Agile methodologies are approaches to managing software development based  on  short-term, iterative,  and  incremental  deliveries,  enabling   continuous feedback and flexible response to change.

Stemming  from  a  rapidly  evolving  business  environment that  demands  faster product improvements and modifications, the agile methodology promotes  the organizational qualities of speed,  responsiveness,  and  adaptability throughout the  entire application management process, from defining product requirements to coding, testing, and, finally, release management.

This article  explains  why agile development cannot  be implemented  effectively without unit  testing—and especially automated unit testing.

The Importance of Code Quality

Developers have known for decades that the further into a project  timeline a bug gets discovered  from its insertion  point, the more costly it is to fix. When a developer finds a bug, it can sometimes  take  minutes  to  fix. If it slips through testing  and finds its way  to  the  customer,  figure 1 shows  that  mitigation can be exponentially more expensive to fix. [1]

Correcting  quality issues can take months of rework, cost millions  of dollars,  or,  if done  too  late,  may  even cost  lives. Take,  for  example,  the  first launch  of the  Ariane  5 rocket  in 1996.  Its flight abruptly terminated just  thirty-seven  seconds after  liftoff,  taking  with  it hundreds of millions  of dollars  in invested effort. Also think of Toyota recalling four hundred thousand vehicles because of a bug in the brake control  system, costing an estimated  three billion dollars.

While  we  can’t  eliminate  all  bugs,  we  can  fight  them  by baking  quality  into  the  code.  There  are  many  ways to  define code quality  depending  on the perspective  of the customer  or the developer.

The  customer   expects   working   software.   Customers   do not  care how the code is written—they just need the software to  work.  When  developers  talk  about  code quality,  they  talk about  code that  is easy to maintain, easy to read, and risk-adverse to change. Each perspective takes the cost of bugs into consideration. The  customer  knows  that  for  each  bug,  he’ll lose precious business hours or days. The developer knows that each returning bug means considerable time spent fixing it in- stead of working  on new features.

Agile methodologies take working  software  and combine  it with early feedback. For example, early releases can get user feedback about  how well the software  operates.  To give the developers  confidence that  their code works,  unit testing gives the fastest available quality feedback.

The  earlier  defects are  found,  the  cheaper  they  are  to  fix. As agile methodologies encourage  high code quality,  the team should run lots of unit tests. Similarly, automated tests give the developer early feedback on the quality of the software in a repeatable fashion prior to release.

What Is Unit Testing?

Unit testing is a methodology where individual units of software,  associated  data,  and  usage procedures are tested  to determine  whether  they  operate  correctly.  The  unit  is usually  a small piece of code—for  example,  a single function.  The unit test is a short  function  that  tests the behavior  of the unit  that produces  a pass/fail  result.  This is achieved by performing the tested  function  on a known  value with  a single correct  result. Unit tests often use mock objects to simulate the behavior of dependencies  in a predictable way.

The main  purpose  of unit  testing  is to allow  developers  to identify as many problems  as possible at the development stage and  to do it in an automated, repeatable fashion  that  can be applied for every code change.

This makes developers directly responsible for producing working   code,  even  before  it  reaches  the  quality   assurance team.
 

What Does Unit Testing Have to Do with Agile Development?

I think the two are closely linked. In fact, I believe you can’t be truly agile without implementing  automated unit testing as an  integral  part  of the  development process.  Automated unit testing has several benefits that align closely with agile development principles.

The   central   benefit   of  unit   testing   is  that   it  produces working  code  faster  and  with  fewer  bugs.  The  ability  to  automate  tests and  catch  bugs at the development stage reduces a huge amount of overhead  that  is otherwise  spent on releases that  are immediately  rejected  by QA due to basic functionalities being broken. Unit testing increases the chances of a new feature  working  correctly  upon  first delivery, as it becomes the developer’s responsibility to verify that he is delivering working code.

Another  reason  that  unit  tests  cut  down  on  development time is that  their  fine resolution allows  them  to pinpoint pre- cisely the  location  of a problem.  A failed unit  test can  direct the developer to the exact location  of the problem  in the code, allowing him to quickly resolve it. This minimizes or even eliminates  the time that  would  otherwise  be spent locating  the problem.

Unit testing may not be able to catch all bugs, but it is highly effective in catching regression bugs that are defects that break existing  functionality. These bugs hamper  progress  and  waste valuable  development and  QA resources  as code is sent back and forth between the two departments, delaying new versions of existing  products and  new product releases. Without automated  testing,  it is virtually  impossible  to  detect  bugs during the development phase.  This causes sprints  to become bogged down as developers need to spend more and more time fixing regression bugs in order to keep producing working software.  It becomes impossible  to maintain a steady and predictable software delivery schedule while also maintaining quality.  When a release date draws  near and the product is not working,  panic sets in, software  is released without enough  time to test it, and more bugs are introduced, creating a vicious cycle.

Code that  is not properly  maintained very quickly becomes legacy code that  developers  either refuse to change or insist on rewriting  themselves.  To keep code alive, you need to be able to change it and be confident that your changes won’t break anything.   Unit  testing  promotes   this  confidence.  Without it, you  end  up  either  refusing  to  change  older  code or  investing large amounts of time rewriting  it every so often.  In order  to respond  quickly  to change,  you need to be able to modify  all parts  of your  code  quickly  and  confidently.  Some tools  even allow you to develop  unit  tests for older  code without having to change the code itself.

Agility through Automation

The  platform for  unit  testing  is implicit,  and  we  usually omit  the  word  automated before  it. In reality,  unit  testing  is a collection  of processes,  skills, and  tools  that  support agility. For example,  writing  the tests is an actual  skill. I look at tests I wrote  five years ago and  think,  “How would  anyone  let me write this?” (I’m sure I’ll feel the same in five more years about what I’m writing now.)

In addition, using isolation  and mock objects correctly  is a capability  that  improves  over  time.  Refactoring of the  tested code or changing code design can fill up a three-day  workshop, and much like design, it can be improved  and lead to maintain- able test design.

When  we improve  our  skills,  we can  move  more  quickly and  change  directions  as we go with  agility.  But without automation, we won’t be able to use our skills effectively in a repeatable fashion.

Automation is the  foundation that  gives the  power  to  get quick  feedback  from  running  tests.  It gives us  the  ability  to cover more  code and  know  we didn’t  break  anything.  And it gives us the independence to change our design when we need to without risk and to mold the software  the way we want it.

In the  end,  the  Agile Manifesto favors  working  software. Automated unit tests bring us close to that  point  quicker  than other processes.

Conclusion

The benefits of unit testing are closely aligned with the principles  of agile software  development. Unit  testing  allows you to make code changes while remaining  confident that  they will not  break  existing  functionality and  that  the  major  part of new  functionality will work  on  first delivery.  This  enables frequent, timely delivery of working software, which in turn enables swift response  to changes in requirements. Automated unit  testing  also  promotes  a transparent view into  the  code’s health by producing reports  that allow anyone to see which problems  occur and their precise locations  in the code. Further, automated unit testing reduces the number  of regression  bugs, preventing  development sprints  from  becoming  bogged  down and  enabling  developers  to  maintain a  constant,  sustainable work pace.

Together  with the agile methodology, an integrated, automated  unit  testing  tool  that  works  well within  your  programming environment is a crucial  necessity for managing  modern software  development.