My Stuff

2010 Conferences

OSGi DevCon @ JAX London

February 23 - Keynote titled OSGi in the Enterprise: Agility, Modularity, and Architecture’s Paradox


March 22 - 25 - Tutorial on Modular Architecture

Über Conf

June 14 - 17 - Sessions titled Turtles and Architecture and Patterns of Modular Architecture


July 26 - 30 - Two sessions on rich mobile applications and one on agile development. Half day tutorial on software process improvement.

Tweets @ Twitter

re: #apple event "We sold more iPads than any PC manufacturer sold of their entire PC line." 2012-09-12

re: #Apple Event ""Our notebooks now rank #1 in the US in Market share in the last three months." 2012-09-12

Right on. I just won a Best Buy drawing worth $1000. Either that or I won a shiny new virus by clicking the link. Hmm...what to do. 2012-08-29

The #osgi alliance response ( to the @mreinhold #jigsaw announcement ( 2012-08-29

Good Q&A with @mreinhold on project #jigsaw. Modularity will change the way we design and run apps! 2012-08-28

LinkedIn Profile

The opinions expressed on this site are my own, and not necessarily those of my employer.

Back to the Future

Filed Under Architecture & Design, Java, OSGi, Platforms | 5 Comments 


This is my last post of the year, and I want to take a moment to thank everyone that spends their precious time reading my long-winded entries. In 2009, I saw a four fold increase in blog traffic over 2008, and December 2009 saw a five fold increase in traffic over January 2009. Out of a total of 74 posts this year, more than half were related to modularity. I’d like to think the increase in traffic is related to the topics I’ve been writing about. Modularity is something that folks are interested in. But we still have a ways to go.

Future of Modularity

Interestingly, I was speaking to a conference organizer yesterday, and I sensed his mild surprise when I began to talk about modularity. He noted that modularity is certainly not a new concept. I agreed. It’s an idea that has been around since at least the early 1970’s when David Parnas published his essay. But things are different now. Until recently, modularity was not a first class concept on the Java platform. If we wanted to develop software with a modular architecture, we were required to do so without much framework or platform support. In fact, we hadn’t even identified any standard definition of module on the platform.

That’s changing. As the application platform vendors continue to bake OSGi into their products, it’s clear that a module is a JAR file and there’s a framework to back it up. In 2010 modularity on the Java platform will gain considerable visibility in our industry. The stackless stack is coming to fruition, and it’s a game changer…a disruptor…that reaches from the developer to the data center. It’s not something we should ignore. Really, it’s not something we can ignore anymore.  So, as we close out one decade and usher in another, modularity will celebrate at least its 40th birthday. Maybe we’re finally starting to get it.

All Posts Page

A final note before closing out 2009. For those interested, I’ve created a custom Wordpress page that shows a summary of all posts published on this blog. Enjoy!

OSGi DevCon Keynote

Filed Under General | Leave a Comment 

I’m excited to be given the opportunity to present the keynote at OSGi DevCon London on February 23, 2010. The event is hosted in conjunction with JAX London. The sessions for the conference look great, with a great lineup of speakers and 15 sessions, including two in-depth tutorials by Neil Bartlett and Peter Kriens, devoted exclusively to OSGi.

Here’s the abstract for my session titled OSGi in the Enterprise: Agility, Modularity, and Architecture’s Paradox.

Attempts to architect more flexible software often results in the opposite - brittle software fraught with complexity. Something is missing. Complexity is the beast we must tame, and modularity is part of the answer. In this keynote, we’ll examine the challenges of traditional approaches to software architecture, probe the paradox of architecture, and explore the inextricable link between structural and temporal architectural decisions. From the highest level applications and services to the code that exists in the bowels of the system, we expose the advantages of a modular architecture. Come learn new ways that large software systems can be organized to increase flexibility, reusability, adaptability, maintainability, testability, and extensibility. Come discover the importance of OSGi in the Enterprise.

Yes, come discover the important of OSGi in the Enterprise!

That Rotting Design

Filed Under Agile, Architecture & Design, Java, OSGi, Platforms | 6 Comments 

Note: This is a re-post, with slight modifications, from an entry in October 2007. And now, two years later, we’re just about there! For the abridged version of this post, focus on the text in bold. You can see the original version here.

