coder · SasSwart · Nov 19, 2025 · Nov 11, 2025 · Nov 11, 2025 · Nov 12, 2025
diff --git a/agent/unit/graph.go b/agent/unit/graph.go
@@ -58,7 +58,7 @@ func (g *Graph[EdgeType, VertexType]) AddEdge(from, to VertexType, edge EdgeType
 	toID := g.getOrCreateVertexID(to)
 
 	if g.canReach(to, from) {
-		return xerrors.Errorf("adding edge (%v -> %v) would create a cycle", from, to)
+		return xerrors.Errorf("adding edge (%v -> %v): %w", from, to, ErrCycleDetected)
 	}
 
 	g.gonumGraph.SetEdge(simple.Edge{F: simple.Node(fromID), T: simple.Node(toID)})

diff --git a/agent/unit/graph_test.go b/agent/unit/graph_test.go
@@ -148,8 +148,7 @@ func TestGraph(t *testing.T) {
 			graph := &testGraph{}
 			unit1 := &testGraphVertex{Name: "unit1"}
 			err := graph.AddEdge(unit1, unit1, testEdgeCompleted)
-			require.Error(t, err)
-			require.ErrorContains(t, err, fmt.Sprintf("adding edge (%v -> %v) would create a cycle", unit1, unit1))
+			require.ErrorIs(t, err, unit.ErrCycleDetected)
 
 			return graph
 		},
@@ -160,8 +159,7 @@ func TestGraph(t *testing.T) {
 			err := graph.AddEdge(unit1, unit2, testEdgeCompleted)
 			require.NoError(t, err)
 			err = graph.AddEdge(unit2, unit1, testEdgeStarted)
-			require.Error(t, err)
-			require.ErrorContains(t, err, fmt.Sprintf("adding edge (%v -> %v) would create a cycle", unit2, unit1))
+			require.ErrorIs(t, err, unit.ErrCycleDetected)
 
 			return graph
 		},
@@ -341,7 +339,7 @@ func TestGraphThreadSafety(t *testing.T) {
 		// Verify all attempts correctly returned cycle error
 		for i, err := range cycleErrors {
 			require.Error(t, err, "goroutine %d should have detected cycle", i)
-			require.Contains(t, err.Error(), "would create a cycle")
+			require.ErrorIs(t, err, unit.ErrCycleDetected)
 		}
 
 		// Verify graph remains valid (original chain intact)

