@@ -0,0 +1,267 @@
+package dag
+
+import (
+	"fmt"
+	"log"
+	"sync"
+	"time"
+
+	"github.com/hashicorp/go-multierror"
+)
+
+// walker performs a graph walk
+type walker struct {
+	Callback WalkFunc
+
+	vertices  *Set
+	edges     *Set
+	vertexMap map[Vertex]*walkerVertex
+
+	wait       sync.WaitGroup
+	changeLock sync.Mutex
+
+	errMap  map[Vertex]error
+	errLock sync.Mutex
+}
+
+type walkerVertex struct {
+	sync.Mutex
+
+	DoneCh       chan struct{}
+	CancelCh     chan struct{}
+	DepsCh       chan struct{}
+	DepsUpdateCh chan chan struct{}
+
+	deps         map[Vertex]chan struct{}
+	depsCancelCh chan struct{}
+}
+
+// Wait waits for the completion of the walk and returns any errors (
+// in the form of a multierror) that occurred. Update should be called
+// to populate the walk with vertices and edges.
+func (w *walker) Wait() error {
+	// Wait for completion
+	w.wait.Wait()
+
+	// Grab the error lock
+	w.errLock.Lock()
+	defer w.errLock.Unlock()
+
+	// Build the error
+	var result error
+	for v, err := range w.errMap {
+		result = multierror.Append(result, fmt.Errorf(
+			"%s: %s", VertexName(v), err))
+	}
+
+	return result
+}
+
+// Update updates the currently executing walk with the given vertices
+// and edges. It does not block until completion.
+//
+// Update can be called in parallel to Walk.
+func (w *walker) Update(v, e *Set) {
+	// Grab the change lock so no more updates happen but also so that
+	// no new vertices are executed during this time since we may be
+	// removing them.
+	w.changeLock.Lock()
+	defer w.changeLock.Unlock()
+
+	// Initialize fields
+	if w.vertexMap == nil {
+		w.vertexMap = make(map[Vertex]*walkerVertex)
+	}
+
+	// Calculate all our sets
+	newEdges := e.Difference(w.edges)
+	newVerts := v.Difference(w.vertices)
+	oldVerts := w.vertices.Difference(v)
+
+	// Add the new vertices
+	for _, raw := range newVerts.List() {
+		v := raw.(Vertex)
+
+		// Add to the waitgroup so our walk is not done until everything finishes
+		w.wait.Add(1)
+
+		// Initialize the vertex info
+		info := &walkerVertex{
+			DoneCh:       make(chan struct{}),
+			CancelCh:     make(chan struct{}),
+			DepsCh:       make(chan struct{}),
+			DepsUpdateCh: make(chan chan struct{}, 5),
+			deps:         make(map[Vertex]chan struct{}),
+		}
+
+		// Close the deps channel immediately so it passes
+		close(info.DepsCh)
+
+		// Add it to the map and kick off the walk
+		w.vertexMap[v] = info
+	}
+
+	// Remove the old vertices
+	for _, raw := range oldVerts.List() {
+		v := raw.(Vertex)
+
+		// Get the vertex info so we can cancel it
+		info, ok := w.vertexMap[v]
+		if !ok {
+			// This vertex for some reason was never in our map. This
+			// shouldn't be possible.
