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Context is key: Test Coverage

Tuesday, December 7th, 2010

The notion that Test Coverage alone can provide an adequate metric for determining how well a particular piece of code or, an entire codebase, is being tested has always troubled me. In my experience this can be a very misleading assumption.

Like many of my previous themes, with Test Coverage context trully is key. The issue I find is that relying on a predetermined percentage threshold to provide a “true” measure of successful testing simply fails to take into account the numerous factors involved. For example, the preceived thoroughness of a systems tests can easily be increased – beit mistakenly or intentionally – by testing parts of the system which could be considered irrelevant, or provide very little tangible value; such as getters, setters and the like.

Personally, I advocate focusing first on testing the most critical behaviors and state of a particular piece of code. The tests should not be limited to just testing the expected cases but also, and of equal importance, the testing of exceptional and negative tests.

I could go on about this in great detail; however, I recently came across a really good post from googletesting (which I found via Mike Labriola) which I think pretty much sums it up.

Domain Models and Value Objects

Thursday, August 5th, 2010

The other day a friend asked me what is the difference between a Value Object and a Domain Model, and when I would suggest using one over the other?

Since I have been asked this very same question quite a few times, I thought it might prove useful to provide a brief definition in the context of a language agnostic idiom which could serve as a point of reference for others as well. Thus, below is general definition of each.

Domain Models

A Domain Model is anything of significance which represents a specific business concept within a problem domain. Domain Models are simply classes which represent such concepts by defining all of the state, behavior, constraints and relationships to other Domain Models needed to do so. Essentially, a Domain Model “models” a domain concept, such as a Product, a User, or anything which could be defined within a problem domain itself, outside of the context of code.

Domain Models promote reuse and eliminate redundancy by defining specific classes which encapsulate business logic, state, behaviors and relationships. As business domain concepts change, so to do the implementations of the Domain Models.

Value Objects

As the name implies, a Value Object, more commonly referred to as a VO, is an object which simply provides values, nothing more.

Value Objects are entirely immutable; that is, all properties are read-only and assignments to those properties are specified only during object creation; after which, properties can not be modified and, by design, should not require changes.

Value Objects are typically used to provide an aggregation of conceptually related properties whose values describe the initial state of the object when instantiated and do not require any real concept of identity or uniqueness. While there are some edge cases (such as validation), more commonly than not, Value Objects do not implement any specific behavior. Conceptually, think of a Value Objects as being nothing more than an object which holds a value, or series of related values, which describe something about the object when created.

It is important to make the distinction between Value Objects and Domain Models, as a Value Objects is not a Model, but rather, it is nothing more than an object which holds values and could be used to describe any particular context. Perhaps a good example of a Value Object could be a JSON object returned from the server. That is a Value Object. A Domain Model could then wrap the Value Object in order to provide state changes, validation and behaviors.

And that’s it

Hopefully the above descriptions of both Domain Models and Value Objects will clear up any confusion surrounding the two concepts; ideally, making it easier to understand when to use each.

The point to keep in mind is that Domain Models simply model a business concept, including it’s rules, constraints and behaviors, while Value Objects simply describe a contextual state.

Misplaced Code

Monday, May 31st, 2010

Often I come across what I like to call “Misplaced Code”, that is, code which should be refactored to a specific, independent concern rather than mistakenly being defined in an incorrect context.

For instance, consider the following example to get a better idea of what I mean:

Taking the above example into a broader context, it is quite common to see code such as this scattered throughout a codebase; particularly in the context of view concerns. At best this could become hard to maintain and, at worst, it will result in unexpected bugs down the road. In most cases (as in the above example) the actual code itself is not necessarily bad, however it is the context in which it is placed which is what I would like to highlight as it will almost certainly cause technical debt to some extent.

Considering the above example, should code such as this become redundantly implemented throughout a codebase it is quite easy to see how it can become a maintenance issue as, something as simple as a change to a hostname would require multiple refactorings. A much more appropriate solution would be to encapsulate this logic within a specific class whose purpose is to provide a facility from which this information can be determined. In this manner unnecessary redundancy would be eliminated (as well as risk) and valuable development time would be regained as the code would need only be tested and written once – in one place.

So again, using the above example, this could be refactored to a specific API and client code would leverage the API as in the following:

This may appear quite straightforward, however, I have seen examples (this one in particular) in numerous projects over the years and it is worth pointing out. Always take the context to which code is placed into consideration and you will reap the maintenance benefits in the long run.

Some useful Tips to keep in mind

Sunday, February 28th, 2010

Throughout my career I have always been drawn to books which provide a practical way of thinking about software. Books of this nature tend to have an emphasis on fundamental principles which apply to all software engineering disciplines, and form much of the basis of the Agile methodologies many of us have come to appreciate.

