# `Arc.Watch`
[🔗](https://github.com/Cratis/Arc.Elixir/blob/main/lib/arc/watch.ex#L4)

A source of change notifications for observable queries.

`@observe watch: [...]` accepts Chronicle read model modules, and `{module, target}` pairs where
`module` implements this behaviour. Persistence packages use it to re-run a query when their
data changes, for example `ArcMongo.watch/1` over a MongoDB change stream.

`subscribe/3` runs when the subscription starts, with the context of the caller (so a watcher
can follow the caller's tenant), and must arrange for `{:arc_notify, target}` to be sent to
`owner` whenever the watched data may have changed. Arc debounces bursts and re-runs the query.
`unsubscribe/1` is called with the returned handle when the subscription ends.

# `subscribe`

```elixir
@callback subscribe(target :: term(), ctx :: Arc.Context.t(), owner :: pid()) ::
  {:ok, handle :: term()} | {:error, term()}
```

# `unsubscribe`

```elixir
@callback unsubscribe(handle :: term()) :: :ok
```

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