Design Rot

We’ve all experienced that sinking feeling when maintaining a piece of crappy software. Has my change broken the system in some unintended way? What is the ramification of my change on other parts of the system? If you’re lucky, and the system has a robust suite of unit tests, they can offer some support in proving your work. In practice, however, few systems have thorough automated test coverage. Mostly we’re alone, left to verify our changes as best as possible. We might privately criticize the original developers for creating such garbage. It certainly lends a plausble excuse in explaining why the maintenance effort is so costly or time-consuming. Or it might serve as the basis upon which we recommend a re-write. But mostly, we should wonder how it happened.

For sure, most software doesn’t start out this way. Most software starts out clean, with a clear design strategy. But as the system grows over time, strange things begin to happen. Business rules change. Deadline pressures mount. Test coverage slips. Refactoring is a forgotten luxury. And the inherent flaws present in every initial design begin to surface. Reality has proven that few enterprise development teams have the time or resources to fix a broken design. More often, we are left to work within the constraints of the original design. As change continues, our compromises exacerbate the problem. The consequence of rotting design is seen throughout the enterprise on a daily basis. Most apparent is the affect on software maintenance. But rotting design leads to buggy software and performance degradation, as well. Over time, at least a portion of every enterprise software system experiences the problem of rotting design. A quote from Brook’s sums it well:

All repairs tend to destroy the structure, to increase the entropy and disorder of the system. Less and less effort is spent on fixing the original design flaws; more and more is spent on fixing flaws introduced by earlier fixes. As time passes, the system becomes less and less well-ordered. Sooner or later the fixing ceases to gain any ground. Each forward step is matched by a backward one. Although in principle usable forever, the system has worn out as a base for progress.

The most obvious question is, “How do we prevent rotting design?” Unfortunately, rotting design is not preventable, only reducable. Of the past ten years, the design patterns movement has provided insight to the qualities of good design. Dissecting design patterns reveals many important design principles that contribute to more resilient software design. Favor object composition over class inheritance, and program to an interface, not an implementation, are two examples. Of the 23 patterns in the GOF book, all adhere to these fundamental statements. Alone however, design patterns that emphasize class structure are not enough to help reduce rotting design.

Reducing Rot

Most patterns emphasize class design, and present techniques that can be used in specific contexts to minimize dependencies between classes. Teasing apart the underlying goal of most patterns shows us that each aim to manage the dependencies between classes through abstract coupling. Conceptually, classes with the fewest dependencies are highly reusable, extensible, and testable. The greatest influence in reducing design rot is minimizing unnecessary dependencies. Yet enterprise development involves creating many more entities beyond only classes. Teams must define the package structure in which those classes live, and the module structure in which they are deployed. Increasing the survivability of your design involves managing dependencies between all software entities - classes, packages, and modules.

But if minimal dependencies were the only traits of great design, developers would lean towards creating very heavy, self-contained software entities with a rich API. While these entities might have minimal dependencies, extreme attempts to minimize dependencies results in excessive redundancy across entities with each providing its own built-in implementation of common behavior. Ironically, avoiding redundant implementations, thereby maximizing reuse, requires that we delegate to external entities, increasing dependencies. Attempts to maximize reuse results in excessive dependencies and attempts to minimize dependencies results in excessive redundancy. Neither is ideal, and a gentle balance must be sought when defining the behavior, or granularity, of all software entities - classes, packages, and modules. For more on the use/reuse paradox, see Reuse: Is the Dream Dead?

Software design is in a constant quandary. Any single element key to crafting great designs, if taken to its individual extreme, results in directly the opposite - a brittle design. The essential complexity surrounding design is different for every software development effort. The ideal design for a software system is always the product of it’s current set of behavioral specifications. As behavior changes, so too must the granularity of the software entities and the dependencies between them. The most successful designs are not characterized by their initial brilliance, but instead through their ability to withstand the test, and evolve over the course, of time. As the complexity of software design is an essential complexity surrounding software development, our hopes lie with technologies and principles that help increase the ability of your design to survive. Such is the reason why agile architecture is so important.