Often, I find myself going back to the seminal text The Pragmatic Programmer as it provides a great source of some important things I like to keep in mind from day to day. And so, I just wanted to take a moment to share some of the best tips from the book which I have found to be particularly useful, and inspiring.

Care About Your Craft
Why spend your life developing software unless you care about doing it well?

Provide Options, Don’t Make Lame Excuses
Instead of excuses, provide options. Don’t say it can’t be done; explain what can be done.

Critically Analyze What You Read and Hear
Don’t be swayed by vendors, media hype, or dogma. Analyze information in terms of you and your project.

Design with Contracts
Use contracts to document and verify that code does no more and no less than it claims to do.

Refactor Early, Refactor Often
Just as you might weed and rearrange a garden, rewrite, rework, and re-architect code when it needs it. Fix the root of the problem.

Costly Tools Don’t Produce Better Designs
Beware of vendor hype, industry dogma, and the aura of the price tag. Judge tools on their merits.

Start When You’re Ready
You’ve been building experience all your life. Don’t ignore niggling doubts.

Don’t Be a Slave to Formal Methods
Don’t blindly adopt any technique without putting it into the context of your development practices and capabilities.

It’s Both What You Say and the Way You Say It
There’s no point in having great ideas if you don’t communicate them effectively.

You Can’t Write Perfect Software
Software can’t be perfect. Protect your code and users from the inevitable errors.

Build Documentation In, Don’t Bolt It On
Documentation created separately from code is less likely to be correct and up to date.

Put Abstractions in Code, Details in Metadata
Program for the general case, and put the specifics outside the compiled code base.

Work with a User to Think Like a User
It’s the best way to gain insight into how the system will really be used.

Program Close to the Problem Domain
Design and code in your user’s language.

Use a Project Glossary
Create and maintain a single source of all the specific terms and vocabulary for a project.

Be a Catalyst for Change
You can’t force change on people. Instead, show them how the future might be and help them participate in creating it.

DRY – Don’t Repeat Yourself
Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

Eliminate Effects Between Unrelated Things
Design components that are self-contained, independent, and have a single, well-defined purpose.

Iterate the Schedule with the Code
Use experience you gain as you implement to refine the project time scales.

Use the Power of Command Shells
Use the shell when graphical user interfaces don’t cut it.

Don’t Panic When Debugging
Take a deep breath and THINK! about what could be causing the bug.

Don’t Assume It – Prove It
Prove your assumptions in the actual environment—with real data and boundary conditions.

Write Code That Writes Code
Code generators increase your productivity and help avoid duplication.

Test Your Software, or Your Users Will
Test ruthlessly. Don’t make your users find bugs for you.

Don’t Gather Requirements—Dig for Them
Requirements rarely lie on the surface. They’re buried deep beneath layers of assumptions, misconceptions, and politics.

Abstractions Live Longer than Details
Invest in the abstraction, not the implementation. Abstractions can survive the barrage of changes from different implementations and new technologies.

Don’t Think Outside the Box—Find the Box
When faced with an impossible problem, identify the real constraints. Ask yourself: “Does it have to be done this way? Does it have to be done at all?”;

Some Things Are Better Done than Described
Don’t fall into the specification spiral—at some point you need to start coding.

Don’t Use Manual Procedures
A shell script or batch file will execute the same instructions, in the same order, time after time.

Test State Coverage, Not Code Coverage
Identify and test significant program states. Just testing lines of code isn’t enough.

Gently Exceed Your Users’ Expectations
Come to understand your users’ expectations, then deliver just that little bit more.

Don’t Live with Broken Windows
Fix bad designs, wrong decisions, and poor code when you see them.

Remember the Big Picture
Don’t get so engrossed in the details that you forget to check what’s happening around you.

Make It Easy to Reuse
If it’s easy to reuse, people will. Create an environment that supports reuse.

There Are No Final Decisions
No decision is cast in stone. Instead, consider each as being written in the sand at the beach, and plan for change.

Estimate to Avoid Surprises
Estimate before you start. You’ll spot potential problems up front.

Use a Single Editor Well
The editor should be an extension of your hand; make sure your editor is configurable, extensible, and programmable.

Fix the Problem, Not the Blame
It doesn’t really matter whether the bug is your fault or someone else’s—it is still your problem, and it still needs to be fixed.

“select” Isn’t Broken
It is rare to find a bug in the OS or the compiler, or even a third-party product or library. The bug is most likely in the application.

Learn a Text Manipulation Language
You spend a large part of each day working with text. Why not have the computer do some of it for you?

