Using Iii

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# Engine


## Engine configuration

The iii engine starts from a `config.yaml` file at your project root. Pass `--config <path>` to
point at a different file, or `--use-default-config` to start with a default set of workers (handy
for first-run and scratch work).

```bash
iii --config config.yaml
```

## Configuration file structure

`config.yaml` has a single top-level key, `workers:`, that lists the workers the engine should load.
Each entry has a `name` (a registry slug or local worker name) and a `config` block whose shape is
defined by that worker.

```yaml
workers:
  - name: iii-http
    config:
      port: 3111
      host: 127.0.0.1

  - name: iii-state
    config:
      adapter:
        name: kv
        config:
          store_method: file_based
          file_path: ./data/state_store.db
```

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Per-worker config schemas live on each worker's Worker Docs page. See
[Worker Registry](./workers-registry) for where to find a worker's config reference.

<Note>
  Workers do not need to be running alongside iii; configuring them in config.yaml is a convenience. A worker
  can be deployed anywhere and only needs a connection string to the iii instance. See
  [Creating Workers / Connecting to the engine](../creating-workers/workers#connecting-to-the-engine) for more information.
</Note>

## Environment variable expansion

Values in `config.yaml` support `${VAR:default}` syntax. The expansion uses the value of the
environment variable `VAR`, falling back to `default` when the variable is not set. Use this to swap
ports, URLs, and feature flags per environment without forking the config file.

```yaml
workers:
  - name: iii-http
    config:
      port: ${HTTP_PORT:3111}
      host: ${HTTP_HOST:127.0.0.1}
```

## Default configuration

Run `iii --use-default-config` to start the engine with a default set of workers without writing a
`config.yaml`. Useful for first-run and scratch work. Once you need to customize ports, adapters, or
the set of workers, switch to a real `config.yaml`.