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

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

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 the report, its checks' results kept as the terms they are and healthy decided from them (a warning keeps it healthy, an error doesn't):

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

Text is made only at the edge: Macaw.Host.Health.to_wire/1 words each result ("ok", "warning: <detail>", "error: <detail>"), and the two edges that send a report, Macaw.Host.Health.Plug (JSON, 200 when healthy and 503 when not) and Macaw.Host.Release.status/1 (the line <app>-ctl status reads), call it.

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. A check that returns anything else (a bare string, "ok" included) is an error naming the check: its text is never read for what it might mean. 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.