# `NoNoncense.MachineId.Strategy.EnvironmentVariable`
[🔗](https://github.com/juulSme/NoNoncense/blob/v2.0.0/lib/no_noncense/machine_id/strategy/environment_variable.ex#L1)

Strategy to acquire the machine ID from the last digits in an environment variable value.
Useful for using Kubernetes StatefulSets for determining the machine ID.

A StatefulSet gives each pod a stable, zero-based ordinal in its name, such as
`nonce-worker-0` and `nonce-worker-1`. Expose the pod name through the Downward API:

```yaml
env:
  - name: POD_NAME
    valueFrom:
      fieldRef:
        fieldPath: metadata.name
```

Then configure the strategy when starting `NoNoncense.MachineId`:

    children = [
      {NoNoncense.MachineId,
       strategy: NoNoncense.MachineId.Strategy.EnvironmentVariable,
       strategy_opts: [variable_name: "POD_NAME"],
       instances: [[base_key: System.fetch_env!("BASE_KEY")]]}
    ]

The default `ConflictGuard` detects duplicate IDs among connected Erlang nodes.

This strategy is suitable only when every nonce-generating pod has a globally unique numeric
suffix. A single StatefulSet provides that property, but separate StatefulSets reuse their
ordinals. Ensure the resulting IDs remain in the supported range of 0..511.

> #### Be careful with other orchestrators {: .warning}
>
> Do not assume a task or allocation index from an orchestrator such as Nomad is an exclusive,
> globally unique machine ID. During rolling deployments, an old and replacement allocation may
> overlap, or a deployment may reuse an index. Use this strategy only when the orchestrator
> guarantees unique IDs for all concurrently running instances; otherwise use a coordinating
> lease strategy.

# `opt`

```elixir
@type opt() :: {:variable_name, String.t()}
```

---

*Consult [api-reference.md](api-reference.md) for complete listing*