+			continue
+		}
+
+		// Cancel the vertex
+		close(info.CancelCh)
+
+		// Delete it out of the map
+		delete(w.vertexMap, v)
+	}
+
+	// Add the new edges
+	var changedDeps Set
+	for _, raw := range newEdges.List() {
+		edge := raw.(Edge)
+
+		// waiter is the vertex that is "waiting" on this edge
+		waiter := edge.Target()
+
+		// dep is the dependency we're waiting on
+		dep := edge.Source()
+
+		// Get the info for the waiter
+		waiterInfo, ok := w.vertexMap[waiter]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Get the info for the dep
+		depInfo, ok := w.vertexMap[dep]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Add the dependency to our waiter
+		waiterInfo.deps[dep] = depInfo.DoneCh
+
+		// Record that the deps changed for this waiter
+		changedDeps.Add(waiter)
+	}
+
+	for _, raw := range changedDeps.List() {
+		v := raw.(Vertex)
+		info, ok := w.vertexMap[v]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Create a new done channel
+		doneCh := make(chan struct{})
+
+		// Create the channel we close for cancellation
+		cancelCh := make(chan struct{})
+		info.depsCancelCh = cancelCh
+
+		// Build a new deps copy
+		deps := make(map[Vertex]<-chan struct{})
+		for k, v := range info.deps {
+			deps[k] = v
+		}
+
+		// Update the update channel
+		info.DepsUpdateCh <- doneCh
+
+		// Start the waiter
+		go w.waitDeps(v, deps, doneCh, cancelCh)
+	}
+
+	// Kickstart all the vertices
+	for _, raw := range newVerts.List() {
+		v := raw.(Vertex)
+		go w.walkVertex(v, w.vertexMap[v])
+	}
+}
+
+// walkVertex walks a single vertex, waiting for any dependencies before
+// executing the callback.
+func (w *walker) walkVertex(v Vertex, info *walkerVertex) {
+	// When we're done executing, lower the waitgroup count
+	defer w.wait.Done()
+
+	// When we're done, always close our done channel
+	defer close(info.DoneCh)
+
+	// Wait for our dependencies
+	depsCh := info.DepsCh
+	for {
+		select {
+		case <-info.CancelCh:
+			// Cancel
+			return
+
+		case <-depsCh:
+			// Deps complete!
+			depsCh = nil
+
+		case depsCh = <-info.DepsUpdateCh:
+			// New deps, reloop
+		}
+
+		if depsCh == nil {
+			// One final check if we have an update
+			select {
+			case depsCh = <-info.DepsUpdateCh:
+			default:
+			}
+
+			if depsCh == nil {
+				break
+			}
+		}
+	}
+
+	// Call our callback
+	if err := w.Callback(v); err != nil {
+		w.errLock.Lock()
+		defer w.errLock.Unlock()
+
+		if w.errMap == nil {
+			w.errMap = make(map[Vertex]error)
+		}
+		w.errMap[v] = err
+	}
+}
+
+func (w *walker) waitDeps(
+	v Vertex,
+	deps map[Vertex]<-chan struct{},
+	doneCh chan<- struct{},
+	cancelCh <-chan struct{}) {
+	// Whenever we return, mark ourselves as complete
+	defer close(doneCh)
+
+	// For each dependency given to us, wait for it to complete
+	for dep, depCh := range deps {
+	DepSatisfied:
+		for {
+			select {
+			case <-depCh:
+				// Dependency satisfied!
+				break DepSatisfied
+
+			case <-cancelCh:
+				// Wait cancelled
+				return
+
+			case <-time.After(time.Second * 5):
+				log.Printf("[DEBUG] vertex %q, waiting for: %q",
+					VertexName(v), VertexName(dep))
+			}
+		}
+	}
+}
@@ -0,0 +1,43 @@
+package dag
+
+import (
+	"reflect"
+	"sync"
+	"testing"
+)
+
+func TestWalker_basic(t *testing.T) {
+	var g Graph
+	g.Add(1)
+	g.Add(2)
+	g.Connect(BasicEdge(1, 2))
+
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var order []interface{}
+		w := &walker{Callback: walkCbRecord(&order)}
+		w.Update(g.vertices, g.edges)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1, 2}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+// walkCbRecord is a test helper callback that just records the order called.
+func walkCbRecord(order *[]interface{}) WalkFunc {
+	var l sync.Mutex
+	return func(v Vertex) error {
+		l.Lock()
+		defer l.Unlock()
+		*order = append(*order, v)
+		return nil
+	}
+}