when maglevd rehashes its config:

- when a backend gets newly added or restarted, an event should fire; perhaps transition to 'unknown'
- when a backend gets removed, an event should fire; perhaps transition to 'removed'
- when a backend is in 'unknown' state, fast-interval is appropriate

internal/checker/checker.go

@@ -67,10 +67,11 @@ func (c *Checker) Run(ctx context.Context) error {
 
 	c.mu.Lock()
 	names := activeBackendNames(c.cfg)
+	maxHistory := c.cfg.HealthChecker.TransitionHistory
 	for i, name := range names {
 		b := c.cfg.Backends[name]
 		hc := c.cfg.HealthChecks[b.HealthCheck]
-		c.startWorker(ctx, name, b, hc, i, len(names))
+		c.startWorker(ctx, name, b, hc, i, len(names), maxHistory)
 	}
 	c.mu.Unlock()
 
@@ -86,21 +87,25 @@ func (c *Checker) Reload(ctx context.Context, cfg *config.Config) error {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
+	maxHistory := cfg.HealthChecker.TransitionHistory
+
 	desired := map[string]struct{}{}
 	for _, name := range activeBackendNames(cfg) {
 		desired[name] = struct{}{}
 	}
 
-	// Stop workers no longer needed.
+	// Stop workers no longer needed; emit a removed event using the old frontends.
 	for name, w := range c.workers {
 		if _, ok := desired[name]; !ok {
 			slog.Info("backend-stop", "backend", name)
+			t := w.backend.Remove(maxHistory)
+			c.emitForBackend(name, w.backend.Address, t, c.cfg.Frontends)
 			w.cancel()
 			delete(c.workers, name)
 		}
 	}
 
-	// Add new or restart changed workers.
+	// Add new or restart changed workers; emit an unknown event using the new frontends.
 	names := activeBackendNames(cfg)
 	for i, name := range names {
 		b := cfg.Backends[name]
@@ -113,11 +118,12 @@ func (c *Checker) Reload(ctx context.Context, cfg *config.Config) error {
 			}
 			slog.Info("backend-restart", "backend", name)
 			w.cancel()
-			c.startWorker(ctx, name, b, hc, i, len(names))
+			c.startWorker(ctx, name, b, hc, i, len(names), maxHistory)
 		} else {
 			slog.Info("backend-start", "backend", name)
-			c.startWorker(ctx, name, b, hc, i, len(names))
+			c.startWorker(ctx, name, b, hc, i, len(names), maxHistory)
 		}
+		c.emitForBackend(name, c.workers[name].backend.Address, c.workers[name].backend.Transitions[0], cfg.Frontends)
 	}
 
 	c.cfg = cfg
@@ -231,7 +237,7 @@ func (c *Checker) PauseBackend(name string) (BackendSnapshot, bool) {
 	maxHistory := c.cfg.HealthChecker.TransitionHistory
 	if w.backend.Pause(maxHistory) {
 		slog.Info("backend-pause", "backend", name)
-		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0])
+		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0], c.cfg.Frontends)
 	}
 	return BackendSnapshot{Health: w.backend, Config: w.entry}, true
 }
@@ -247,7 +253,7 @@ func (c *Checker) ResumeBackend(name string) (BackendSnapshot, bool) {
 	maxHistory := c.cfg.HealthChecker.TransitionHistory
 	if w.backend.Resume(maxHistory) {
 		slog.Info("backend-resume", "backend", name)
-		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0])
+		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0], c.cfg.Frontends)
 	}
 	return BackendSnapshot{Health: w.backend, Config: w.entry}, true
 }
@@ -256,7 +262,7 @@ func (c *Checker) ResumeBackend(name string) (BackendSnapshot, bool) {
 
 // startWorker creates a Backend and launches a probe goroutine.
 // Must be called with c.mu held.
-func (c *Checker) startWorker(ctx context.Context, name string, entry config.Backend, hc config.HealthCheck, pos, total int) {
+func (c *Checker) startWorker(ctx context.Context, name string, entry config.Backend, hc config.HealthCheck, pos, total, maxHistory int) {
 	rise, fall := hc.Rise, hc.Fall
 	if entry.HealthCheck == "" {
 		// No healthcheck: one synthetic pass drives the backend to Up immediately.
@@ -269,6 +275,7 @@ func (c *Checker) startWorker(ctx context.Context, name string, entry config.Bac
 		entry:   entry,
 		cancel:  cancel,
 	}
+	w.backend.Start(maxHistory)
 	c.workers[name] = w
 	go c.runProbe(wCtx, name, pos, total)
 }
@@ -369,16 +376,16 @@ func (c *Checker) runProbe(ctx context.Context, name string, pos, total int) {
 				"code", result.Code,
 				"detail", result.Detail,
 			)
-			c.emitForBackend(name, addr, t)
+			c.emitForBackend(name, addr, t, c.cfg.Frontends)
 		}
 		c.mu.Unlock()
 	}
 }
 