diff --git a/agent/unit/manager.go b/agent/unit/manager.go
@@ -0,0 +1,255 @@
+package unit
+
+import (
+	"errors"
+	"sync"
+
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/util/slice"
+)
+
+var (
+	ErrUnitNotFound             = xerrors.New("unit not found")
+	ErrUnitAlreadyRegistered    = xerrors.New("unit already registered")
+	ErrCannotUpdateOtherUnit    = xerrors.New("cannot update other unit's status")
+	ErrDependenciesNotSatisfied = xerrors.New("unit dependencies not satisfied")
+	ErrSameStatusAlreadySet     = xerrors.New("same status already set")
+	ErrCycleDetected            = xerrors.New("cycle detected")
+	ErrFailedToAddDependency    = xerrors.New("failed to add dependency")
+)
+
+// Status represents the status of a unit.
+type Status string
+
+// Status constants for dependency tracking.
+const (
+	StatusNotRegistered Status = ""
+	StatusPending       Status = "pending"
+	StatusStarted       Status = "started"
+	StatusComplete      Status = "completed"
+)
+
+// ID provides a type narrowed representation of the unique identifier of a unit.
+type ID string
+
+// Unit represents a point-in-time snapshot of a vertex in the dependency graph.
+// Units may depend on other units, or be depended on by other units. The unit struct
+// is not aware of updates made to the dependency graph after it is initialized and should
+// not be cached.
+type Unit struct {
+	id     ID
+	status Status
+	// ready is true if all dependencies are satisfied.
+	// It does not have an accessor method on Unit, because a unit cannot know whether it is ready.
+	// Only the Manager can calculate whether a unit is ready based on knowledge of the dependency graph.
+	// To discourage use of an outdated readiness value, only the Manager should set and return this field.
+	ready bool
+}
+
+func (u Unit) ID() ID {
+	return u.id
+}
+
+func (u Unit) Status() Status {
+	return u.status
+}
+
+// Dependency represents a dependency relationship between units.
+type Dependency struct {
+	Unit           ID
+	DependsOn      ID
+	RequiredStatus Status
+	CurrentStatus  Status
+	IsSatisfied    bool
+}
+
+// Manager provides reactive dependency tracking over a Graph.
+// It manages Unit registration, dependency relationships, and status updates
+// with automatic recalculation of readiness when dependencies are satisfied.
+type Manager struct {
+	mu sync.RWMutex
+
+	// The underlying graph that stores dependency relationships
+	graph *Graph[Status, ID]
+
+	// Store vertex instances for each unit to ensure consistent references
+	units map[ID]Unit
+}
+
+// NewManager creates a new Manager instance.
+func NewManager() *Manager {
+	return &Manager{
+		graph: &Graph[Status, ID]{},
+		units: make(map[ID]Unit),
+	}
+}
+
+// Register adds a unit to the manager if it is not already registered.
+// If a Unit is already registered (per the ID field), it is not updated.
+func (m *Manager) Register(id ID) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.registered(id) {
+		return xerrors.Errorf("registering unit %q: %w", id, ErrUnitAlreadyRegistered)
+	}
+
+	m.units[id] = Unit{
+		id:     id,
+		status: StatusPending,
+		ready:  true,
+	}
+
+	return nil
+}
+
+// registered checks if a unit is registered in the manager.
+func (m *Manager) registered(id ID) bool {
+	return m.units[id].status != StatusNotRegistered
+}
+
+// Unit fetches a unit from the manager. If the unit does not exist,
+// it returns the Unit zero-value as a placeholder unit, because
+// units may depend on other units that have not yet been created.
+func (m *Manager) Unit(id ID) Unit {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	return m.units[id]
+}
+
+func (m *Manager) IsReady(id ID) bool {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if !m.registered(id) {
+		return false
+	}
+
+	return m.units[id].ready
+}
+
+// AddDependency adds a dependency relationship between units.
+// The unit depends on the dependsOn unit reaching the requiredStatus.
+func (m *Manager) AddDependency(unit ID, dependsOn ID, requiredStatus Status) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if !m.registered(unit) {
+		return xerrors.Errorf("checking registration for unit %q: %w", unit, ErrUnitNotFound)
+	}
+
+	// Add the dependency edge to the graph
+	// The edge goes from unit to dependsOn, representing the dependency
+	err := m.graph.AddEdge(unit, dependsOn, requiredStatus)
+	if err != nil {
+		return xerrors.Errorf("adding edge for unit %q: %w", unit, errors.Join(ErrFailedToAddDependency, err))
+	}
+
+	// Recalculate readiness for the unit since it now has a new dependency
+	m.recalculateReadinessUnsafe(unit)
+
+	return nil
+}
+
+// UpdateStatus updates a unit's status and recalculates readiness for affected dependents.
+func (m *Manager) UpdateStatus(unit ID, newStatus Status) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if !m.registered(unit) {
+		return xerrors.Errorf("checking registration for unit %q: %w", unit, ErrUnitNotFound)
+	}
+
+	u := m.units[unit]
+	if u.status == newStatus {
+		return xerrors.Errorf("checking status for unit %q: %w", unit, ErrSameStatusAlreadySet)
+	}
+
+	u.status = newStatus
+	m.units[unit] = u
+
+	// Get all units that depend on this one (reverse adjacent vertices)
+	dependents := m.graph.GetReverseAdjacentVertices(unit)
+
+	// Recalculate readiness for all dependents
+	for _, dependent := range dependents {
+		m.recalculateReadinessUnsafe(dependent.From)
+	}
+
+	return nil
+}
+
+// recalculateReadinessUnsafe recalculates the readiness state for a unit.
+// This method assumes the caller holds the write lock.
+func (m *Manager) recalculateReadinessUnsafe(unit ID) {
+	u := m.units[unit]
+	dependencies := m.graph.GetForwardAdjacentVertices(unit)
+
+	allSatisfied := true
+	for _, dependency := range dependencies {
+		requiredStatus := dependency.Edge
+		dependsOnUnit := m.units[dependency.To]
+		if dependsOnUnit.status != requiredStatus {
+			allSatisfied = false
+			break
+		}
+	}
+
+	u.ready = allSatisfied
+	m.units[unit] = u
+}
+
+// GetGraph returns the underlying graph for visualization and debugging.
+// This should be used carefully as it exposes the internal graph structure.
+func (m *Manager) GetGraph() *Graph[Status, ID] {
+	return m.graph
+}
+
+// GetAllDependencies returns all dependencies for a unit, both satisfied and unsatisfied.
+func (m *Manager) GetAllDependencies(unit ID) ([]Dependency, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if !m.registered(unit) {
+		return nil, xerrors.Errorf("checking registration for unit %q: %w", unit, ErrUnitNotFound)
+	}
+
+	dependencies := m.graph.GetForwardAdjacentVertices(unit)
+
+	var allDependencies []Dependency
+
+	for _, dependency := range dependencies {
+		dependsOnUnit := m.units[dependency.To]
+		requiredStatus := dependency.Edge
+		allDependencies = append(allDependencies, Dependency{
+			Unit:           unit,
+			DependsOn:      dependsOnUnit.id,
+			RequiredStatus: requiredStatus,
+			CurrentStatus:  dependsOnUnit.status,
+			IsSatisfied:    dependsOnUnit.status == requiredStatus,
+		})
+	}
+
+	return allDependencies, nil
+}
+
+// GetUnmetDependencies returns a list of unsatisfied dependencies for a unit.
+func (m *Manager) GetUnmetDependencies(unit ID) ([]Dependency, error) {
+	allDependencies, err := m.GetAllDependencies(unit)
+	if err != nil {
+		return nil, err
+	}
+
+	var unmetDependencies []Dependency = slice.Filter(allDependencies, func(dependency Dependency) bool {
+		return !dependency.IsSatisfied
+	})
+
+	return unmetDependencies, nil
+}
+
+// ExportDOT exports the dependency graph to DOT format for visualization.
+func (m *Manager) ExportDOT(name string) (string, error) {
+	return m.graph.ToDOT(name)
+}