May 2, 2017 2:19:03 PM | What Is Behavior-Driven Development and How To Use It

A close examination of the behavior-driven development software development methodology with example user stories, scenarios, and more.

The other week we covered the first part of a miniature series covering a trifecta of software development life cycle articles beginning with Object-Oriented Analysis and Design - What is it and how do you use it?. Shortly after its introduction, Object-Oriented Analysis and Design was the stepping off point for another new software design pattern, which we covered a few days later in Domain-Driven Design - What is it and how do you use it?. A few weeks prior, we also explored Test-Driven Development, which is also related to today's topic.

The next logical step in this series of models is behavior-driven development (BDD), which is the topic we'll be exploring today. Throughout this article we'll examine just what BDD is, how it is commonly put into practice, and any potential advantages or disadvantages you might encounter when implementing behavior-driven developmenttechniques into your own projects. Let's get crackin'!

What is Behavior-Driven Development?

At its core, behavior-driven development is a software development methodology that combines practices from test-driven development (TDD) and domain-driven design (DDD).

As we saw in our previous article, test-driven development focuses development on short, cyclical iterations in which (failing) tests are initially created that define the desired functionality, and only then is actual code written that ensures those previously-failing tests can now pass. When performed consistently and rapidly, this TDD structure can dramatically reduce overall development time and lower reported bug counts, since code bases tend to be stronger and more stable throughout the software development life cycle.

As we also explored in a prior article, domain-driven design centers on the concept of domain and domain logic, which simply encompass the overall "sphere of knowledge or activity around which application logic revolves." Thus, DDD practices attempt to simplify the terminology within the project scope by focusing and defining everything in the application as real-world objects or concepts that most people are familiar with. This largely simplifies communication and encourages the team to develop an application that precisely fits the needs of the particular domain in question.

Thus, by combining parts of both TDD and DDD, behavior-driven development aims to simplify development through the use of a common domain-specific language (DSL), which is used to adapt natural language sentences and phrases into executable tests. Whether it is recognized or not, this practice of using natural language sentences to describe and define tests is used in a plethora of programming languages and testing suites. As a simple example, the built-in testing framework for Ruby on Rails allows tests to be defined like so:

test "User should have a valid email address." do
# Test code to be executed to check for valid email address.

While this is treated behind-the-scenes as a normal method, to the end-user when executing the test it's easy to quickly differentiate between this test and others, while also immediately recognizing what this test aims to prove or disprove.

The Principles of Behavior-Driven Development

At its core, behavior-driven development expands on TDD and DDD by narrowing in on the notion of behavior. While "loose" TDD allows for tests to focus on all requirement levels within the application, BDD states that tests should be defined in terms of the "desired behavior of the unit." As we learned about from our DDD article, this behavior is best defined as the relevant business logic or domain logic for that particular software unit.

Beyond that overall focus on behavior, BDD also specifies a handful of principles that should be put into practice.

Behavioral Specifications

Typically, defining behaviors within BDD is accomplished through user stories. These are written-out scenarios that include some sort of baseline title that summarizes the intent, a narrative section that describes the who and what that should be involved in achieving this story requirement, and the scenarios section that describes a series of specific scenarios via if-then-style conditions.

While BDD doesn't enforce any one particular syntax or format for these user stories, it does suggest that your team standardize a format to abide by. This will ensure that your team can continue to easily discuss and modify stories, and multiple team members can create stories without the need to work closely together.

Here is a typical user story format used in BDD projects, as recommended by Dan North, who is considered to more or less be the "founder" of BDD:

Title (one line describing the story)

As a [role]
I want [feature]
So that [benefit]

Acceptance Criteria: (presented as Scenarios)

Scenario 1: Title
Given [context]
And [some more context]...
When [event]
Then [outcome]
And [another outcome]...

Scenario 2: ...

