# `Statifier.Supervisor`
[🔗](https://github.com/riddler/statifier-ex/blob/v2.6.0/lib/statifier/supervisor.ex#L1)

The ADR-0027 session runtime: a module-based supervisor holding the
library's named session registry and the flat dynamic supervisor sessions
started through `Statifier.start_session/2` land on.

## The embedder places this, the library does not

Statifier-ex ships no `mod:` application callback (ADR-0027 decision 1). A
library that started processes on load would tax every host that only
ever compiles a document, and this module names the seam ADR-0003's
"embedders can supply their own effect interpreter" already opened: an
embedder adds `Statifier.Supervisor` (or `{Statifier.Supervisor, []}`) to
their own supervision tree when they want cross-session
`#_scxml_<sessionid>` delivery (via `Statifier.start_session/2`), and
omits it entirely when they do not. A host that never places this
supervisor starts no session-runtime processes and pays nothing.

## Children, in order, under `:rest_for_one`

1. `{Registry, keys: :unique, name: Statifier.Registry}` - `Session.init/1`
   registers under the `sess_` id once it is generated (`Session`'s own
   moduledoc, "`<send>` routing" section).
2. `{DynamicSupervisor, name: Statifier.SessionSupervisor}` - the flat
   supervisor `Statifier.start_session/2` starts sessions on.

`:rest_for_one` means a registry crash takes the `DynamicSupervisor` (and
every session under it) down with it rather than leaving live sessions
behind a dead registry as unreachable orphans; ADR-0027 decision 1 records
that those sessions restart fresh rather than continuing as amnesiacs
(each session is `restart: :temporary`, so "restart" here means the
embedder's own supervision tree deciding what happens next, not this
supervisor bringing a session back - see decision 4). A `DynamicSupervisor`
crash on its own does not take the registry with it, since registrations
belong to the session processes, not to the supervisor that started them.

## One instance, no `:name` option

ADR-0027 defers multiple named runtime instances as mechanism with no
caller. `Statifier.Registry` and `Statifier.SessionSupervisor` are fixed,
module-qualified names; starting a second `Statifier.Supervisor` in the
same node is a name collision, not a second isolated session population.
The test suite places one runtime for the whole run in
`test/test_helper.exs`, for the same one-instance reason, rather than each
test standing up its own registry/supervisor pair.

# `child_spec`

Returns a specification to start this module under a supervisor.

See `Supervisor`.

# `start_link`

```elixir
@spec start_link(init_arg :: term()) :: Supervisor.on_start()
```

Starts the session runtime. `init_arg` is unused and exists only because
`use Supervisor` expects a `start_link/1` of this shape; pass anything
(`[]` is conventional) or use the generated `child_spec/1` directly.

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