|By Piram Manickam, Subrahmanya SV||
|April 10, 2012 01:30 PM EDT||
This article introduces the concepts of Component Oriented Development and Assembly (CODA) using the OSGi Service platform with an example application. The article starts with an introduction to software components, elaborates with an example application, followed by an overview of the OSGi Service platform, and an implementation of the example application using this platform.
Introduction to Software Components
Components are parts that can be assembled to form a larger system. Electronic components such as ICs (Integrated Circuits) are assembled together to build an electronic system; similarly software components are assembled together to build a software system. Software systems have a static form as well as a dynamic runtime form. Software components can be assembled either in static form or dynamic form. In either case, the software component is an independent unit of development, deployment, and assembly. Using components to build software systems will provide many architectural advantages apart from promoting ease of reuse.
Age Calculation - An Example Application
Consider an application meant for calculating the age of people. Assume this application prompts the user to provide her personal details such as name and date of birth. Once the user provides the details, the application calculates the age of the person as of today and displays it with a personal greeting. Such an application can be built from the following software components:
- UserProfile Component: Responsible for collecting the personal details of the user such as name and date of birth
- Age Component: Responsible for calculating the age, given the date of birth
- Greeting Component: Responsible for displaying the greeting message along with age of the person; needs to make use of the UserProfile component to obtain input from the user and Age component to calculate the age
- AgeCalculationApp Component: Responsible for invoking the Greeting service provided by Greeting component
The components can be represented in a UML component diagram as shown in Figure 1. The dotted arrow lines in the diagram denote dependency.
Figure 1: Components in the Age Calculation application
Interfaces of Software Components
A software interface is a collection of software operations. Software components interact with one another by invoking the software operations. To facilitate structured interactions across components, each component exposes one or more software interfaces through which other components can interact. The exposed interfaces of a component are called as its "provided interfaces." Internals of the component implement the service contracts exposed by the provided interfaces.
Apart from providing services, a component needs to consume services provided by other components. The set of services that a component wants to consume is specified through "required interfaces". A component can work with any other component that provides the required interfaces. Hence there is no tight coupling between one component and another; the dependency of a component is only with interfaces and not with other components.
Interfaces in the Age Calculation Application
Let us identify the interfaces provided and required by each of the four components in the Age Calculation application.
The Age component is responsible for calculating the age, given the date of birth. Hence it needs to provide a service for calculating the age. Let's expose this service through a provided interface IAgeCalculation. The Java code for this interface is given in Figure 2.
Figure 2: AgeCalculation Java interface code
In the IAgeCalculation interface definition, Calendar is a data type defined in java.util package to represent the date. IAge is a custom-defined data type to hold the age. The Java code for IAge is shown in Figure 3.
Figure 3: Java code for data type IAge
The Age Component can be represented in a UML component diagram as shown in Figure 4. The lollipop connector from the component denotes provided interface.
Figure 4: UML representation of Age component
The UserProfile component is responsible for collecting the personal information from end user and passing it back to the consuming component. Let us define an interface called as IUserProfileCollection to expose this service. The Java code for this interface is shown in Figure 5.
Figure 5: Java code for IUserProfileCollection interface
When the method collectUserProfile() is called, the component starts an interactive session with the end user to collect her personal information. During the user interaction session, the method isProfileCollected() will return false; after the user interaction session has finished and the user profile has been collected, this method will return true. At this time, the user profile is ready for pick-up by the consuming component and it can do so by invoking giveUserProfile() method.
giveUserProfile() method returns the user profile information through a custom defined data type IUserProfile. Java code for IUserProfile data type is given in Figure 6.
Figure 6: Java code for IUserProfile data type
The UserProfile component in a UML component diagram notation is shown in Figure 7.
Figure 7: UML representation of UserProfile component
The Greeting component is responsible for displaying the greeting message along with the age. We'll expose this service through the IGreeting interface. The Java code for IGreeting interface is shown in Figure 8.
Figure 8: Java code for interface IGreeting
The Greeting component provides the IGreeting service, and it requires the IAgeCalculation and IUserProfileCollection services. The required and provided interfaces of the Greeting component can be represented using the UML component diagram as shown in Figure 9. The receptacle sockets in the component diagram denote required interfaces.
Figure 9: UML representation of Greeting component
The AgeCalculationApp component is responsible for invoking the IGreeting service of Greeting component. The AgeCalculationApp itself does not provide any service. The UML diagram for this component is shown in Figure 10.