Use Exceptions for Exceptional Problems
Exceptions can suffer from all the readability and maintainability problems of classic spaghetti code. Reserve exceptions for exceptional things.

Minimize Coupling Between Modules
Avoid coupling by writing shy” code and applying the Law of Demeter.

Design Using Services
Design in terms of services: independent, concurrent objects behind well-defined, consistent interfaces.

Don’t Program by Coincidence
Rely only on reliable things. Beware of accidental complexity, and don’t confuse a happy coincidence with a purposeful plan.

Organize Teams Around Functionality
Don’t separate designers from coders, testers from data modelers. Build teams the way you build code.

Test Early. Test Often. Test Automatically.
Tests that run with every build are much more effective than test plans that sit on a shelf.

Find Bugs Once
Once a human tester finds a bug, it should be the last time a human tester finds that bug. Automatic tests should check for it from then on.

Sign Your Work
Craftsmen of an earlier age were proud to sign their work. You should be, too.

It is my hope that you will find some of these tips helpful and, if so, I suggest keeping those which resonate with you (as well as some of your own) someplace visible for reference as it will help serve as a nice reminder of the more important things we should always keep in mind.

Guiding Design with Behavior Verification and Mock Objects

Tuesday, November 24th, 2009

At some point every developer who has disciplined themselves in the ritualistic like art and science of Test Driven Development soon discovers that the collaborators on which a class under test depend introduce an additional layer of complexity to consider when writing your tests – and designing your APIs.

For example, consider a simple test against a class Car which has an instance of class Engine. Car implements a start method which, when invoked, calls the Engine object’s run method. The challenge here lies in testing the dependency Car has on Engine, specifically, how one verifies that an invocation of Car.start results in the Engine object’s run method being called.

There are two ways of testing the above example of Car, which in unit testing nomenclature is called the System Under Test (SUT), and it’s Engine instance which is Car's Depended-on Component (DOC). The most common approach is to define assertions based on the state of both the SUT and it’s DOC after being exercised. This style of testing is commonly referred to as State Verification, and is typically the approach most developers initially use when writing tests.

Using the above Car example, a typical State Verification test would be implemented as follows:

Figure 1. CarTest, State Verification.

From a requirements perspective and therefore a testing and implementation perspective as well, the expectation of calling start on Car is that it will A.) change it’s running state to true, and B.) invoke run on it’s Engine instance. As you can see in Figure 1, in order to test the start method on Car the Engine object must also be tested. In the example, using the State Verification style of testing, Car exposes the Engine instance in order to allow the state of Engine to be verified. This has lead to a less than ideal design as it breaks encapsulation and violates Principle of Least Knowledge. Obviously, a better design of Car.isStarted could be implemented such that it determines if it’s Engine instance is also in a running state; however, realistically, Engine.run will likely need to do more than just set its running state to true; conceivable, it could need to do much, much more. More importantly, while testing Car one should only be concerned with the state and behavior of Car – and not that of its dependencies. As such, it soon becomes apparent that what really needs to be tested with regards to Engine in Car.start is that Engine.run is invoked, and nothing more.

With this in mind, the implementation details of Engine.run are decidedly of less concern when testing Car; in fact, a “real” concrete implementation of Engine need not even exist in order to test Car; only the contract between Car and Engine should be of concern. Therefore, State Verification alone is not sufficient for testing Car.start as, at best, this approach unnecessarily requires a real Engine implementation or, at worst, as illustrated in Figure 1, can negatively guide design as it would require exposing the DOC in order to verify its state; effectively breaking encapsulation and unnecessarily complicating implementation. To reiterate an important point: State Verification requires an implementation of Engine and, assuming Test First is being followed (ideally, it is), the concern when testing Car should be focused exclusively on Car and it’s interactions with its DOC; not on their specific implementations. And this is where the second style of testing – Behavior Verification – plays an important role in TDD.

The Behavior Verification style of testing relies on the use of Mock Objects in order to test the expectations of an SUT; that is, that the expected methods are called on it’s DOC with the expected parameters. Behavior Verification is most useful where State Verification alone would otherwise negatively influence design by requiring the implementation of needless state if only for the purpose of providing a more convenient means of testing. For example, many times an object may not need to be stateful or the behavior of an object may not always require a change in it’s state after exercising the SUT. In such cases, Behavior Verification with Mock Objects will lead to a simpler, more cohesive design as it requires careful design considerations of the SUT and it’s interactions with its DOC. A rather natural side-effect of this is promoting the use of interfaces over implementations as well as maintaining encapsulation.

For testing with Behavior Verification in Flex, there are numerous Mock Object frameworks available, all of which are quite good in their own right and more or less provide different implementations of the same fundamental concepts. To name just a few, in no particular order, there are asMock, mockito-flex, mockolate and mock4as.

