| name | defense-in-depth |
| description | Multi-layer validation pattern - validates data at EVERY layer it passes through to make bugs structurally impossible, not just caught. |
| trigger | - Bug caused by invalid data reaching deep layers - Single validation point can be bypassed - Need to prevent bug category, not just instance |
| skip_when | - Validation already exists at all layers → check other issues - Simple input validation sufficient → add single check |
| related | [object Object] |
Defense-in-Depth Validation
Overview
When you fix a bug caused by invalid data, adding validation at one place feels sufficient. But that single check can be bypassed by different code paths, refactoring, or mocks.
Core principle: Validate at EVERY layer data passes through. Make the bug structurally impossible.
Why Multiple Layers
Single validation: "We fixed the bug" Multiple layers: "We made the bug impossible"
Different layers catch different cases:
- Entry validation catches most bugs
- Business logic catches edge cases
- Environment guards prevent context-specific dangers
- Debug logging helps when other layers fail
The Four Layers
| Layer | Purpose | Example |
|---|---|---|
| 1. Entry Point | Reject invalid input at API boundary | if (!workingDir || !existsSync(workingDir)) throw new Error(...) |
| 2. Business Logic | Ensure data makes sense for operation | if (!projectDir) throw new Error('projectDir required') |
| 3. Environment Guards | Prevent dangerous ops in contexts | if (NODE_ENV === 'test' && !path.startsWith(tmpdir())) throw... |
| 4. Debug Instrumentation | Capture context for forensics | logger.debug('About to git init', { directory, cwd, stack }) |
Applying the Pattern
Steps: (1) Trace data flow (origin → error) (2) Map all checkpoints (3) Add validation at each layer (4) Test each layer (try to bypass layer 1, verify layer 2 catches it)
Example
Bug: Empty projectDir caused git init in source code
Flow: Test setup ('') → Project.create(name, '') → WorkspaceManager.createWorkspace('') → git init in process.cwd()
Layers added: L1: Project.create() validates not empty/exists/writable | L2: WorkspaceManager validates not empty | L3: Refuse git init outside tmpdir in tests | L4: Stack trace logging
Result: 1847 tests passed, bug impossible to reproduce
Key Insight
All four layers necessary - each caught bugs others missed: different code paths bypassed entry validation | mocks bypassed business logic | edge cases needed environment guards | debug logging identified structural misuse.
Don't stop at one validation point. Add checks at every layer.