Figure 10: UML representation of AgeCalculationApp
Components are assembled together to build a software system. Components for an assembly should be chosen such a way that, amongst the chosen components the set of required interfaces is fulfilled by the set of provided interfaces. The four components of the example application can be assembled to result in Age Calculation application. UML component diagram denoting this assembly is shown in Figure 11. Connectors with a lollipop and a receptacle together show how two components assemble over a common interface (one component provides the interface and another component requires it).
Figure 11: UML representation of Age Calculation application assembly
Once an application is assembled, specific components from the assembly can be replaced with compatible newer components. Let us say we have two new components - GUIUserProfile and GUIGreeting, which have same interfaces as UserProfile and Greeting components, respectively. Then these new components can replace the old components to provide us with a GUI application. The application assembly would then look like Figure 12.
Figure 12: Age Calculation GUI Application Assembly
Introduction to the OSGi Service Platform
OSGi Service is a standards based software development platform. The platform standards are specified by the OSGi Alliance (http://www.osgi.org), formerly known as the Open Services Gateway initiative. OSGi alliance is an industry backed nonprofit organization.
OSGi platform provides a modularity and component model on top of Java. Figure 13 illustrates how OSGi logically fits in a development / deployment stack.
Figure 13: OSGi From Development / Deployment Perspective
At the core of the OSGi platform is the OSGi framework defined by standards. OSGi framework is the runtime environment on which OSGi applications can be executed. OSGi framework specifies a common API using which application developers can develop OSGi applications. The OSGi framework specifications is used by OSGi framework developers also, who build implementations of the framework as per the specifications.
There are a number of framework implementations available from different developers. A few well known framework implementations are listed below:
- Apache Felix (http://felix.apache.org)
- Eclipse Equinox (http://www.eclipse.org/equinox)
- Knoplerfish (http://www.knoplerfish.org)
The functionality of OSGi framework is divided into many layers. For our purposes, we shall focus on Modules Layer and Services Layer. Figure 14 illustrates the layers we would like to focus.
The modules layer is the bottom most layer interacting with the runtime framework. The modules layer defines a modularity framework on top of standard Java so that modules expose and interact via well-defined APIs. Individual module is called as a "bundle" in OSGi. OSGi bundle is the atomic unit of deployment on an OSGi framework. An OSGi bundle is nothing but a Java Archive (JAR) File with some special manifest information that would be used by the OSGi framework when the bundle is deployed. Unlike a normal JAR file, which exposes all its code when it is deployed on a Java Virtual Machine (JVM), the bundle does not expose all the code contained within. Only code (Java packages) marked for explicit export is exposed outside the bundle.
Figure 14: OSGi Framework Functional Layers
Above the Module layer is the Services layer. An OSGi Service is defined by a Java interface. Any bundle can implement the service, and register the service with the OSGi Service Registry. OSGi Service Registry, defined as part of framework standards, supports a publish, subscribe, and look-up mechanism of exchanging services across components. A bundle can register the service that it has implemented with the OSGi Service Registry. A bundle requiring a service can query the OSGi Service Registry to look-up any registered service implementation. Instead of looking-up, a bundle can also subscribe to be notified on service registration / unregistration events. These mechanisms make the bundles that provide service and bundles that consume service unaware of each other, leading to independent development and deployment of these components. Once deployed, they can be assembled dynamically using the OSGi Service Registry.
Implementing Age Calculation Using OSGi
The Age Calculation application has one bundle for each of the components. Each bundle has a Java package that is exposed for access from outside and an internal Java package. The exposed package defines the interfaces and the internal package contains implementation classes. The package diagram for all the bundles is shown in Figure 15.
Figure 15: Age Calculation Application bundles
The service provided by each bundle is exposed by using the OSGi Declarative Services framework. Using the same framework, each bundle also specifies the list of services it requires. The OSGi Declarative Services framework (also known as Service Component Runtime), will perform the work of registering the provided service with the OSGi Service Registry and obtaining the required service through look-up into OSGi Service Registry. Thus, the Declarative Services framework provides automatic assembly of deployed components.
As an example, let us consider the Greeting bundle. The Declarative Services specification of the components in this bundle is done through an XML file, which is given in Figure 16.
Figure 16: Greeting component Declarative Service Definition
The XML file contains following metadata about the Greeting component:
- A component by name com.demo.greeting is defined
- The component is implemented by the class com.demo.greeting.internal.Greeting
- The component requires two interfaces - IuserProfileCollection and IAgeCalculation. These required interfaces should be dynamically wired to this component using setUserProfileCollection() and setAgeCalculation() methods on the component implementation class.
- The component provides a service through the provided interface com.demo.greeting.IGreeting.
