Behavior Driven Development

Behavior-driven development (or BDD) is an agile software development technique devised by Dan North as a response to the issues he encountered whilst teaching Test-Driven Development:

• Where to start
• What to test and what not to test
• How much to test in one go
• What to call the tests
• How to understand why a test fails

At the heart of BDD is a rethinking of the approach to unit testing and acceptance testing: unit test names are whole sentences starting with the word "should" and they are written in order of business value. Acceptance tests are written using the standard agile framework of a User story: "As a [role] I want [feature] so that [benefit]". Acceptance criteria are written in terms of scenarios and implemented as classes: Given [initial context], when [event occurs], then [ensure some outcomes].

BDD encourages collaboration between developers, QA and non-technical or business participants in a software project. It has evolved over the last few years.

On the "Agile specifications, BDD and Testing eXchange" in November 2009 in London, Dan North gave the following description of BDD:

BDD is a second-generation, outside–in, pull-based, multiple-stakeholder, multiple-scale, high-automation, agile methodology. It describes a cycle of interactions with well-defined outputs, resulting in the delivery of working, tested software that matters.

BDD focuses on obtaining a clear understanding of desired software behavior through discussion with stakeholders. It extends TDD by writing test cases in a natural language that non-programmers can read. Behavior-driven developers use their native language in combination with the ubiquitous language of domain-driven design to describe the purpose and benefit of their code. This allows the developers to focus on why the code should be created, rather than the technical details, and minimizes translation between the technical language in which the code is written and the domain language spoken by the business, users, stakeholders, project management, etc.

Dan North created the first ever BDD framework, JBehave, followed by a story-level BDD framework for Ruby called RBehave which was later integrated into the RSpec project. He also worked with David Chelimsky, Aslak Hellesøy and others to develop RSpec and also to write "The RSpec Book: Behaviour Driven Development with RSpec, Cucumber, and Friends". The first story-based framework in RSpec was later replaced by Cucumber mainly developed by Aslak Hellesøy.

In 2008, Chris Matts, who was involved in the first discussions around BDD, came up with the idea of Feature Injection, allowing BDD to cover the analysis space and provide a full treatment of the software lifecycle from vision through to code and release.

BDD practices

The practices of BDD include:

• Establishing the goals of different stakeholders required for a vision to be implemented
• Drawing out features which will achieve those goals using feature injection
• Involving stakeholders in the implementation process through outside–in software development
• Using examples to describe the behavior of the application, or of units of code
• Automating those examples to provide quick feedback and regression testing
• Using 'should' when describing the behavior of software to help clarify responsibility and allow the software's functionality to be questioned
• Using 'ensure' when describing responsibilities of software to differentiate outcomes in the scope of the code in question from side-effects of other elements of code.
• Using mocks to stand-in for collaborating modules of code which have not yet been written

Outside–in

BDD is driven by business value; that is, the benefit to the business which accrues once the application is in production. The only way in which this benefit can be realized is through the user interface(s) to the application, usually (but not always) a GUI.

In the same way, each piece of code, starting with the UI, can be considered a stakeholder of the other modules of code which it uses. Each element of code provides some aspect of behavior which, in collaboration with the other elements, provides the application behavior.

The first piece of production code that BDD developers implement is the UI. Developers can then benefit from quick feedback as to whether the UI looks and behaves appropriately. Through code, and using principles of good design and refactoring, developers discover collaborators of the UI, and of every unit of code thereafter. This helps them adhere to the principle of YAGNI, since each piece of production code is required either by the business, or by another piece of code already written.

Application examples in the Gherkin language

The requirements of a retail application might be, "Refunded or exchanged items should be returned to stock."

In BDD, a developer or QA engineer might clarify the requirements by breaking this down into specific examples, e.g.

Note: The language of the examples below is called Gherkin and is used in cucumber for ruby , lettuce for python , specflow for dotnet and behat for php .

Scenario 1: Refunded items should be returned to stock

• Given a customer previously bought a black sweater from me
• and I currently have three black sweaters left in stock
• when he returns the sweater for a refund
• then I should have four black sweaters in stock

Scenario 2: Replaced items should be returned to stock

• Given that a customer buys a blue garment
• and I have two blue garments in stock
• and three black garments in stock.
• When he returns the garment for a replacement in black,
• Then I should have three blue garments in stock
• and two black garments in stock

Each scenario is an exemplar, designed to illustrate a specific aspect of behavior of the application.

