Lesson 08 · Starting is idempotent - Build a Process Supervisor

Starting is idempotent

Asking to start a service that is already up must not launch a second copy. A supervisor is declarative - "I want web running" - so a redundant start is simply satisfied already. Today you make Start a no-op when the service is not Stopped.

The goal

Make Start on an already-running service do nothing - no second process is spawned.

Start here - the target

TO DO

Scenario: Starting a Running service spawns nothing

Givena service "web" that has been started and marked ready (State Running, holding one handle from the runtime)

WhenStart("web") is called again

Thenthe runtime was asked to Start exactly once in total, and the service still holds its original handle (State stays Running)

Andcalling Start on a service that is Starting also spawns nothing - only a Stopped or Exited service may be started

Background

A supervisor expresses desired state, not one-off commands. When you say a service should run, asking twice does not mean “run two copies” - it means the same thing said again. So Start on a service that is already Running (or Starting) must do nothing: no new process, no new handle, no state change. Spawning a second copy would leak a process the supervisor has lost track of, because a service holds exactly one handle.

The transition guard from lesson 4 gives you this almost for free: Starting is only reachable from Stopped or Exited, so a service that is Running simply fails the CanTransition check and Start returns early. This idempotence is the quiet foundation of the reconcile loop much later, where the supervisor repeatedly calls Start on everything that should be up and relies on the already-up ones staying untouched.

Make it work

func (s *Supervisor) Start(name string) {
  svc, _ := s.Get(name)
  // guard: a start is only meaningful from a stopped/exited service
  if !CanTransition(svc.State, Starting) {
    return // already up (or coming up); nothing to do
  }
  svc.Handle = s.rt.Start(svc.Command)
  svc.State = Starting
}

CheckpointDONE

Starting an already-running service is a harmless no-op. Commit and stop here.