diff --git a/osbase/supervisor/supervisor_metrics.go b/osbase/supervisor/supervisor_metrics.go
new file mode 100644
index 0000000..d83b7a7
--- /dev/null
+++ b/osbase/supervisor/supervisor_metrics.go
@@ -0,0 +1,70 @@
+package supervisor
+
+import (
+	"sync"
+	"time"
+)
+
+// Metrics is an interface from the supervisor to any kind of metrics-collecting
+// component.
+type Metrics interface {
+	// NotifyNodeState is called whenever a given runnable at a given DN changes
+	// state. Called synchronously from the supervisor's processor loop, so must not
+	// block, but is also guaranteed to only be called from a single goroutine.
+	NotifyNodeState(dn string, state NodeState)
+}
+
+// metricsFanout is used internally to fan out a single Metrics interface (which
+// it implements) onto multiple subordinate Metrics interfaces (as provided by
+// the user via WithMetrics).
+type metricsFanout struct {
+	sub []Metrics
+}
+
+func (m *metricsFanout) NotifyNodeState(dn string, state NodeState) {
+	for _, sub := range m.sub {
+		sub.NotifyNodeState(dn, state)
+	}
+}
+
+// InMemoryMetrics is a simple Metrics implementation that keeps an in-memory
+// mirror of the state of all DNs in the supervisor. The zero value for
+// InMemoryMetrics is ready to use.
+type InMemoryMetrics struct {
+	mu  sync.RWMutex
+	dns map[string]DNState
+}
+
+// DNState is the state of a supervisor runnable, recorded alongside a timestamp
+// of when the State changed.
+type DNState struct {
+	// State is the current state of the runnable.
+	State NodeState
+	// Transition is the time at which the runnable reached its State.
+	Transition time.Time
+}
+
+func (m *InMemoryMetrics) NotifyNodeState(dn string, state NodeState) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.dns == nil {
+		m.dns = make(map[string]DNState)
+	}
+	m.dns[dn] = DNState{
+		State:      state,
+		Transition: time.Now(),
+	}
+}
+
+// DNs returns a copy (snapshot in time) of the recorded DN states, in a map from
+// DN to DNState. The returned value can be mutated.
+func (m *InMemoryMetrics) DNs() map[string]DNState {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	res := make(map[string]DNState)
+	for k, v := range m.dns {
+		res[k] = v
+	}
+	return res
+}