To understand how the Greeting component works, some portions of the implementation class is presented in Figure 17. The Greeting class has two field variables referring to the services required by this component. In the component Declarative Service metadata setter methods of these two variables are linked to the service reference (required interface). Hence when the Service Component Runtime (SCR) activates this component, it would call the setter methods to point these variables to objects which provide the service. For example, an instance of AgeCalculator object from Age component would be passed to ageCalculation_ member variable.
Figure 17: Implementation of Greeting component
The Greeting class's implementation of greet() method obtains the user profile by invoking IUserProfileCollection service. From the obtained profile, date of birth is extracted and IAgeCalculation service is used to calculate the age. Finally the method displays the age along with a greeting to the user.
Output from the Example Application
The Age Calculation application, with console mode user interaction components, has the following output.
osgi> What is your first name?
What is your last name?
What is your title (Mr./Ms./Mrs./Prof./Dr.)?
What is your year of birth?
What is your month of birth (1-12)?
What is your date of birth (1-31)?
Hello Mr. Albert Einstein, you are 132 years, 10 months, and 3 days old.
If we replace the Greeting and UserProfile components in the application assembly with GUIGreeting and GUIUserProfile components, the application output is as below:
The source code of the application can be downloaded as a zip file containing an Eclipse workspace folder. To open the source code and run the application simply unzip the folder and switch the Eclipse workspace to point to the unzipped folder. Choose the appropriate components in the Run Configuration to run either console version or the GUI version of the assembly. For example, for the GUI version, the following run configuration will be applicable:
|newuniverse 01/31/12 03:10:00 AM EST|
DevOps at Cloud Expo, taking place Nov 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 19th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long dev...
Sep. 30, 2016 04:00 PM EDT Reads: 3,572
With online viewership and sales growing rapidly, enterprises are interested in understanding how they analyze performance to positively impact business metrics. Deeper insight into the user experience is needed to understand why conversions are dropping and/or bounce rates are increasing or, preferably, to understand what has been helping these metrics improve. The digital performance management industry has evolved as application performance management companies have broadened their scope beyo...
Sep. 30, 2016 03:30 PM EDT Reads: 1,384
Video experiences should be unique and exciting! But that doesn’t mean you need to patch all the pieces yourself. Users demand rich and engaging experiences and new ways to connect with you. But creating robust video applications at scale can be complicated, time-consuming and expensive. In his session at @ThingsExpo, Zohar Babin, Vice President of Platform, Ecosystem and Community at Kaltura, will discuss how VPaaS enables you to move fast, creating scalable video experiences that reach your ...
Sep. 30, 2016 03:00 PM EDT Reads: 1,234
In this strange new world where more and more power is drawn from business technology, companies are effectively straddling two paths on the road to innovation and transformation into digital enterprises. The first path is the heritage trail – with “legacy” technology forming the background. Here, extant technologies are transformed by core IT teams to provide more API-driven approaches. Legacy systems can restrict companies that are transitioning into digital enterprises. To truly become a lea...
Sep. 30, 2016 02:45 PM EDT Reads: 651
Just over a week ago I received a long and loud sustained applause for a presentation I delivered at this year’s Cloud Expo in Santa Clara. I was extremely pleased with the turnout and had some very good conversations with many of the attendees. Over the next few days I had many more meaningful conversations and was not only happy with the results but also learned a few new things. Here is everything I learned in those three days distilled into three short points.
Sep. 30, 2016 02:30 PM EDT Reads: 5,253
24Notion is full-service global creative digital marketing, technology and lifestyle agency that combines strategic ideas with customized tactical execution. With a broad understand of the art of traditional marketing, new media, communications and social influence, 24Notion uniquely understands how to connect your brand strategy with the right consumer. 24Notion ranked #12 on Corporate Social Responsibility - Book of List.
Sep. 30, 2016 02:30 PM EDT Reads: 353
Information technology is an industry that has always experienced change, and the dramatic change sweeping across the industry today could not be truthfully described as the first time we've seen such widespread change impacting customer investments. However, the rate of the change, and the potential outcomes from today's digital transformation has the distinct potential to separate the industry into two camps: Organizations that see the change coming, embrace it, and successful leverage it; and...
Sep. 30, 2016 01:15 PM EDT Reads: 1,287
Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 19th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devices - comp...
Sep. 30, 2016 01:15 PM EDT Reads: 5,164
It's been a busy time for tech's ongoing infatuation with containers. Amazon just announced EC2 Container Registry to simply container management. The new Azure container service taps into Microsoft's partnership with Docker and Mesosphere. You know when there's a standard for containers on the table there's money on the table, too. Everyone is talking containers because they reduce a ton of development-related challenges and make it much easier to move across production and testing environm...
