Reputation: 841

TDD and BDD Differences

I honestly don't see the difference between BDD and TDD. I mean, both are just tests if what is expected happens. I've seen BDD Tests that are so fleshed out they practically count as TDD tests, and I've seen TDD tests that are so vague that they black box a lot of code. Let's just say I'm pretty convinced that having both is better.

Here's a fun question though. Where do I start? Do I start out with high level BDD tests? Do I start out with low level TDD tests?

Upvotes: 59

Answers (10)

Liberty Lover

Reputation: 1002

Overall

BDD is really Design-by-Contract using different terms. Generally speaking, BDD is in the form of Given-When-Then, which is roughly analogous to Preconditions (Given), Check-conditions/Loop-invariants (When), and Post-conditions/Invariants (Then).

Notice

Note that BDD is very much Hoare-logic (i.e. {P}C{Q} or {P}recondition-[C]ommand-{Q}Post-condition). Therefore:

Preconditions (Given) must hold true for the command (method/function) to compute correctly. Any violation of the Given (precondition) signals a fault in the calling Client code.
Command(s) (When) are what happens after the precondition(s) are met. In Eiffel, they can be punctuated within the method or function code with other contracts. Think about these as though they are QA/QC checks along a process assembly line.
Post-conditions (Then) must hold true once the Command (When) is finished.

Moral of the Story

Because BDD is just DbC (Hoare-logic) repackaged in different words, this means it is not TDD. Why? Because TDD is not about preconditions/checks/post-condition contracts tied directly to methods, functions, properties, and class-state. TDD is the next step up the ladder in testing methods, functions, properties, and classes with their discrete states. Once you see this and fully appreciate that TDD is not BDD and BDD is not TDD, but that they are separate and complementary technologies for software correctness proofs—THEN—you will finally understand these topics correctly. You will also use and apply them correctly.

Conclusion

Eiffel is the only language I am aware of where BDD (Design-by-Contract) is baked raw into both the language specification and compiler. It is not a Frankenstein bolt-on monster with limitations. In Eiffel, BDD (aka DbC) is an elegant, helpful, useful, and direct participant in the software correctness toolbox.

Behavior Driven Development is an extension/revision of Test Driven Development. Its purpose is to help the folks devising the system (i.e., the developers) identify appropriate tests to write -- that is, tests that reflect the behavior desired by the stakeholders. The effect ends up being the same -- develop the test and then develop the code/system that passes the test. The hope in BDD is that the tests are actually useful in showing that the system meets the requirements.

UPDATE

Units of code (individual methods) may be too granular to represent the behavior represented by the behavioral tests, but you should still test them with unit tests to guarantee they function appropriately. If this is what you mean by "TDD" tests, then yes, you still need them.

Upvotes: 6

k3b

Reputation: 14755

BDD is from customers point of view and focuses on excpected behavior of the whole system.

TDD is from developpers point of view and focuses on the implementation of one unit/class/feature. It benefits among others from better architecture (Design for testability, less coupling between modules).

From technical point of view (how to write the "test") they are similar.

I would (from an agile point of view) start with one bdd-userstory and implement it using TDD.

Upvotes: 43

Rahul Garg

Reputation: 4329

The main difference is just the wording. BDD uses a more verbose style so that it can be read almost like a sentence.

Upvotes: -1

Anshuman

Reputation: 31

BDD is about getting your TDD right. It provides "structure and diciplene" to your TDD. It guides you in testing the right thing and doing the right amount of test. Here is a fantastic small post on BDD and TDD,

http://codingcraft.wordpress.com/2011/11/12/bdd-get-your-tdd-right/

Upvotes: 3

Huan

Reputation: 297

Terminology are different, but in my work, i use TDD to dev detail, mainly for unit test, and the BDD is more high level, for customer, QA or no-tech man .

Upvotes: 1

Mark Mayo

Reputation: 12585

A fantastic article on the differences between TDD and BDD:

http://www.lostechies.com/blogs/sean_chambers/archive/2008/12/07/starting-with-bdd-vs-starting-with-tdd.aspx

Should give you everything you need to know, including problems with both, and examples.

Upvotes: 2

Jörg W Mittag

Reputation: 369428

I honestly don't see the difference between BDD and TDD.

That's because there isn't any.

I mean, both are just tests if what is expected happens.

That's wrong. BDD and TDD have absolutely nothing whatsoever to do with testing. None. Nada. Zilch. Zip. Nix. Not in the slightest.

Unfortunately, TDD has the word "test" in pretty much everything (not only in its name, but also in test framework, unit test, TestCase (the class you tpyically inherit from), FooTest (the class which typically holds your tests), testBar (the typical naming pattern for a test method), plus a lot test-related terminology such as "assertion" and "verification") which leads some people to believe that it actually does have something to do with tests. So, some smart people said: "Hey, let's just change the name" to remove any potential for confusion.

And that's what BDD is. It's just TDD with any test-related terminology replaced by examples-of-behavior-related terminology:

Test → Example
Assertion → Expectation
assert → should
Unit → Behavior
Verification → Specification
… and so on

BDD is just TDD with different words. If you do TDD right, you are doing BDD. The difference is that – provided you believe at least in the weak form of the Sapir-Whorf Hypothesis – the different words make it easier to do it right.

Upvotes: 88

Daniel Fath

Reputation: 18059

From what I've gathered on Wikipedia, BDD includes acceptance and QA test that can't be done without stakeholders/user input. Also BDD uses a natural language to specify its test while TDD usually uses programming language. There might be some overlap between the two but I think it's not the vagueness but BDD's language that is the main difference.

As for where you are to start, well that really depends on your development process, doesn't it? I assume if you are doing bottom-up that you're going to write TDD first and once you reach higher level you'll use BDD to test if those features work as expected.

As k3b noted: main difference would be that BDD is problem-domain oriented while TDD is more oriented solution-domain.

Upvotes: 8