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

Compares a discovery report with what the kernel already knows, before registration.

Three kinds of drift are detected:

  * `:schema_changed_without_generation` — a registered event type generation's stored
    schema no longer has the same coarse shape (see `Chronicle.Discovery.EventSchemaShape`)
    as the module that currently declares it — its struct changed without a bumped
    `:generation`.
  * `:observer_missing_from_code` — the kernel has a client-owned reactor, reducer or
    projection/read-model observer on the `event-log` sequence whose id is no longer
    declared by any current artifact. Always `:info`: an event store can be shared by
    several clients, and their observers are also client-owned, so this is evidence only,
    never a reason to fail startup.
  * `:observer_event_types_changed` — a still-declared observer's handled event type ids no
    longer match what the kernel has recorded for it.

Does not decide what to do about a finding — `drift_policy` on `Chronicle.Client` and
`Chronicle.Registration.Coordinator` do that.

# `detect`

```elixir
@spec detect(term(), Chronicle.Discovery.Report.t(), keyword()) ::
  {:ok, [Chronicle.Discovery.Drift.t()]} | {:error, term()}
```

Detects likely artifact identity drift for the given report.

`opts` requires `:event_store` and accepts `:namespace` (default `"Default"`). Returns
`{:ok, findings}` (possibly empty) or `{:error, reason}` when the kernel could not be
reached — an indeterminate result, never to be treated the same as a clean one.

The check makes one `AllEventTypes` call, then `AllEventTypeGenerations` only for event
types whose local generation is not the kernel's latest (bounded-parallel), all within a
total deadline (5 seconds, `:deadline` in milliseconds) after which it returns
`{:error, :timeout}`. A raise inside the check is returned as
`{:error, {:exception, exception}}`.

`:fetch_all_event_types_fun`, `:fetch_event_type_generations_fun` and
`:fetch_observers_fun` are test-only seams replacing the real kernel calls; production
code never passes them.

---

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