Technique: Defense in Depth (Swiss Cheese Model)

Concept: After fixing the root cause, you must add "Armor" to the system to catch this specific class of error in the future.

The Three Layers of Defense

Apply at least two of these layers for every critical bug.

Layer 1: The API Boundary (The "Bouncer")

Stop bad data from entering your house.

• •Technique: Schema Validation (Zod, Joi, Pydantic).
• •Action: If price must be positive, enforce price > 0 at the JSON parsing layer.
• •Result: The system rejects the request with 400 Bad Request instantly. No internal logic runs.

Layer 2: The Service Boundary (The "Contract")

Stop bad data from moving between rooms.

• •Technique: Domain Types / Value Objects.
• •Action: Don't pass string email. Pass a EmailAddress class that throws an error in its constructor if the format is invalid.
• •Result: You cannot instantiate an invalid object deep in the business logic.

Layer 3: The Database Constraint (The "Vault")

Stop bad data from being written to history.

• •Technique: SQL Constraints / Foreign Keys.
• •Action: Add NOT NULL constraints. Add CHECK (price >= 0).
• •Result: Even if code fails, the database refuses to accept the corruption.

The Checklist

When closing a bug, ask:

1. • Did I fix the root cause?
2. • Defense: Did I add a validation check so this inputs fails early next time?
3. • Defense: Did I verify the database prevents this state?

Example:

• •Bug: User verified with empty email.
• •Fix: Updated regex in signup logic.
• •Defense 1: Added class EmailAddress { ... } which validates on creation.
• •Defense 2: Added CHECK (length(email) > 5) to Postgres migration.