A Promising Future

I’m hopeful that all software developers have experienced the pleasure of a design that, through the course of time, has withstood the test of time. Unfortunately, many enterprise development teams have too few of these experiences. Likewise, few enterprise development teams devote adequate effort to package and module design. It’s unreasonable to believe that even the most flexible class structure can survive should the higher level software entities containing those classes not exhibit similarily flexible qualities. The problems are rampant. Increased dependencies between packages and modules inhibit reusability, hinder maintenance, prevent extensibility, restrict testability, and limit a developer’s ability to understand the ramification of change.

Services offer some promise to remedy our failures with object-oriented development. Yet, while services may offer tangible business value, within each awaits a rotting design. There exists a world between class design and web services that deserves more exploration, and as an industry, we are beginning to notice. OSGi is a proven module system for the Java platform, while Jigsaw aims to modularize the JDK. JSR-294 aims to improve modularity on the Java platform. While some friction might exist between the constituencies involved, it’s only because they too recognize that something has been missing, and are passionate about fixing the problem. Of course, it doesn’t stop there. A plethora of application servers and tools are also including support for modularity using OSGi, which has grown into the defacto standard module system on the Java platform.

All aim to help manage the complexity, from design through deployment, of enterprise software development. With each, new practices, heuristics, and patterns will emerge that increase the ability of a design to grow and adapt.

Top 5 Essays You Should Read

Filed Under General | 1 Comment 

Certainly, there are many landmark books in software development that have shaped our industry. Design Patterns: Elements of Reusable Object-Oriented Software (the GOF book) is one really good example. Unfortunately, a good share of people don’t have the opportunity to read some of these great works because, well…it can get expensive pretty quickly stocking a bookshelf. But there exists a treasure trove of published content available online that is equally impactful. Here, in no particular order, are 5 essays that have helped shaped our industry, for better or worse.

  • Cathedral and the Bazaar by Eric Raymond - Discusses the evolution of Linux and provides amazing insight to lessons learned.
  • Code as Design by Jack Reeves - Presents the notion that programming is fundamentally a design activity and that the only final and true representation of “the design” is the source code itself.
  • Managing the Development of Large Software Systems (pdf) by Winston Royce - Paper widely regarded as that which gave birth to the waterfall development lifecycle.
  • No Silver Bullet by Frederick Brooks - We’re still looking, but as this paper points out, there is no silver bullet. The essential complexity Brook’s speaks of is largely why we continue to struggle with the same problems today that we did a decade ago.
  • On the Criteria to be Used in Decomposing Systems into Modules by David Parnas - Discusses the important design decisions that impact how we modularize our software systems. Important because modularity is coming to the Java platform, and we need to know how to use it effectively.

I’ll give an honorable mention to Design Principles and Design Patterns by Bob Martin, which discusses key principles of object-oriented design. Many of the patterns in the GOF book adhere to these principles.

These essays are sure to provide a positive and lasting influence. But I’m sure there are more. What am I missing? What do you consider the most impactful software development essays? What would you add to this list?

Architecture All the Way Down

Filed Under Agile, Architecture & Design, Development, OSGi, SOA | 8 Comments 

In Turtles and Architecture, I talked about how important it is that we “architect all the way down”. It helps increase transparency and understanding between developers and architects by emphasizing a lot of the middle ground that noone ever seems to focus on. It’s just another reason why modularity is so important. I used the diagram at right to illustrate the point (click to enlarge). Look at the huge gap that exists if we don’t focus on the stuff highlighted by the gray bar in the middle.

One reason I like this visual is that it illustrates the social aspect of software architecture. Yet, there are other significant advantages to architecture all the way down that we haven’t explored yet. Another is structural flexibility.

Structural Flexibility - Different Entities, Different Purpose

Another benefit of module design in filling that middle ground is that modules can offer different benefits than classes and services. The diagram at right (click to enlarge) illustrates some of the capabilities of different types of entities.

For example, classes are pretty easily reused within an application, but because classes aren’t a unit of deployment, it’s difficult to use them across applications. Of course, intra-application reuse is a sweet spot of services, since they’re method of invocation is through some distributed protocol (SOAP, HTTP, even RMI/IIOP). Yet because services are invoked remotely, we typically like to make them coarser-grained because of the performance implications of distributed requests. So this begs the question - If a service is too coarse-grained to reuse (ie. it does more than what we want), how do I reuse some desirable behavior across applications? Well, without modules, our only other choice is the class. Since a class can’t be reused intra-process, we do one of either two things. Expose it as a service or copy the class (ie. the code). Given the circumstances, neither of these may be ideal. Another option is desirable.