Sep. 30, 2016 01:00 PM EDT Reads: 6,006
19th Cloud Expo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterpri...
Sep. 30, 2016 12:45 PM EDT Reads: 4,499
Apache Hadoop is a key technology for gaining business insights from your Big Data, but the penetration into enterprises is shockingly low. In fact, Apache Hadoop and Big Data proponents recognize that this technology has not yet achieved its game-changing business potential. In his session at 19th Cloud Expo, John Mertic, director of program management for ODPi at The Linux Foundation, will explain why this is, how we can work together as an open data community to increase adoption, and the i...
Sep. 30, 2016 12:00 PM EDT Reads: 1,212
About a year ago we tuned into “the need for speed” and how a concept like "serverless computing” was increasingly catering to this. We are now a year further and the term “serverless” is taking on unexpected proportions. With some even seeing it as the successor to cloud in general or at least as a successor to the clouds’ poorer cousin in terms of revenue, hype and adoption: PaaS. The question we need to ask is whether this constitutes an example of Hype Hopping: to effortlessly pivot to the ...
Sep. 30, 2016 11:30 AM EDT Reads: 1,204
SYS-CON Events announced today that LeaseWeb USA, a cloud Infrastructure-as-a-Service (IaaS) provider, will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. LeaseWeb is one of the world's largest hosting brands. The company helps customers define, develop and deploy IT infrastructure tailored to their exact business needs, by combining various kinds cloud solutions.
Sep. 30, 2016 10:15 AM EDT Reads: 3,379
Large enterprises today are juggling an enormous variety of network equipment. Business users are asking for specific network throughput guarantees when it comes to their critical applications, legal departments require compliance with mandated regulatory frameworks, and operations are asked to do more with shrinking budgets. All these requirements do not easily align with existing network architectures; hence, network operators are continuously faced with a slew of granular parameter change req...
Sep. 30, 2016 10:15 AM EDT Reads: 882
DevOps at Cloud Expo – being held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA – announces that its Call for Papers is open. Born out of proven success in agile development, cloud computing, and process automation, DevOps is a macro trend you cannot afford to miss. From showcase success stories from early adopters and web-scale businesses, DevOps is expanding to organizations of all sizes, including the world's largest enterprises – and delivering real results. Am...
Sep. 30, 2016 10:00 AM EDT Reads: 4,625
There is little doubt that Big Data solutions will have an increasing role in the Enterprise IT mainstream over time. Big Data at Cloud Expo - to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA - has announced its Call for Papers is open. Cloud computing is being adopted in one form or another by 94% of enterprises today. Tens of billions of new devices are being connected to The Internet of Things. And Big Data is driving this bus. An exponential increase is...
Sep. 30, 2016 10:00 AM EDT Reads: 2,774
SYS-CON Events announced today the Enterprise IoT Bootcamp, being held November 1-2, 2016, in conjunction with 19th Cloud Expo | @ThingsExpo at the Santa Clara Convention Center in Santa Clara, CA. Combined with real-world scenarios and use cases, the Enterprise IoT Bootcamp is not just based on presentations but with hands-on demos and detailed walkthroughs. We will introduce you to a variety of real world use cases prototyped using Arduino, Raspberry Pi, BeagleBone, Spark, and Intel Edison. Y...
Sep. 30, 2016 09:15 AM EDT Reads: 3,066
With the rise of Docker, Kubernetes, and other container technologies, the growth of microservices has skyrocketed among dev teams looking to innovate on a faster release cycle. This has enabled teams to finally realize their DevOps goals to ship and iterate quickly in a continuous delivery model. Why containers are growing in popularity is no surprise — they’re extremely easy to spin up or down, but come with an unforeseen issue. However, without the right foresight, DevOps and IT teams may lo...
Sep. 30, 2016 09:15 AM EDT Reads: 1,228
Much of the value of DevOps comes from a (renewed) focus on measurement, sharing, and continuous feedback loops. In increasingly complex DevOps workflows and environments, and especially in larger, regulated, or more crystallized organizations, these core concepts become even more critical. In his session at @DevOpsSummit at 18th Cloud Expo, Andi Mann, Chief Technology Advocate at Splunk, showed how, by focusing on 'metrics that matter,' you can provide objective, transparent, and meaningful f...
Sep. 30, 2016 09:00 AM EDT Reads: 2,915
Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterprises are using some form of XaaS - software, platform, and infrastructure as a service.
Sep. 30, 2016 08:30 AM EDT Reads: 911