Cucumber, A Brief Overview


I’ve found Cucumber to be a pretty nice way of doing

high level acceptance/integration testing. It’s also the first time I’ve ever thought that ATDD (acceptance test driven

development) is actually achievable. The idea is that you write your “stories” in plain english, preferrably

with your client/customer/product owner using the given/when/then style of syntax. Once written, these stories can be

run through the Cucumber framework where it is parsed and coupled with custom ruby code (some of which you have to

write) to essentially give you executable specifications. Of all the attempts I’ve seen out there to achieve ATDD and

executable specifications, Cucumber really seems to have it nailed the best so far. Writing the stories does take some

getting used to as you do somewhat have to learn to speak the “language” of Cucumber. But after writing your first

couple “features”, as Cucumber calls them, you pretty much get the hang of it.


Here is an example of what a Cucumber feature looks like.


Feature: Add new user
    In order to allow a new user to access the system
    As an admin
    I want to add a new user to the system

    Scenario: New link is clicked from the users page
        Given I am logged in as an admin
        And I am on the users page
        When I follow "New"
        Then I should be on the new user page

    Scenario Outline: Required text fields are blank 
        Given I am logged in as an admin
        And I am on the new user page
        When I fill in "" for ""
        And I press "Save"
        Then I should see an error for ""

            | field_name |
            | First name |
            | Last name  |
            | Email      |
            | Password   |

The first section under the feature name is simply text to describe the feature in a typical As a/I want to/So that

format. That text is for display purposes only and is not used during execution.

Next comes the scenarios which are executed when run through Cucumber. The first scenario simply verifies that

when a link named “New” is clicked from the main users page, that it takes you to the “New User” page. This is a pretty

simple one, but it’s nice to have some of these as kind of smoke tests.

The second one is interesting in that it acts as an outline to keep your cucumber features DRY. If there is one thing

you’ll notice about the Ruby/Rails community and the tools/frameworks used is that there is an extreme emphasis on

keeping things DRY, which I love. What this outline does is use tokens in the actual scenario steps, then specify a

simple table representation that is used to plug in each “example” value when the scenario is run. So this scenario

outline actually gets run 4 different times, once with each value for listed. The closest parallel I can

think of in .NET land is RowTests with MBUnit, but I’m sure there are others such as Fitness, which I haven’t used too


I’ve used Cucumber to drive out almost every feature so far in this Rails app I’m building. And I can definitely

say it’s been worth it. Especially when I go to update my gems to a new version or even try out the edge version

straight from the Git repo. I’ve been able to lean on my Cucumber features to give me a nice sense at a high level

as to the health of my application from the user’s point of view.

Great, so how does all this work?

Under the hood, Cucumber is actually spinning up the Rails app, opening the pages you tell it and clicking links and

filling out text fields on the actual page. It does this using a web testing framework such as Capybara or Webrat.

Each one comes with a set of pre-defined steps that you can use to cover a lot of the common actions you need to do

on a web page such as clicking links/buttons, filling out forms and inspecting the content on the page using css or

xpath helpers. Capybara seems to be taking over Webrat’s long rule in this space and is the default option when

getting Cucumber set up. There is also the ability to test javascript/AJAX using Capybara with either EnvJs or

Culerity/Celerity. Unfortunately I haven’t been able to get this working yet, but I plan to very soon because

I’m already starting to build some non-trival client side stuff now that I’d really like to get some automated

tests around.

No, really, how DOES this work?!?

Ok, so the “magic” of Cucumber is all in the step definitions, which are defined using Ruby. There is also some

supporting Ruby code that you get out of the box with Cucumber that can help get you started. For example, it

creates a file named “paths.rb” for you which is where you tell Cucumber where to navigate given a set of text.

So when Cucumber encounters a line like this in your plain text scenario, “And I am on the users page”, it

automatically knows the phrase “I am on” and signals to Cucumber that you’re wanting to navigate somewhere.

Then it looks at the text after that, which is “the users page” in this case. This is where you need to give

Cucumber a little help.

Here is an example of how to help a cuke out.


module NavigationHelpers
    def path_to(page_name)
      case page_name
        when /the users page/
          users_path  # rails routing helper method
        when /the new user page/
          new_user_path  # rails routing helper method

Since the Cucumber feature files are plain text and you are encouraged to use whatever language feels

natural to you and your customer, you’re going to inevitably write something that Cucumber just doesn’t

know how to do out of the box. This is where custom step definitions come into play. But don’t worry,

they’re not as scary as they may look.

From the scenario above, Cucumber has no idea how to interpret “Given I am logged in as an admin” without

a little help. Below you can see that I’m specifiying a regular expression for the text that Cucumber

needs help with and then simply using Capybara to visit a page, fill in a couple fields and click a button

to perform the login using the actual login page. Don’t let the regular expressions stuff scare you off.

The awesome thing about running your Cucumber features is that when it comes across something it doesn’t

understand it gives you the exact snippet of Ruby code that you can literally copy/paste from the terminal

into a custom step file like the one show below. Since I’m still not a regular expression guru, I LOVE that

Cucumber helps me with that!


Given /^I am logged in as an admin$/ do
    user = Factory.create(:admin) # I'll get to this later (factory_girl)
    fill_in('Email', :with =>
    fill_in('Password', :with => user.password)

Cucumber integration with Capybara (or Webrat) comes with a nice set of web related steps that cover a lot

of the common cases. But at some point you’ll definitely want to define your own web related steps. I just

created a file named “custom_web_steps.rb” to place them in. This is a simple web step I defined to allow me

to assert that an error is shown on the page containing the text I supply, which comes from my plain text scenario.

Notice that you can chain together other web steps within each other like I’m doing below. My custom web step below

is simply leveraging one of the built in Capybara web steps to do what it needs.


Then /^I should see an error for "([^"]*)"$/ do |text|
    Then "I should see "#{text}" within ".error""

Clear as mud?

Well hopefully that gives you a little insight into how to get going with using Cucumber to drive top-down development

using acceptance testing. At least, this has been my experience with it so far. I’d be very interested to hear your

feedback and some of the ways you’ve been successful with acceptance testing using Cucumber or any other framework for

that matter.

The Ruby/Rails Life – My Rails 3 Stack – Part 2