mechanoid · v0.3.2

Heartbeat

A self-driving health machine: a timeout schedules the next check; .producing runs the check and feeds the result back as an event. Degraded and critical tiers use shorter or recovery deadlines.

(The timeout event is named HeartbeatTick here so it does not clash with this page’s object name.)

enum ServiceState derives Finite:
  case Stopped, Started, Degraded, Critical

enum ServiceEvent derives Finite:
  case Start, Stop, ManualReset
  case HeartbeatTick, DegradedCheck
  case Healthy, Unstable, Failed

val machine = Machine(
  assemblyAll[ServiceState, ServiceEvent]:
    (Stopped via Start to Started) @@ Aspect.timeout(10.seconds, HeartbeatTick)
    (Started via HeartbeatTick to Started)
      .producing { (_, _) => ZIO.succeed(Healthy) } @@ Aspect.timeout(10.seconds, HeartbeatTick)
    (Started via Healthy to Started) @@ Aspect.timeout(10.seconds, HeartbeatTick)
    (Started via Unstable to Degraded) @@ Aspect.timeout(3.seconds, DegradedCheck)
    (Degraded via DegradedCheck to Degraded)
      .producing { (_, _) => ZIO.succeed(Healthy) } @@ Aspect.timeout(3.seconds, DegradedCheck)
    (Degraded via Healthy to Started) @@ Aspect.timeout(10.seconds, HeartbeatTick)
    (Degraded via Failed to Critical) @@ Aspect.timeout(30.seconds, ManualReset)
    anyOf(Started, Degraded, Critical) via Stop to Stopped
    Critical via ManualReset to Started
)

The graph

Mermoid.diagram(
  MermaidVisualizer.flowchart(machine),
  DocsDiagrams.diagramConfig,
)
StartHeartbeatTickHealthyUnstableDegradedCheckHealthyFailedStopStopStopManualReset

Features in play: assemblyAll, .producing, @@ Aspect.timeout, anyOf. Full stack with Postgres stores and TimeoutSweeper: examples/.../heartbeat.

A timeout tick

DocSpecs send the timeout event the sweeper would fire. After HeartbeatTick, the producing effect returns Healthy and the machine stays in Started with a fresh deadline.

ZIO.scoped {
  for
    fsm   <- machine.start(Stopped)
    _     <- fsm.send(Start)
    _     <- fsm.send(HeartbeatTick)
    _     <- ZIO.sleep(50.millis)
    state <- fsm.currentState
  yield state
}.asDoc
Started

Next: Orders for rich event[T] payloads, or Durable Timeouts for the production sweeper story.