hmm, let me rephrase, if your system can’t handle this model, it will fail in the end anyway.

In certain cases, it would do no harm to replace a service on the fly. If an application uses the Log Service, it is likely not interested which Log Service it uses. If this service is unregistered, it could easily be unbound and then bound again with a new instance. Most stateless services can be replaced this way. Binding and unbinding during the active life of the application is called the dynamic model.

Usage
Let us now take a look at the possible combinations and see what they mean, and where they are applicable.

• static, 1..1 – The simplest case. The service must be present before the application is activated and the application is deactivated when the service disappears. The world looks perfectly simple and static for the application. The typical use case for this policy is the Http Service. Many applications should only be started when the Http Service is present and when it goes away the application has no purpose anymore. A once chosen Http Service can not just be replaced because the application must register the servlet with the Http Service; the Http Service is not stateless with respect to the application.


• static, 0..1 – The common case when the service is not really required to operate the application and the programmer does not want to bother when the service shows up later. The application is therefore unconditionally activated, regardless if the service is present. However, if a service is bound to the application before activation, the application is deactivated when that specific service goes away, even if another becomes available. This cardinality could for example be used for the Configuration Admin service because it is likely to be there or not.


• static, 1..n – This case is not very useful, tracking multiple services require a dynamic policy to make sense.


• static, 0..n – See previous.


• dynamic, 1..1 – The service must be present before the application is activated, however, the service can be unbound and bound again if a replacement can be found. This mode is useful when you have a stateless service but you require its presence to operate. For example, if the application needs to send messages then it can not function without a message service. However, it does not really care which message service is used.


• dynamic, 0..1 – An optional service that can dynamically be replaced. The application is unconditionally activated and never deactivated on behalf of this service. This policy is typically used for a Log Service. Logging is nice, but not generating a fire alarm because you can not log it is bound to make some unhappy customers.


• dynamic, 1..n – At least one service is needed, but many are handled. The application is only activated when at least one service is present and is deactivated when no more services can be found. This can be useful if you perform an expensive function that is only relevant when there is at least one consumer. For example, assume you run a GPS device in a phone so battery power is at premium. If there are no positioning listeners, it is no use to run the GPS chip thereby reducing power.


• dynamic, 0..n – The typical tracking case. The application is unconditionally activated and never deactivated on behalf of this service. Services are bound and unbound as they come and go. For example, if you are tracking a set of Bluetooth devices in the service registry then this is your policy to use.

Spring
The Spring-OSGi spec can copy the concepts from the declarative services because they have exactly the same issues. The discussion in the mail group got confused because the interaction between the policies for proxying (static/dynamic) were confused with optionality.

Spring adds the concept of proxies. It is my advice that the rules are as close as possible to the declarative services model which means that in most cases the application must participate in the binding and unbinding. It is tempting to automatically replace the service reference in a proxy but most of the time services are stateful and then it is easy to confuse them. Associating proxying with dynamic does not make sense because normally the application must do some action during binding. Maybe Spring needs an extra policy to indicate that replacing the service in the proxy is allowed or not, for example statefull and stateless. Then again, I am starting to have serious doubts how useful proxy rebinds are in reality.

Anyway, this year will be an interesting year for declarative services. It is clear from this issue that declarative services did a good job in this area but the possibilities of Spring will likely encompass all the important features of declarative and then add more. I am not sure if it is a good idea to have declarative services compete with Spring. My personal preference is that the Spring-OSGi specification becomes an official OSGi R5 spec … Let me know if you have ideas how we should handle this.

Peter Kriens