The Fettle supervisor starts two servers; Fettle.ScoreBoard a GenServer which responds to queries about the current health status, and Fettle.RunnerSupervisor a supervisor which starts worker Fettle.Runner servers running the health-checks themselves, and takes care of restarting them if they exit abnormally.

The RunnerSupervisor starts a worker process for each configured Health.Spec, configuring them so they report results back to the ScoreBoard process.

Supervision Tree

• Fettle.Supervisor

  • Registry (Fettle.Registry)
  • Fettle.ScoreBoard

  • Fettle.RunnerSupervisor

    • Fettle.Runner

Config

By default Fettle loads configuration from the :fettle OTP application key; alternatively you can pass a :config option with either a {M,F,A}, a zero-args anonymous function, or some in-line configuration as a Keyword list:

Fettle.Supervisor.start_link(config: {MyMod, :fettle, []}) # calls `MyMod.fettle/0`
# or
Fettle.Supervisor.start_link(config: [
  system_code: "app",
  name: "My Application",
  checks: [...]
]) # inline config

See Fettle.Config for format details.

When using application configuration under the :fettle key, Fettle uses DeferredConfig to resolve {:system, "ENV_VAR"} style tuples using shell enviroment variables; this works for all config values: values will be automatically cast to integers as required.