Modules represent that other option. They are a finer level unit of granularity than services, and are a unit of deployment. Since each of these different types of entities are units of composition, we have tremendous flexibility in how we assemble applications. Possibly more important though is our increased ability to accommodate the architectural shifts that naturally occur as requirements evolve. Let’s look at a more concrete example.

A Bit More Concretely

Let’s say I have a business function called Pay Bill for which I develop a web service that can be invoked by many different consumers. That service happens to be relatively coarse-grained, and performs all the steps involved in paying the bill. These happen to include the following:

  • audit bill - apply a discount to the bill based on payee
  • check for duplicate - ensure the bill hasn’t already been paid
  • remit payment - cut the check
  • reconcile payment - reconcile with accounts payable financials

This seems reasonable. We have a nice little service that we can reuse any time we want to pay a bill. Unfortunately, the real world often gets in the way of our idealistic solutions. In fact, there are two problems that will eventually surface, and modularity benefits both scenarios. Let’s start by looking at the first scenario.

What should I do when a different scenario arises that demands I follow a slightly modified Pay Bill function?

As part of the remit step, I have a new payee that demands electronic payment. This is pretty easy actually. I simply modify the service to support electronic payments and then configure the service to context for that specific payee. So how does modularity help here?

If the service is composed of modules, it’s going to be much easier for me to understand the structure of the service, assess the impact of what’s it’s going to cost to change the service, and then introduce a new module (or modify the existing module) to provide the new capability. Without modules, I’m simply wading through the code trying to figure out all of these things. Now, the 2nd scenario.

What should I do when I want to reuse just one step of the Pay Bill function?

Let’s say another new requirement emerges. Whereas traditionally bills were entered by data entry personnel, we now have to support electronic delivery of bills. We also know that bills delivered electronically are often duplicates. It’s just one of those business things, you know? If we don’t pay the bill on the day it’s received, the billing party sends us the bill again, asking for payment. So we need to check for duplicates before we save the bill to the database and prepare it for processing. What do we do?

We could take the same approach as before and modify the Pay Bill service so that the duplicate check could be invoked separately from the higher level pay bill function. But that’s a bastardized design solution. Why? We are exposing behavior of the Pay Bill service that shouldn’t be exposed. The API is coarse-grained in some areas and fine-grained in others.

Maybe exposing the finer-grained capabilities of the Pay Bill function isn’t a severe compromise, but it is a compromise nonetheless. And we are forced to compromise because of architectural inflexibility. We don’t have architecture all the way down, and are therefore left with limited choice as to how we support the new requirement. We can either modify the service to reuse what we know is already encapsulated within the service, or copy the code to create something new. But those are the two options we have, and neither may be ideal.

As we continue to hack the system in this manner, with a multitude of other similar changes, the design will rot. Eventually, our Pay Bill service is transformed into a utility that performs all general-purpose bill functions. It’s API is a mix of coarse-grained and fine-grained operations. It’s become the monolith that we’re trying to avoid. While the Pay Bill service is still pretty reusable (it does everything now), it isn’t that usable. That tension between reuse and use surfaces again.

Our decision to modify the Pay Bill service to expose the duplicate check was driven by one thing - ease. It was our easiest option. Really, it was our only option. But it isn’t the best option.

If we architect all the way down, we have another option. A Pay Bill service composed of modules that audit the bill, check for duplicates, remit payment, and reconcile the payment means we can choose the desirable solution over the easiest short term choice. We can avoid the long term compromises that degrade architectural integrity.

Shown at right (click to enlarge), we see the service composed of modules. If we have a check for duplicates module, we can simply reuse that module in the new service or application that’s going to process electronic bills. Or we might expose the capabilities of the check for duplicates module as a separate service. We have multiple reuse entry points, as well as different levels of granularity through which our software entities can be managed, deployed, built, and much more.

To Conclude

My point here isn’t to debate the original design nor the decisions made to reuse the check for duplicates functionality in another service or application. Certainly, debating architectural and design decisions is a favorite past-time of developers. There are a variety of different ways that we can support new requirements as they emerge. Some are better than others and all are contextual.

