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RSpec vs Cucumber comparison of testing frameworks
What are the differences between RSpec and Cucumber?

RSpec

https://rspec.info

Cucumber

https://github.com/cucumber/cucumber-ruby
Programming language

Ruby

Ruby

Category

Unit Testing, Intergration Testing

Acceptance Testing

General info

Domain Specific Language (DSL) testing solution for Ruby code

It focuses on empowering Test Driven Development (TDD). RSpec contains multiple smaller libraries, which may be independently used with other testing frameworks.

An automation tool for Behavior-Driven Development

The specifications are written in plain texts, which allows them to be easily understandable for all stakeholders. Cucumber Framework also supports languages beyond Ruby e.g. Java, JavaScript and Scala.
xUnit
Set of frameworks originating from SUnit (Smalltalk's testing framework). They share similar structure and functionality.

Yes

Yes, but it differs from more traiditional xUnit solutions. Its tests are written in a "Tests as Specification" manner. This means that the terminology used in RSpec is adjusted to a language more fitting for specification.

No

Client-side
Allows testing code execution on the client, such as a web browser

Yes

RSpec is used to primarily test the behaviour of applications or individual components so it can test front-end behaviour as well - you can use capybara gem with RSpec for that.

Yes

You can test the front-end part like the GUI using cucumber and selenium, they integrate well to test your front-end.
Server-side
Allows testing the bahovior of a server-side code

Yes

You can test server-side behaviour with Rspec

Yes

You can test back-end components such as APIs using rest & soap clients, and databases using whatever client libraries were provided by the libraries that existed in those stacks
Fixtures
Allows defining a fixed, specific states of data (fixtures) that are test-local. This ensures specific environment for a single test

Yes

RSpec does contain fixture methods

Group fixtures
Allows defining a fixed, specific states of data for a group of tests (group-fixtures). This ensures specific environment for a given group of tests.

Yes

Group fixture methods are supported

Yes

Using the cucumber extension aruba you can create fixures in two steps: 1.Create a fixtures-directory; 2.Create fixture files in this directory
Generators
Supports data generators for tests. Data generators generate input data for test. The test is then run for each input data produced in this way.

Yes

RSpec contains generators for example intergration_'test_name' which will save a spec inside the spec/requests folder

Yes

You can group your fixtures inside your fixtures directories
Licence
Licence type governing the use and redistribution of the software

MIT License

MIT License

Mocks
Mocks are objects that simulate the behavior of real objects. Using mocks allows testing some part of the code in isolation (with other parts mocked when needed)

Yes

Available through rspec-mocks gem.

Yes

By using all of RSpec’s supported mocking frameworks (RSpec, Mocha, RR, Flexmock)
Grouping
Allows organizing tests in groups

Yes

Allows declaring example groups and contexts.

Yes

Cucumber allows the use of tag Cucumber feature files or individual tests to group tests. Then, you can pass a Cucumber tagged expression at test execution that specifies the tag (grouped) tests to run. https://www.toolsqa.com/cucumber/cucumber-tags/
Other
Other useful information about the testing framework