# Modules

Reusable building blocks for cloud infrastructure. There are two kinds:

- **Primitives** — a single cloud resource or a small group of related
  resources (e.g. a VPC with subnets and routing). Each primitive has an
  `interface.json` declaring its inputs and outputs.
- **Modules** — a pattern that references multiple primitives to deploy a
  complete stack (e.g. an ECS Fargate microservice). Each module has a
  `composition.json` declaring its children and wires.

The engine adapter compiles a module instance to infrastructure. Each
module's README documents which resources it creates.

## Primitives

| Module | What it creates | Source |
|--------|----------------|--------|
| `s3` | `aws_s3_bucket` — a single S3 bucket | [modules/l1/s3/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/s3/README.md) |
| `vpc` | `aws_vpc` + `aws_subnet` + `aws_route_table` + `aws_internet_gateway` — VPC with subnets and routing | [modules/l1/vpc/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/vpc/README.md) |
| `ecs-cluster` | `aws_ecs_cluster` — ECS Fargate cluster | [modules/l1/ecs-cluster/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/ecs-cluster/README.md) |
| `ecs-service` | `aws_ecs_task_definition` + `aws_ecs_service` — Fargate service with task definition | [modules/l1/ecs-service/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/ecs-service/README.md) |
| `iam-role` | `aws_iam_role` — IAM role with assume-role policy | [modules/l1/iam-role/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/iam-role/README.md) |
| `alb` | `aws_lb` + `aws_lb_target_group` + `aws_lb_listener` — Application Load Balancer | [modules/l1/alb/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/alb/README.md) |
| `ecr` | `aws_ecr_repository` — ECR container image repository | [modules/l1/ecr/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/ecr/README.md) |
| `cloudfront` | `aws_cloudfront_distribution` + `aws_cloudfront_origin_access_control` — CloudFront distribution with S3 origin via OAC | [modules/l1/cloudfront/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/cloudfront/README.md) |
| `waf` | `aws_wafv2_web_acl` — WAFv2 Web ACL (CloudFront-scoped) | [modules/l1/waf/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/waf/README.md) |
| `rds` | `aws_db_instance` — RDS database instance (multi-engine: postgres, mysql, etc.) | [modules/l1/rds/README.md](https://github.com/acdl/acdl/blob/main/modules/l1/rds/README.md) |

## Modules

| Module | What it references | Source |
|--------|--------------------|--------|
| `static-assets` | 3 primitives (s3, cloudfront, waf) — a production static asset stack | [modules/l2/static-assets/README.md](https://github.com/acdl/acdl/blob/main/modules/l2/static-assets/README.md) |
| `microservice` | 6 primitives (vpc, cluster, ecr, iam-role, alb, ecs-service) — an ECS Fargate microservice | [modules/l2/microservice/README.md](https://github.com/acdl/acdl/blob/main/modules/l2/microservice/README.md) |

## Registry

Module versions are tracked in
[`registry.json`](https://github.com/acdl/acdl/blob/main/modules/registry.json).
Both primitives and modules are registered.

## Examples

Each module has a `examples/` directory containing validated consumer
contract examples (`simple.yaml` + `complex.yaml` + variation files). The
platform-test pipeline validates them against
[`schemas/contract.schema.json`](https://github.com/acdl/acdl/blob/main/schemas/contract.schema.json).
See each module's `## Examples` section for the excerpts.

## Versioning

Primitives and modules use semver: interface → MAJOR, behavior → MINOR,
lifecycle → PATCH. A MAJOR bump requires a new registry entry (immutable
publication); the old entry enters a 12-month deprecation window. See
[Versioning](../pipeline/versioning) for the deploy-pipeline versioning.

## Module patterns (roadmap)

The current `composition.json` mechanism is a thin pattern layer. A future
redesign will let a consumer dynamically create a module directly from the
contract file (an agentic "composition" flow). That is on the roadmap, not
implemented today.