# `Chronicle.Discovery.DefaultedIdWarning`
[🔗](https://github.com/Cratis/Chronicle.Elixir/blob/main/lib/chronicle/discovery/defaulted_id_warning.ex#L4)

Warns, at most once per module for the lifetime of the BEAM node, that a reactor, reducer
or projection is relying on its module name as its identifier.

Renaming or moving the module then silently resets Chronicle's observation position for
it, because the id changes along with the name. Discovery can run more than once in a
client's lifetime (once while logging at startup, again through
`Chronicle.Client.discovery_report/1` or `mix chronicle.discover`), so the warning is kept
behind a `:persistent_term` guard keyed by module rather than emitted unconditionally.

# `warn_once`

```elixir
@spec warn_once(module(), :reactor | :reducer | :projection, boolean()) :: :ok
```

Emits the one-time warning for `module` when its id was defaulted and warnings are enabled.

`kind` is `:reactor`, `:reducer` or `:projection` — only those three default an id from the
module name today. `warn?` is the client's `warn_defaulted_ids` option; passing `false`
suppresses the warning (and skips the `:persistent_term` write) without otherwise affecting
discovery.

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