Expanding on this template, Mr. North provides a filled out user story example in his Introducing BDD article:

Story: Account Holder withdraws cash

As an Account Holder
I want to withdraw cash from an ATM
So that I can get money when the bank is closed

Scenario 1: Account has sufficient funds
Given the account balance is \$100
And the card is valid
And the machine contains enough money
When the Account Holder requests \$20
Then the ATM should dispense \$20
And the account balance should be \$80
And the card should be returned

Scenario 2: Account has insufficient funds
Given the account balance is \$10
And the card is valid
And the machine contains enough money
When the Account Holder requests \$20
Then the ATM should not dispense any money
And the ATM should say there are insufficient funds
And the account balance should be \$20
And the card should be returned

Scenario 3: Card has been disabled
Given the card is disabled
When the Account Holder requests \$20
Then the ATM should retain the card
And the ATM should say the card has been retained

Ubiquitous Language

Just as we discussed in our Domain-Driven Design article, behavior-driven development heavily emphasizes the importance of a ubiquitous language, which is also commonly referred to as domain-specific language or DSL. The DSL should be clearly defined and agreed upon by all team members early in the development life cycle. DSLallows for easy communication about the domain of the project, and should be both simple and robust enough to support discussion between all types of personnel, from developers and team leaders to customers and business executives.

Using Specialized Tools

Behavior-driven development is heavily supported by specialized tools that aid in the creation and execution of testing suites. Just like automated testing tools used in test-driven development, BDD tools will similarly perform automated tests in an aim to streamline the development process. The big difference, however, between TDD and BDD testing tools is that BDD tools are tightly linked to the DSL that has been defined for the project. As such, test specifications inside typical BDD testing tools will aim to directly copy the language and phrases from the DSL user stories that have already been defined.

Looking back at Scenario 1 from Dan North's Account Holder withdraws cash story above, for example, we can see how that scenario might be translated into actual automated test code (using the Ruby programming language, in this particular case):

Scenario 1: Account has sufficient funds
Given the account balance is \$100
And the card is valid
And the machine contains enough money
When the Account Holder requests \$20
Then the ATM should dispense \$20
And the account balance should be \$80
And the card should be returned

And here is the pseudocode that tests for that scenario in Ruby (using Cucumber syntax):

Given /^the account balance is \$100$/ do
Account.balance = 100

And /^the card is valid$/ do
Account.card.valid = true

And /^the machine contains enough money$/ do
Machine.balance = 10000

When /^the Account Holder requests \$20$/ do

Then /^the ATM should dispense \$20$/ do
expect(Account.dispensation).to be 20

And /^the account balance should be \$80$/ do
expect(Account.balance).to be 80

And /^the card should be returned$/ do
expect(Machine.card?).to be false

Advantages of Behavior-Driven Development

Since behavior-driven development is heavily derived from and influenced by test-driven development, many of the same benefits that apply to TDD also apply to BDD.

  • Reduces Regression: With a full suite of tests being continually executed, and with new tests always being added, BDD dramatically reduces the likelihood of regression bugs popping up, since the codebase will be in a constant state of monitoring and testing.
  • Improves Team Communication: The reliance on a well-defined ubiquitous language/DSL means that BDDcan often improve communication across the entirety of the team, or even among organizations, since there is a common, real-life basis for phrases and terminology when discussing the project.

Disadvantages of Behavior-Driven Development

  • Requires Specification Before Development: For better or worse, behavior-driven development requires that the team sit down and write out both the DSL and in-depth specification documentation (user stories) for each particular scenario or feature, before even a single line of functional code can be written. For many teams, particularly dealing with larger projects, this restriction may not be an issue, but for smaller teams and rapid projects, that extra effort and requirement may be more hindrance than help.
  • Relies on Constant Outside Feedback: While staying in touch with users, customers, or domain experts may not be a problem for some teams, for many organizations this requirement of constant contact with outside people can have a negative impact on development time. In cases where feedback is required to flesh out a new user story or scenario prior to tests being written -- let alone functional code -- if the relevant domain expert is unavailable at that time, development can grind to a halt (or be forcibly redirected, which is effectively the same thing in many cases).

Written By: Frances Banks