The gist of my point is that architecture all the way down gives us options, and these options help maintain architectural integrity. They increase our options when making decisions and allow our system to accommodate unforeseen architectural shifts. I can modify an existing application or service to give me what I need. Or I can reuse an existing module that’s already available for me. Or I can compose a new service from an existing set of modules. Or I can break apart existing modules to create new modules that result in new services. And I can do a lot of this refactoring without significant impact on the existing code. This was an important point illustrated in the series of posts on Applied Modularity.

In other words, architecture all the way down helps increase architectural agility, and modularity is a key ingredient.

The Secret Sauce

Filed Under Agile, Development | Leave a Comment 

All too often, software process improvement initiatives fail. In a recent post discussing SEMAT, Ralph Johnson provided some words of wisdom that serve as a wonderful guide to any team about to embark on that much vaunted software process improvement initiative.

The state of the practice in software development is pretty dismal. Some groups do a great job, but most do not.  As I tell the students in my software engineering course, if you manage requirements, make sure the developers talk to each other, release working code regularly, have some sort of a systematic testing process, use build and version control tools, and periodically stop and see how you are doing and how you can improve, you will be better than 90% of the groups out there. Of course, I could be exaggerating.  Maybe it is only better than 75%.

I suppose that pretty much sums it up! Amazing how difficult we tend to make things though, heh?

The Enterprise is Getting Its OSGi

Filed Under Architecture & Design, OSGi | 5 Comments 

There have been a few folks who believe that OSGi isn’t something that enterprise developers want. I’ve heartily disagreed with this, and have stated that the enterprise wants it’s OSGi. In The Two Faces of Modularity and OSGi, I stated:

While there is some dispute over whether vendors will expose the virtues of OSGi to the enterprise, eventually they will (SpringSource dm Server currently does). They will because the enterprise stands to gain considerably from a more modular architecture. There is benefit in a world void of classpath hell and monolithic applications. There is benefit in modularity, and eventually the enterprise will ask for OSGi. In the end, the enterprise will get what they’re asking for…

Well, it looks like that day is finally arriving. IBM recently announced the IBM Websphere Application Server V7 OSGi Application Open Alpha Program. From the program’s homepage, I quote the following:

The OSGi Applications Open Alpha brings the modularity, dynamism, and versioning of the OSGi service platform to enterprise application developers. It delivers a simple to use, lightweight programming model for web applications that combines the standard Blueprint component model with familiar Java enterprise technologies. TheOSGi Applications Open Alpha enables enterprise applications to be deployed to the WebSphere Application Server as collections of OSGi bundles, leveraging WebSphere platform enterprise qualities of service to provide the most complete and robust enterprise server for modular web applications. In addition to being easy to use, theWebSphere OSGi applications implementation addresses many of the challenges of developing and maintaining extensible Web applications through OSGi technology benefits of modularity, dynamism, and versioning.

Additionally, a post to the IBM developerWorks forum discusses some of the capabilities, which includes:

  • a built-in bundle repository which can host common and versioned bundles shared between multiple applications so that each application does not deploy its own copy of each common library.
  • a standardized (by the OSGI Alliance) and IBM-supported evolution of the Spring framework called the Blueprint component model. This provides the declarative assembly and simplified unit test of the Spring framework but in a standardized form that is provided as part of the WAS runtime rather than being a 3rd party library deployed as part of the application
  • familiar Java EE programming model with the option of deploying web applications as a set of versioned OSGi bundles with dynamic lifecycle.
  • full integration with WAS Admin and WAS platform enterprise qualities of service

This is great for enterprise developers who are tied to the Websphere Applicatin Server (WAS) platform. But don’t confuse WAS support of OSGi with WAS being a next generation application platform. Other vendors have done a great job providing platforms built atop OSGi, including SpringSource and Paremus.

And don’t confuse support of OSGi as a promise that you’ll immediately realize the benefits of modularity. We’ve made that mistake before with other technologies. There is still a lot to learn about how to design more modular software applications. While WAS support for OSGi brings many of the runtime and management benefits of modularity to the enterprise, it’s still our responsibility to craft systems with a modular architecture.

