# Component Inheritance

_Atmos Design Pattern_

The **Component Inheritance** pattern allows an Atmos component to inherit configuration from a base component, then override specific values. This creates DRY configurations where common settings are defined once and reused.

## Use-cases

Use the **Component Inheritance** pattern when:

- Multiple components share common configuration

- You want to define defaults once and override only what differs

## How It Works

A component inherits from a base component using `metadata.inherits`:

**File:** `stacks/catalog/vpc/defaults.yaml`

```yaml
components:
  terraform:
    vpc/defaults:
      vars:
        enabled: true
        nat_gateway_enabled: true
        max_subnet_count: 3
```

**File:** `stacks/deploy/dev.yaml`

```yaml
import:
  - catalog/vpc/defaults

components:
  terraform:
    vpc:
      metadata:
        component: vpc
        inherits:
          - vpc/defaults
      vars:
        # Override only what's different
        max_subnet_count: 2
```

The `vpc` component inherits all vars from `vpc/defaults`, then overrides `max_subnet_count`.

## Inheritance Order

When multiple sources define the same value, later sources win:

```
base component → inherited components (in order) → inline vars
```

This allows progressive specialization from general defaults to specific overrides.

## Related Patterns

- [Multiple Inheritance](/design-patterns/inheritance-patterns/multiple-inheritance) - Inherit from multiple base components
- [Abstract Component](/design-patterns/inheritance-patterns/abstract-component) - Prevent base components from being deployed
- [Multiple Component Instances](/design-patterns/inheritance-patterns/multiple-component-instances) - Deploy multiple instances of the same component

## References

- [Component Inheritance](/howto/inheritance)