Lesson 24 · A dependent waits for its dependency

A dependent waits for its dependency

Start order is not enough - a dependent must not launch until its dependency is genuinely Running, not merely Starting. Today StartAll walks the order but holds a service back until every dependency has reached Running.

The goal

In StartAll, start a service only once all of its dependencies are Running.

Start here - the target

TO DO

Scenario: A dependent is blocked until its dependency is ready

Given"web" requires "db"; StartAll() is called, which starts "db" (now Starting, not yet ready)

WhenStartAll evaluates "web" while "db" is still Starting

Then"web" is not started - it stays Stopped because its dependency is not Running yet

Andafter MarkReady("db") makes db Running, a further StartAll() call starts "web" (web becomes Starting)

Background

Ordering the starts is necessary but not sufficient. Starting db before web means little if web launches the instant db is merely spawned - db might still be opening its files, and web would fail to connect. The real rule is readiness: a dependent may start only once every dependency has reached Running, the state that means “ready to serve” from lesson 7. This is precisely why that Starting-versus-Running distinction was worth modelling.

So StartAll walks the topological order but skips any service whose dependencies are not all Running yet, leaving it Stopped for now. As dependencies become ready - a later StartAll pass, or the reconcile loop ahead - the services waiting on them are released to start. Pin both sides: while db is only Starting, web is held back; once db is Running, web is allowed up. Starting climbs the graph bottom-up; stopping, as you will see next, must unwind it top-down.

Make it work

func (s *Supervisor) StartAll() {
  for _, name := range order { // topological order
    svc, _ := s.Get(name)
    if !depsRunning(svc) { continue } // hold back until deps are Running
    s.Start(name)
  }
}
func depsRunning(svc *Service) bool {
  // every dep must be in state Running, not just Starting
}

CheckpointDONE

A dependent stays down until all its dependencies are Running. Commit and stop here.