I’m still unsure of the actual release date for WAS V7, and I don’t have information surrounding all the prickly details of OSGi in WAS 7 and the Open Alpha program. But I intend to find out, and have already reached out to IBM to gather more details. There is still more to learn, so stay tuned. Maybe…just maybe…2010 will be the year that OSGi (and modularity) hits the enterprise.

Applied Modularity - Retrospectives

Filed Under Agile, Architecture & Design, Java, OSGi | 1 Comment 

We’ve completed our four part series on Applied Modularity, and I wanted to put a final wrap on the posts by highlighting a few things that may not be obvious. First, a brief review on the series.

  • In Part 1, we introduced the sample application and applied PhysicalLayers pattern to separate our logical layers out into physical layers.
  • In Part 2, we applied the AbstractModules (second refactoring) and AcyclicRelationships (third refactoring) patterns.
  • In Part 3, we applied the SeparateAbstractions (fourth refactoring) pattern.
  • In Part 4, we applied the CollocateException (fifth refactoring), IndependentDeployment (sixth refactoring), and ImplementationFactory (sevent refactoring) patterns.

Through this series of refactorings, we made considerable progress in modularizing the application using a few of the modularity patterns. The diagram at right (click to enlarge) illustrates the progress we made. We started with everything bundled into a single WAR file and wound up with a highly modularized system that satisfied the evolutionary requirements. Aside from the many advantages we spoke about in each post, I want to take a moment to explore a few other thoughts.

A Note On Module Testing

If you’ve explored (and built) the system by getting the code from the Google code repository, you’ll notice that there are a corresponding set of test modules for each module that we’ve created. These can be found in the bin directory (shown at right). Like we do with unit testing, I’ve tried to create a test component for each module in the system. Unfortunately, there’s a flaw in the billtest.jar module.

Similar to unit testing, where we create mocks and stubs to avoid undesirable dependencies, a test module shouldn’t pull in other modules that contain implementation classes. Instead, we should create mocks or stubs to avoid this situation. In other words, a test module should only be dependent on the same set of modules as the module it’s testing. Unfortunately, the billtest.jar module breaks this rule by leveraging the the AuditFacade implementations. That means the billtest.jar module is also dependent on the audit1.jar and audit2.jar modules, but the bill.jar module is not. So billtest.jar is really a module integration test, not a module unit test. It could easily be fixed by creating a mock AuditFacade implementation that lived in the billtest.jar module.

This begs another question….

How do we keep track of module relationships so that we recognize when something bad like this happens?

Even for small systems, without a module system like OSGi, it can be incredibly challenging.

A Note On Managing Modules

Modularizing a system on a platform that doesn’t support modularity is challenging. Hell, modularizing a system on a platform that does support modularity is challenging! One of the greatest challenges is in managing module dependencies. Tracking the dependencies between modules is really quite difficult.

This is where module systems like OSGi really help by enforcing the declared module dependencies. In plain ole Java today, there is no notion of module so there is nothing to help enforce modularity. And the first unwanted dependency that creeps into our system compromises architectural integrity. This is where JarAnalyzer can be helpful. By incorporating JarAnalyzer into my build script, I’m able to more easily manage the dependencies between modules.

JarAnalyzer has two output formats. The first is a GraphViz compliant dot file that can be easily converted to an image showing module relationships. The image at right (click to enlarge), which includes the test modules, clearly illustrates the problem with the billtest.jar module discussed above.

As can be seen, the bill.jar module has only a single outgoing dependency on the auditspec.jar module. So the module that tests the bill.jar module should not be dependent on any other modules, either. However, if you look at the billtest.jar module, you’ll see that it depends upon the audit1.jar and audit2.jar modules. So instead of using a mock or stub to test the bill.jar module, I got lazy and used the various AuditFacade implementations. Look at a few of the other modules, and you’ll discover that none include additional dependencies beyond the dependencies already present within the modules they test.

The second output format for JarAnalyzer is an html file that provides some key design quality metrics, as well as listing the dependencies among modules. Essentially, it’s a textual view of the same information provided by the visual diagram. I’ve included the Summary header of the JarAnalyzer report for the system below (click to enlarge). You can also browse the complete JarAnalyzer HTML report for the final version of the system. There is also a version that omits the test modules.

