# Component Overrides

_Atmos Design Pattern_

The **Component Overrides** pattern lets you apply configuration to a specific group of components without affecting other components in the same stack.

## The Problem

When you set `vars` at the stack level, it applies to **every component** in the final merged stack:

```yaml title="stacks/deploy/prod.yaml"
import:
  - teams/platform    # imports: vpc, eks, rds
  - teams/security    # imports: waf, security-groups

vars:
  environment: prod
  # This tag applies to ALL components (vpc, eks, rds, waf, security-groups)
  tags:
    Team: Platform
```

But what if the Platform team and Security team each want their own `Team` tag? You can't use regular `vars` because it applies globally.

## The Solution: Overrides

The `overrides` section is **scoped to the current file** and its imports. It doesn't leak into sibling imports or get inherited by files that import this one.

:::tip Key Insight
**Overrides are file-scoped and don't get inherited.** They only apply to components visible from that file's import tree.
:::

**File:** `stacks/teams/platform.yaml`

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

# Only applies to vpc, eks, rds (not security team's components)
terraform:
  overrides:
    vars:
      tags:
        Team: Platform
```

**File:** `stacks/teams/security.yaml`

```yaml
import:
  - catalog/waf/defaults
  - catalog/security-groups/defaults

# Only applies to waf, security-groups (not platform team's components)
terraform:
  overrides:
    vars:
      tags:
        Team: Security
```

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

```yaml
import:
  - teams/platform   # vpc, eks, rds get Team: Platform
  - teams/security   # waf, security-groups get Team: Security
  - mixins/stage/prod

vars:
  environment: prod
```

Now each team's components get their own `Team` tag, even though they're all in the same final stack.

## When to Use Overrides

| Scenario | Use `vars` | Use `overrides` |
|----------|-----------|-----------------|
| Setting values for ALL components | ✓ | |
| Setting values for a SUBSET of components | | ✓ |
| Team-specific configuration | | ✓ |
| Layer-specific settings (network, data, compute) | | ✓ |

## Overrides vs Vars

| Aspect | `vars` | `overrides` |
|--------|--------|-------------|
| **Scope** | Final merged stack (global) | Current file + its imports only |
| **Inheritance** | Gets inherited by importing files | Does NOT get inherited |
| **Visibility** | Affects ALL components | Only affects components "visible" from this file |

Think of it this way:

- `vars` = "Apply this to the final result"
- `overrides` = "Apply this only to what I can see from here"

```
platform.yaml
├── imports: vpc, eks, rds
├── overrides: Team=Platform  ← only sees vpc, eks, rds
│
security.yaml
├── imports: waf, firewall
├── overrides: Team=Security  ← only sees waf, firewall
│
prod.yaml
├── imports: platform.yaml, security.yaml
├── vars: env=prod  ← sees ALL components (vpc, eks, rds, waf, firewall)
```

## Example: Layer-Based Overrides

When using [Layered Stack Configuration](/design-patterns/stack-organization/layered-stack-configuration), overrides let each layer tag its components:

**File:** `stacks/layers/network.yaml`

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

terraform:
  overrides:
    vars:
      tags:
        Layer: network
```

**File:** `stacks/layers/data.yaml`

```yaml
import:
  - catalog/rds/defaults
  - catalog/elasticache/defaults

terraform:
  overrides:
    vars:
      tags:
        Layer: data
```

## What You Can Override

The `overrides` section supports:

- `vars` - Input variables
- `env` - Environment variables
- `settings` - Component settings
- `command` - The executable to use (e.g., `tofu` instead of `terraform`)
- `providers` - Terraform provider configuration
- `hooks` - Lifecycle hooks

```yaml
terraform:
  overrides:
    vars:
      tags:
        Team: Platform
    settings:
      spacelift:
        autodeploy: true
    command: tofu  # Use OpenTofu for these components
```

## Related Patterns

- [Layered Stack Configuration](/design-patterns/stack-organization/layered-stack-configuration) - Group components by function
- [Component Catalog](/design-patterns/component-catalog) - Define component defaults

## References

- [Overrides Configuration](/stacks/overrides) - Complete reference documentation