-// emitForBackend emits one Event per frontend that references backendName.
-// Must be called with c.mu held.
-func (c *Checker) emitForBackend(backendName string, addr net.IP, t health.Transition) {
-	for feName, fe := range c.cfg.Frontends {
+// emitForBackend emits one Event per frontend that references backendName,
+// using the provided frontends map. Must be called with c.mu held.
+func (c *Checker) emitForBackend(backendName string, addr net.IP, t health.Transition, frontends map[string]config.Frontend) {
+	for feName, fe := range frontends {
 		for _, name := range fe.Backends {
 			if name == backendName {
 				c.emit(Event{FrontendName: feName, BackendName: backendName, Backend: addr, Transition: t})

internal/health/state.go

@@ -33,6 +33,7 @@ const (
 	StateUp
 	StateDown
 	StatePaused
+	StateRemoved // backend was removed from configuration
 )
 
 func (s State) String() string {
@@ -45,6 +46,8 @@ func (s State) String() string {
 		return "down"
 	case StatePaused:
 		return "paused"
+	case StateRemoved:
+		return "removed"
 	default:
 		return "unknown"
 	}
@@ -118,7 +121,7 @@ func New(name string, addr net.IP, rise, fall int) *Backend {
 // failure means the backend is not yet confirmed reachable), and to StateUp
 // once the counter reaches Rise consecutive passes.
 func (b *Backend) Record(r ProbeResult, maxHistory int) bool {
-	if b.State == StatePaused {
+	if b.State == StatePaused || b.State == StateRemoved {
 		return false
 	}
 	if r.OK {
@@ -157,12 +160,15 @@ func (b *Backend) Resume(maxHistory int) bool {
 }
 
 // NextInterval returns the appropriate probe interval based on state and counter:
-//   - Unknown (no probes yet):        interval  — probe promptly to establish initial state
+//   - Unknown (initial / post-resume): fastInterval (falls back to interval) — probe quickly to establish state
 //   - Fully healthy (counter at max):  interval
 //   - Fully down (counter at 0):       downInterval (falls back to interval)
 //   - Degraded (anywhere in between):  fastInterval (falls back to interval)
 func (b *Backend) NextInterval(interval, fastInterval, downInterval time.Duration) time.Duration {
 	if b.State == StateUnknown {
+		if fastInterval > 0 {
+			return fastInterval
+		}
 		return interval
 	}
 	if b.Counter.Health == b.Counter.Max() {
@@ -180,6 +186,21 @@ func (b *Backend) NextInterval(interval, fastInterval, downInterval time.Duratio
 	return interval
 }
 
+// Start records the initial StateUnknown transition when a backend is first
+// created or restarted. It exists solely to populate the transition history
+// and fire a reload event; the state does not change.
+func (b *Backend) Start(maxHistory int) Transition {
+	b.transition(StateUnknown, ProbeResult{Code: "start"}, maxHistory)
+	return b.Transitions[0]
+}
+
+// Remove transitions the backend to StateRemoved. Returns the transition.
+// After this call no further probe results are accepted.
+func (b *Backend) Remove(maxHistory int) Transition {
+	b.transition(StateRemoved, ProbeResult{Code: "removed"}, maxHistory)
+	return b.Transitions[0]
+}
+
 // transition appends a new Transition and updates State.
 func (b *Backend) transition(to State, r ProbeResult, maxHistory int) {
 	t := Transition{From: b.State, To: to, At: time.Now(), Result: r}

internal/health/state_test.go

@@ -169,9 +169,13 @@ func TestNextInterval(t *testing.T) {
 
 	b := New("test", net.ParseIP("10.0.0.1"), 2, 3) // max=4
 
-	// Unknown (no probes yet): always use interval, never downInterval.
-	if got := b.NextInterval(interval, fast, down); got != interval {
-		t.Errorf("StateUnknown: got %v, want %v (interval)", got, interval)
+	// Unknown: use fast-interval to establish state quickly.
+	if got := b.NextInterval(interval, fast, down); got != fast {
+		t.Errorf("StateUnknown with fast: got %v, want %v (fast)", got, fast)
+	}
+	// Unknown, no fast-interval configured: fall back to interval.
+	if got := b.NextInterval(interval, 0, down); got != interval {
+		t.Errorf("StateUnknown without fast: got %v, want %v (interval)", got, interval)
 	}
 
 	// After first fail: counter=0, state=Down → downInterval.
@@ -291,6 +295,40 @@ func TestTransitionTimestamp(t *testing.T) {
 	}
 }
 
+func TestStartRemove(t *testing.T) {
+	b := newBackend()
+
+	// Start records an unknown→unknown transition.
+	tr := b.Start(5)
+	if tr.From != StateUnknown || tr.To != StateUnknown {
+		t.Errorf("Start transition: got %s→%s, want unknown→unknown", tr.From, tr.To)
+	}
+	if len(b.Transitions) != 1 {
+		t.Errorf("transitions after Start: got %d, want 1", len(b.Transitions))
+	}
+	if b.State != StateUnknown {
+		t.Errorf("state after Start: got %s, want unknown", b.State)
+	}
+
+	// Remove transitions to StateRemoved.
+	b.State = StateUp
+	tr = b.Remove(5)
+	if tr.From != StateUp || tr.To != StateRemoved {
+		t.Errorf("Remove transition: got %s→%s, want up→removed", tr.From, tr.To)
+	}
+	if b.State != StateRemoved {
+		t.Errorf("state after Remove: got %s, want removed", b.State)
+	}
+
+	// Record is a no-op once removed.
+	if b.Record(pass(), 5) {
+		t.Error("Record should not transition a removed backend")
+	}
+	if b.State != StateRemoved {
+		t.Errorf("state changed after Record on removed backend: %s", b.State)
+	}
+}
+
 func TestStateString(t *testing.T) {
 	cases := []struct {
 		s    State
@@ -300,6 +338,7 @@ func TestStateString(t *testing.T) {
 		{StateUp, "up"},
 		{StateDown, "down"},
 		{StatePaused, "paused"},
+		{StateRemoved, "removed"},
 	}
 	for _, c := range cases {
 		if c.s.String() != c.want {