Look at the auditspec.jar module. Note that it has 8 incoming dependencies (afferent coupling) and 0 outgoing dependencies (efferent coupling). It’s abstractness is 0.67 and Instability is 0.00. This is a pretty good sign. Why? It’s instability is very low, implying it’s highly resistant to change. It possesses this resistance to change because of the large number of incoming dependencies. Any change to this module may have serious implications (ie. the ripple effect of change). But because it’s quite abstract, it’s less likely to change than a module with a lot implementation classes. The Distance for the module is 0.33 (ideal is 0.00), so we’re not far from where we ideally want to be.

In case you’re wondering about all these metrics I’m rambling about, you might want to take a look at the Martin Metrics. In general, without a utility like JarAnalyzer (or a module framework like OSGi), it would have been incredibly difficult to manage the modules composing this system.

A Note on Module Reuse

The reuse/release equivalency principles states that the unit of reuse is the unit of release. Modules are a unit of release, and therefore are a unit of reuse. Naturally, the devil is in the details, and we’re going to discuss these details here.

In Reuse: Is the Dream Dead, I spoke of the tension between reuse and use. That tension is evidently at play here. Earlier versions of the system had coarser-grained modules that were easier to use but more difficult to reuse. As we progressed, we broke these coarser-grained modules out into finer-grained modules, increasing their reusability but decreasing their ease of use. A perfect example of this is the bill.jar module. In the final version, it was quite reusable, since it was only dependent on the auditspec.jar module. However, this came at the price of useability.

To elaborate a bit more. In Part 4, the sixth refactoring, we decoupled the bill.jar and financial.jar modules so the two could be deployed independently (ie. increase reuse). But the runtime structure still has some dependencies. In order to reuse bill.jar, we need a BillPayer type. While an alternative BillPayer implementation could be created, the existing implementation is the BillPayAdapter in the mediator.jar module, which also has a relationship to the financial.jar module. This means that to use the bill.jar module without the mediator.jar and financial.jar modules would require a new consuming module to implement the BillPayer interface.

So what do we do if we want to break this runtime coupling? We should move the pay method on the Bill up to the BillPayAdapter class, and get rid of the BillPayer interface. Now the Bill class has no dependency on the BillPayer interface, but it also can’t make payments. Every action has an equal an opposite reaction, heh?

A Note on The Build

The build was a key element in helping enforce modularity (note: JarAnalyzer helped me manage module relationshps; the build enforced module relationships). Even a framework such as OSGi is only going to manage module relationships at runtime. I talk a bit more about this concept in The Two Faces of Modularity & OSGi, and it’s why we need really good tools that help us design more modular software. It’s our responsibility to craft the modules, and the build is one way to help put in place a system of checks and balances that help enforce modularity before discovering at runtime that one module has a relationship to another. In Part 2, as part of the third refactoring, we refactored our build script to a levelized build. Here’s the before and after build script.

This means that as we build each module, we include only the required modules in the build classpath. This is more easily explained by examining the build script for the final version, where you can clearly see what I’m talking about. Look at line 40. When we build the auditspec.jar module, we include nothing else in the build classpath because the auditspec.jar module doesn’t require anything. Now look at line 60, where we build the audit1.jar module. The auditspec.jar module built in the previous step is included in the classpath. This pattern recurs throughout the remainder of the script. Introducing a module dependency that violates the dependency structure enforced by the build results in a failed build.

A Note on Object Orientation

The way we managed, massaged, and modified module relationships was through OO techniques. By introducing interfaces and abstraction and allocating them to their respective modules, we were able to significantly change the module structure of the system. While we used OO to do this, OO is not a prerequisite. We could just as easily have used other techniques, such as aspects (AOP).

The Final Wrap

If you’ve made it this far through the tutorial, you’ve done well. Overall, this was a pretty lengthy and involved tutorial. In fact, I only touched briefly on all that I really had to say. Yeah, you’ve seen the abridged version here! I think I could pretty easily fill a book with the rest. Hmmm…

I use this same system and examples in quite a few of my talks on architecture and modularity. If you have questions or suggestions, feel free to drop me a line via the comments or send me an e-mail (hint: look on the About Page). Or you can track me down at a conference, as I’m always happy to discuss topics related to modularity, architecture, and agility.

I have one final deliverable. As promised way back in Part 1, I intend to show an OSGi-ified version of the system. That will follow shortly.