# ADR-001: Test-Driven Development (TDD) First Approach

## Status
**ACCEPTED** - 2024-09-08

## Context
As a data engineering consultancy specializing in Rust and Leptos, we need to establish a clear technical philosophy that ensures the highest quality deliverables for our clients. Our reputation depends on delivering robust, reliable, and maintainable solutions.

## Decision
We adopt a **Test-Driven Development (TDD) first approach** as our core development methodology.

### TDD Process
1. **Red**: Write a failing test first
2. **Green**: Write minimal code to make the test pass
3. **Refactor**: Improve code while keeping tests green
4. **Repeat**: Continue the cycle for each feature

### TDD Principles
- **No production code without tests**: Every line of production code must have corresponding tests
- **Tests drive design**: Tests define the API and behavior before implementation
- **Fail fast**: Tests catch issues immediately during development
- **Living documentation**: Tests serve as executable documentation
- **Confidence in refactoring**: Comprehensive test coverage enables safe code improvements

## Consequences

### Positive
- **Higher code quality**: TDD forces better design and cleaner code
- **Reduced bugs**: Issues caught early in development cycle
- **Faster debugging**: Immediate feedback on code changes
- **Better API design**: Tests force consideration of usage patterns
- **Client confidence**: Demonstrable quality through comprehensive testing
- **Easier maintenance**: Well-tested code is easier to modify and extend

### Negative
- **Initial development time**: Writing tests first may seem slower initially
- **Learning curve**: Team must be trained in TDD practices
- **Test maintenance**: Tests require ongoing maintenance as code evolves

### Mitigation
- **Training investment**: Comprehensive TDD training for all team members
- **Tooling support**: Automated testing tools and CI/CD integration
- **Code reviews**: Ensure TDD practices are followed in all pull requests

## Implementation

### Development Workflow
1. **Feature planning**: Define acceptance criteria and test scenarios
2. **Test writing**: Write failing tests for new functionality
3. **Implementation**: Write minimal code to pass tests
4. **Refactoring**: Improve code structure while maintaining test coverage
5. **Integration**: Ensure all tests pass in CI/CD pipeline

### Quality Gates
- **Pre-commit hooks**: Run tests before code commits
- **Pull request requirements**: All tests must pass before merge
- **Coverage thresholds**: Maintain minimum 95% test coverage
- **Performance tests**: Include performance benchmarks in test suite

### Tools and Technologies
- **Rust**: Built-in testing framework with `cargo test`
- **Playwright**: End-to-end testing for web applications
- **wasm-bindgen-test**: WebAssembly testing framework
- **Criterion**: Benchmarking and performance testing
- **Tarpaulin**: Code coverage analysis

## Examples

### Rust Unit Test Example
```rust
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_data_pipeline_processes_records() {
        // Given
        let input_data = vec!["record1", "record2", "record3"];
        let pipeline = DataPipeline::new();
        
        // When
        let result = pipeline.process(input_data);
        
        // Then
        assert_eq!(result.len(), 3);
        assert!(result.iter().all(|r| r.is_processed()));
    }
}
```

### Playwright E2E Test Example
```typescript
test('should process data pipeline successfully', async ({ page }) => {
  // Given
  await page.goto('/pipeline');
  
  // When
  await page.fill('[data-testid="input-data"]', 'test-data');
  await page.click('[data-testid="process-button"]');
  
  // Then
  await expect(page.locator('[data-testid="result"]')).toBeVisible();
  await expect(page.locator('[data-testid="success-message"]')).toContainText('Processing complete');
});
```

## Monitoring and Metrics
- **Test coverage**: Track and report test coverage metrics
- **Test execution time**: Monitor test suite performance
- **Bug escape rate**: Measure bugs found in production vs. tests
- **Development velocity**: Track feature delivery with TDD approach

## Review and Updates
This ADR will be reviewed quarterly to ensure TDD practices remain effective and aligned with project needs. Updates will be made based on team feedback and industry best practices.

## Related ADRs
- ADR-002: Testing Pyramid Strategy
- ADR-003: Playwright Testing for Demos
- ADR-004: Code Coverage Standards