Test Driven Development (TDD)

Test Driven Development is a software development practice where tests are written before the production code they verify. The cycle is deceptively simple: write a failing test, write the minimum code to make it pass, then refactor. Repeat.

This practice — Red, Green, Refactor — is the heartbeat of how we develop software at Parlant GmbH. It is not merely a testing strategy; it is a design discipline.

The Cycle: Red, Green, Refactor

1. Red — Write a test for the next small piece of functionality. Run it. It should fail — because the functionality does not exist yet. The failing test confirms that the test is meaningful and that we understand what we are building.
2. Green — Write the simplest code that makes the test pass. Nothing more. No optimisation, no anticipation of future requirements — just enough to turn the test green.
3. Refactor — With a passing test as a safety net, improve the code. Remove duplication, clarify naming, simplify structure. The tests ensure that the behaviour remains unchanged.

This cycle typically takes minutes, not hours. Over the course of a day, a developer will complete dozens of these cycles — each one leaving the codebase slightly better than before.

Why TDD Produces Better Software

Design pressure. Writing the test first forces you to think about the interface before the implementation. How should this function be called? What should it return? What are the edge cases? These questions are answered before a single line of production code is written.

Regression safety. Every test stays in the suite. When a future change breaks something, the tests catch it immediately — not weeks later in production.

Confidence. A comprehensive test suite means you can refactor fearlessly. Change is no longer risky; it is routine.

Documentation. Tests describe what the system does in executable, unambiguous terms. They are the most reliable form of documentation — because they are verified on every build.

Common Misconceptions

“TDD slows us down.” — In the short term, writing tests takes time. In the medium to long term, TDD dramatically reduces debugging, reduces the cost of change, and prevents the accumulation of technical debt. The net effect is faster delivery, not slower.

“We can write tests afterwards.” — Tests written after the fact tend to verify the implementation rather than the behaviour. They miss the design benefits of TDD and are more fragile.

“TDD only works for simple code.” — The opposite is true. The more complex the system, the more valuable TDD becomes. It forces decomposition into testable, well-understood components.

TDD at Parlant

TDD is not optional in our engagements. Every feature, every bug fix, every refactoring begins with a test. This discipline — combined with pair programming and continuous integration — is what allows us to deliver software that is both innovative and reliable.

We have applied TDD in contexts ranging from digital form platforms to industrial planning software to multi-tenant data warehouses. The practice scales; the discipline pays off.