While any of the above Mock Testing Frameworks will do, for the sake of simplicity I will demonstrate re-writing the Cartest using Behavior Verification based on mock4as – if for nothing other than the fact that it requires implementing the actual Mock, which helps illustrate how everything comes together. Moreover, the goal of this essay is to help developers understand the design concepts surrounding TDD with Behavior Verification and Mock Objects by focusing on the basic design concepts; not the implementation specifics of any individual Mock Framework.

Figure 2. CarTest, Behavior Verification approach.

Let’s go through what has changed in CarTest now that it leverages Behavior Verification. First, Car's constructor has been refactored to require an Engine object, which now implements an IEngine interface, which is defined as follows.

Figure 3. IEngine interface.

Note Engine.isRunning is no longer tested, or even defined as, it is simply not needed when testing Car: only the call to Engine.run is to be verified in the context of calling Car.start. Since focus is exclusively on the SUT, only the interactions between Car and Engine are of importance and should be defined. The goal is to focus on the testing of the SUT and not be distracted with design or implementation details of it’s DOC outside of that which is needed by the SUT.

MockEngine provides the actual implementation of IEngine, and, as you may have guessed, is the actual Mock object implementation of IEngine. MockEngine simply serves to provide a means of verifing that when Car.start is exercised it successfully invokes Engine.run; effectively satisfiying the contract between Car and Engine. MockEngine is implemented as follows:

Figure 4. MockEngine implementation.

MockEngine extends org.mock4as.Mock from which it inherits all of the functionality needed to “Mock” an object, in this case, an IEngine implementation. You’ll notice that MockEngine.run does not implement any “real” functionality, but rather it simply invokes the inherited record method, passing in the method name to record for verification when called. This is the mechanism which allows a MockEngine instance to be verified once run is invoked.

CarTest has been refactored to now provide two distinct tests against Car.start. The first, testStartChangesState(), provides the State Verification test of Car; which tests the expected state of Car after being exercised. The second test, testStartInvokesEngineRun(), provides the actual Behavior Verification test which defines the expectations of the SUT and verification of those expectations on the DOC; that is, Behavior Verification tests are implemented such that they first define expectations, then exercise the SUT, and finally, verify that the expectations have been met. In effect, this verifies that the contract between an SUT and its DOC has been satisfied.

Breaking down the testStartInvokesEngineRun() test, it is quite easy to follow the steps used when writing a Behavior Verification test.

And that’s basically it. While much more can be accomplished with the many Mock Testing frameworks available for Flex, and plenty of information is available on the specifics of the subject, this essay quite necessarily aims to focus on the design benefits of testing with Behavior Verification; that is, the design considerations one must make while doing so.

With Behavior Verification and Mock Objects, design can be guided into existence based on necessity rather than pushed into existence based on implementation.

The example can be downloaded here.

Design Considerations: Naming Conventions

Friday, July 31st, 2009

Intuitive naming conventions are perhaps one of the most important factors in providing a scalable software system. They are essential to ensuring an Object Oriented System can easily be understood, and thus modified by all members of a team regardless of their tenure within the organization or individual experience level.

When classes, interfaces, methods, properties, identifiers, events and the like fail to follow logical, consistent and intuitive naming conventions the resulting software becomes significantly more complex to understand, follow and maintain. As such this makes changes much more challenging than they would have been had better naming been considered originally. Of equal concern is the inevitability that poor naming will lead to redundant code being scattered throughout a project as when the intent of code is not clearly conveyed with as little thought as possible developers tend to re-implement existing functionality when the needed API cannot easily be located or identified.

Code is typically read many, many more times than it is written. With this in mind it is important to understand that the goal of good naming is to be as clear and concise as possible so that a reader of the code can easily determine the codes intent and purpose; just by reading it.

Teams should collectively define a set of standard naming conventions which align well with the typical conventions found in their language of choice. In doing so this will help to avoid arbitrary naming conventions which often result in code that is significantly harder to determine intent, and thus maintain. Of equal importance is the need for various teams from within the same engineering department to standardize on domain specific terms which align with the non-technical terms used by business stakeholders. Together this will help to develop a shared lexicon between business owners and engineers, and allow for simplified analysis of requirements etc.

Ideally, code should follow the PIE Principle (Program, Intently and expressively) – that is, code should clearly convey purpose and intent. In doing so the ability to maintain a software application over time becomes significantly easier and limits the possibility of introducing potential risk to project deliverables.

In short, conventions are very important regardless of a teams size; beit a large collaborative team environment, or a single developer who only deals with his own code. Consistency and conventions are a key aspect to ensuring code quality.