When discussing the scenarios, participants question whether the outcomes described always result from those events occurring in the given context. This can help to uncover further scenarios which clarify the requirements. For instance, a domain expert noticing that refunded items are not always returned to stock might reword the requirements as "Refunded or replaced items should be returned to stock, unless faulty.".

This in turn helps participants to pin down the scope of requirements, which leads to better estimates of how long those requirements will take to implement.

The words Given, When and Then are often used to help drive out the scenarios, but are not mandated.

These scenarios can also be automated, if an appropriate tool exists to allow automation at the UI level. If no such tool exists then it may be possible to automate at the next level in, i.e.: if an MVC design pattern has been used, the level of the Controller.

Programmer-domain examples and behavior

The same principles of examples, using contexts, events and outcomes are used to drive development at the level of abstraction of the programmer, as opposed to the business level. For instance, the following examples describe an aspect of behavior of a list:

Example 1: New lists are empty

• Given a new list
• Then the list should be empty.

Example 2: Lists with things in them are not empty.

• Given a new list
• When we add an object
• Then the list should not be empty.

Both these examples are required to describe the behavior of the

list.isEmpty()

method, and to derive the benefit of the method. These examples are usually automated using TDD frameworks. In BDD these examples are often encapsulated in a single method, with the name of the method being a complete description of the behavior. Both examples are required for the code to be valuable, and encapsulating them in this way makes it easy to question, remove or change the behavior.

For instance, using Java and JUnit 4, the above examples might become:

public class ListTest {
 
   @Test
   public void shouldKnowWhetherItIsEmpty() {
      List list1 = new List();
      assertTrue(list1.isEmpty());
 
      List list2 = new List();
      list2.add(new Object());
      assertFalse(list2.isEmpty());
   }
}

Other practitioners, particularly in the Ruby community, prefer to split these into two separate examples, based on separate contexts for when the list is empty or has items in. This technique is based on Dave Astels' practice, "One assertion per test".

Sometimes the difference between the context, events and outcomes is made more explicit. For instance:

public class WindowControlBehavior {
 
    @Test
    public void shouldCloseWindows() {
 
        // Given
        WindowControl control = new WindowControl("My AFrame");
        AFrame frame = new AFrame();
 
        // When
        control.closeWindow();
 
        // Then
        ensureThat(!frame.isShowing());       
    }
}

However the example is phrased, the effect describes the behavior of the code in question. For instance, from the examples above one can derive:

• List should know when it is empty
• WindowControl should close windows

The description is intended to be useful if the test fails, and to provide documentation of the code's behavior. Once the examples have been written they are then run and the code implemented to make them work in the same way as TDD. The examples then become part of the suite of regression tests.

Using mocks

BDD proponents claim that the use of "should" and "ensureThat" in BDD examples encourages developers to question whether the responsibilities they're assigning to their classes are appropriate, or whether they can be delegated or moved to another class entirely. Practitioners use an object which is simpler than the collaborating code, and provides the same interface but more predictable behavior. This is injected into the code which needs it, and examples of that code's behavior are written using this object instead of the production version.

These objects can either be created by hand, or created using a mocking framework such as Mockito, Moq, NMock, Rhino Mocks, JMock, Simple Mocking or EasyMock.

Questioning responsibilities in this way, and using mocks to fulfill the required roles of collaborating classes, encourages the use of Role-based Interfaces. It also helps to keep the classes small and loosely coupled.

External links

• Dan North's article introducing BDD
• Introduction to Behavior Driven Development
• Say Hello To Behavior Driven Development (BDD)- Part 1
• Say Hello To Behavior Driven Development (BDD)- Part 2
• Behavior Driven Development Using Ruby (Part 1)
• Behavior-Driven Development Using Ruby (Part 2)
• In pursuit of code quality: Adventures in behavior-driven development by Andrew Glover
• The RSpec Book: Behaviour Driven Development with RSpec, Cucumber, and Friends
• CBehave: A Behavior Driven Development Framework for C