# Contracts

A consumer declares intent in a **contract** — a small YAML file that names
infrastructure (one or more modules), selects an environment, and supplies
module-specific inputs. The platform validates, resolves, and deploys it.

## The contract file

A minimal example (the `static-assets` module):

```yaml
id: stsi
name: Static Assets Site
environment: dev
infrastructure:
  - module: static-assets
    version: "1.0.0"
    inputs:
      bucket_name: "${env.environment}-${contract.id}-assets"
      index_document: index.html
```

A `microservice` example:

```yaml
id: msvc
name: Microservice
environment: dev
infrastructure:
  - module: microservice
    version: "1.0.0"
    inputs:
      service_name: "${env.environment}-${contract.id}-svc"
      desired_count: 2
```

## Array-based infrastructure (D-015)

The `infrastructure` field is an **array** of module entries, not a map
keyed by module name. This is a locked deviation from the Nova reference
(D-015). Each entry is an object with `module`, `version` (optional), and
`inputs`.

One entry = single-module deploy. N entries = multi-module manifest deployed
in one pipeline run:

```yaml
id: app
name: Pricing Service API
environment: dev
infrastructure:
  - module: microservice
    version: "1.0.0"
    inputs: { ... }
  - module: static-assets
    version: "1.0.0"
    inputs: { ... }
```

All modules deploy to the same `environment` in one pipeline run.

## Fields

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `id` | string | yes | Short operational acronym (`^[a-z][a-z0-9-]{2,5}$`, 3-6 chars). Becomes the stack name used for the Terraform state key and resource naming prefix. Stable across deploys and environment promotions. |
| `name` | string | yes | Full human-readable stack name (min 3 chars). |
| `environment` | string | yes | The platform-managed environment to deploy to (`dev`/`qa`/`prod`/`dr`). See [Environments](../environments/index.md). |
| `infrastructure` | array | yes | Array of modules to deploy (D-015). `minItems: 1`. |

### Infrastructure item fields

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `module` | string | yes | Module name from `modules/registry.json` (`^[a-z][a-z0-9-]*$`). |
| `version` | string | no | Semver pin (`^\d+\.\d+\.\d+$`). Omitted = latest non-deprecated version from the registry. |
| `inputs` | object | yes | Module-specific inputs (see the module's README / `interface.json`). No `aws_*` keys — the contract is engine-agnostic. |

## Validation

The contract is validated against
[`schemas/contract.schema.json`](../../schemas/contract.schema.json). An
invalid contract (missing field, unknown module, wrong type) fails at the
validate-contract stage with a clear error.

## Interpolation tokens (D-016)

The resolver expands `${env.*}` and `${contract.*}` tokens after the
environment is loaded. Unknown tokens raise `ValueError` (fail loud).
Expansion is recursive (nested map/list values expand too).

| Token | Resolves to | Example |
|-------|-------------|---------|
| `${env.name}` | the environment name | `dev` |
| `${env.region}` | the environment's AWS region | `us-east-1` |
| `${env.account_id}` | the environment's AWS account id | `000000000000` |
| `${env.state_backend.bucket}` | the environment's state bucket | `nova-tfstate-dev-us-east-1` |
| `${env.state_backend.lock_table}` | the environment's lock table | `nova-tfstate-locks` |
| `${env.network.vpc_cidr}` | the environment's VPC CIDR | `10.0.0.0/16` |
| `${env.network.azs}` | the environment's availability zones | `["us-east-1a","us-east-1b"]` |
| `${contract.id}` | the contract's operational acronym | `stsi` |
| `${contract.name}` | the contract's name field | `Static Assets Site` |
| `${contract.environment}` | the contract's environment field | `dev` |

## Sample contracts

Two reference examples exist in `contracts/`:

- [`contracts/static-assets.yml`](../../contracts/static-assets.yml) — the
  `static-assets` module (s3 + cloudfront + kms-key, D-038).
- [`contracts/microservice.yml`](../../contracts/microservice.yml) — the
  `microservice` module (vpc + ecs-cluster + ecs-service + iam-role + ecr +
  alb, D-038).

Additionally, every module has a `modules/<name>/examples/` directory with
validated example contracts. See the [module catalog](../modules/index.md)
for the full list.

## Per-environment variants (D-033)

Each sample contract has per-environment variants that differ **only** in
the `environment` field — all other fields are identical. Interpolation
resolves environment-specific values at resolver time.

| File | Environment |
|------|-------------|
| `contracts/static-assets.yml` | dev (default) |
| `contracts/static-assets.dev.yml` | dev |
| `contracts/static-assets.qa.yml` | qa |
| `contracts/static-assets.prod.yml` | prod |
| `contracts/static-assets.dr.yml` | dr |
| `contracts/microservice.yml` | dev (default) |
| `contracts/microservice.dev.yml` | dev |
| `contracts/microservice.qa.yml` | qa |
| `contracts/microservice.prod.yml` | prod |
| `contracts/microservice.dr.yml` | dr |

Promotion = running the pipeline against the matching variant. See the
[Consumer Guide](../consumer-guide.md) for the end-to-end flow.