An app's health, in one shape every <app>-ctl status reads.

A report runs named checks. Each returns :ok, {:error, detail} (the app is unhealthy) or {:warn, detail} (worth showing, not a failure):

Macaw.Host.Health.report([
  database: fn -> Macaw.Host.Health.database(MyApp.Repo) end,
  migrations: fn -> Macaw.Host.Health.migrations(MyApp.Repo) end,
  data_dir: fn -> Macaw.Host.Health.writable(data_dir) end
], version: MyApp.Build.version(), build: MyApp.Build.sha())

gives

%{healthy: true, version: "1.2.0", build: "3f639e84a704",
  checks: %{database: "ok", migrations: "ok", data_dir: "ok"}}

A check that raises is reported as an error naming the exception, never a crash of the report: the point of a health check is to answer while something is broken. The whole report answers within its deadline (5 s unless :deadline says otherwise): the checks run side by side, each given the time left, and one that misses it is reported as timed out. Macaw.Host.Health.Plug serves the report as JSON, 200 when healthy and 503 when not.