# `Ankusa.Instance.Isolated`
[🔗](https://github.com/jamescarr/ankusa/blob/ankusa-v0.7.0/packages/ankusa/lib/ankusa/instance/isolated.ex#L1)

Owns one optional subtree of an instance (dispatch, storage, lifecycle,
metrics, or one of the secondary listeners) and never escalates its failure.

A plain child of `Ankusa.Instance` shares the root's restart budget: a
subtree that crashes in a loop (a sink whose adapter keeps raising at boot, a
port another process took) exhausts it and takes the whole instance down —
edge listener included, so the node stops acking hooks because something it
did not need to ack them is broken. This manager starts the subtree under its
own `:one_for_one` supervisor and, when that supervisor gives up (its restart
intensity is exhausted), does not exit. It waits and starts the subtree again,
backing off from `:base_backoff_ms` doubling up to `:max_backoff_ms` while
restarts keep failing, and starting over once the subtree has stayed up for a
minute.

A child that cannot start when the instance boots still fails the boot: a
misconfigured listener or sink should stop a deploy, not be retried quietly.

Every outage emits `[:ankusa, :instance, :subtree_down]` and every recovery
`[:ankusa, :instance, :subtree_up]` (see `Ankusa.Telemetry`).

## Options

  * `:instance` — the instance name (required)
  * `:domain` — an atom naming the domain, e.g. `:dispatch` (required)
  * `:children` — the child specs of the subtree (required)
  * `:base_backoff_ms` — first restart delay, default `1_000`
  * `:max_backoff_ms` — restart delay ceiling, default `60_000`

The two backoff options exist for tests.

# `child_spec`

Returns a specification to start this module under a supervisor.

See `Supervisor`.

# `start_link`

```elixir
@spec start_link(keyword()) :: GenServer.on_start()
```

# `subtree`

```elixir
@spec subtree(atom(), atom()) :: pid() | nil
```

The pid of the domain's supervisor, or `nil` while it is down (or the
manager is not running).

---

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