plugin.go

/*
Copyright 2015 The Kubernetes Authors All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package scheduler

import (
	"fmt"
	"io"
	"net/http"
	"strconv"
	"sync"
	"time"

	log "github.com/golang/glog"
	mesos "github.com/mesos/mesos-go/mesosproto"
	mutil "github.com/mesos/mesos-go/mesosutil"
	"k8s.io/kubernetes/contrib/mesos/pkg/backoff"
	"k8s.io/kubernetes/contrib/mesos/pkg/offers"
	"k8s.io/kubernetes/contrib/mesos/pkg/queue"
	"k8s.io/kubernetes/contrib/mesos/pkg/runtime"
	annotation "k8s.io/kubernetes/contrib/mesos/pkg/scheduler/meta"
	"k8s.io/kubernetes/contrib/mesos/pkg/scheduler/podtask"
	"k8s.io/kubernetes/pkg/api"
	"k8s.io/kubernetes/pkg/api/errors"
	"k8s.io/kubernetes/pkg/client/cache"
	"k8s.io/kubernetes/pkg/client/record"
	client "k8s.io/kubernetes/pkg/client/unversioned"
	"k8s.io/kubernetes/pkg/fields"
	"k8s.io/kubernetes/pkg/util"
	plugin "k8s.io/kubernetes/plugin/pkg/scheduler"
	"k8s.io/kubernetes/plugin/pkg/scheduler/algorithm"
)

const (
	enqueuePopTimeout   = 200 * time.Millisecond
	enqueueWaitTimeout  = 1 * time.Second
	yieldPopTimeout     = 200 * time.Millisecond
	yieldWaitTimeout    = 1 * time.Second
	pluginRecoveryDelay = 100 * time.Millisecond // delay after scheduler plugin crashes, before we resume scheduling
)

const (
	FailedScheduling = "FailedScheduling"
	Scheduled        = "Scheduled"
)

// scheduler abstraction to allow for easier unit testing
type schedulerInterface interface {
	sync.Locker // synchronize scheduler plugin operations

	SlaveIndex
	algorithm() PodScheduler
	offers() offers.Registry
	tasks() podtask.Registry

	// driver calls

	killTask(taskId string) error
	launchTask(*podtask.T) error

	// convenience

	createPodTask(api.Context, *api.Pod) (*podtask.T, error)
}

type k8smScheduler struct {
	sync.Mutex
	internal *KubernetesScheduler
}

func (k *k8smScheduler) algorithm() PodScheduler {
	return k.internal
}

func (k *k8smScheduler) offers() offers.Registry {
	return k.internal.offers
}

func (k *k8smScheduler) tasks() podtask.Registry {
	return k.internal.taskRegistry
}

func (k *k8smScheduler) createPodTask(ctx api.Context, pod *api.Pod) (*podtask.T, error) {
	return podtask.New(ctx, "", *pod, k.internal.executor)
}

func (k *k8smScheduler) slaveHostNameFor(id string) string {
	return k.internal.slaveHostNames.HostName(id)
}

func (k *k8smScheduler) killTask(taskId string) error {
	killTaskId := mutil.NewTaskID(taskId)
	_, err := k.internal.driver.KillTask(killTaskId)
	return err
}

func (k *k8smScheduler) launchTask(task *podtask.T) error {
	// assume caller is holding scheduler lock
	taskList := []*mesos.TaskInfo{task.BuildTaskInfo()}
	offerIds := []*mesos.OfferID{task.Offer.Details().Id}
	filters := &mesos.Filters{}
	_, err := k.internal.driver.LaunchTasks(offerIds, taskList, filters)
	return err
}

type binder struct {
	api schedulerInterface
}

// implements binding.Registry, launches the pod-associated-task in mesos
func (b *binder) Bind(binding *api.Binding) error {

	ctx := api.WithNamespace(api.NewContext(), binding.Namespace)

	// default upstream scheduler passes pod.Name as binding.Name
	podKey, err := podtask.MakePodKey(ctx, binding.Name)
	if err != nil {
		return err
	}

	b.api.Lock()
	defer b.api.Unlock()

	switch task, state := b.api.tasks().ForPod(podKey); state {
	case podtask.StatePending:
		return b.bind(ctx, binding, task)
	default:
		// in this case it's likely that the pod has been deleted between Schedule
		// and Bind calls
		log.Infof("No pending task for pod %s", podKey)
		return noSuchPodErr //TODO(jdef) this error is somewhat misleading since the task could be running?!
	}
}

func (b *binder) rollback(task *podtask.T, err error) error {
	task.Offer.Release()
	task.Reset()
	if err2 := b.api.tasks().Update(task); err2 != nil {
		log.Errorf("failed to update pod task: %v", err2)
	}
	return err
}

// assumes that: caller has acquired scheduler lock and that the task is still pending
//
// bind does not actually do the binding itself, but launches the pod as a Mesos task. The
// kubernetes executor on the slave will finally do the binding. This is different from the
// upstream scheduler in the sense that the upstream scheduler does the binding and the
// kubelet will notice that and launches the pod.
func (b *binder) bind(ctx api.Context, binding *api.Binding, task *podtask.T) (err error) {
	// sanity check: ensure that the task hasAcceptedOffer(), it's possible that between
	// Schedule() and now that the offer for this task was rescinded or invalidated.
	// ((we should never see this here))
	if !task.HasAcceptedOffer() {
		return fmt.Errorf("task has not accepted a valid offer %v", task.ID)
	}

	// By this time, there is a chance that the slave is disconnected.
	offerId := task.GetOfferId()
	if offer, ok := b.api.offers().Get(offerId); !ok || offer.HasExpired() {
		// already rescinded or timed out or otherwise invalidated
		return b.rollback(task, fmt.Errorf("failed prior to launchTask due to expired offer for task %v", task.ID))
	}

	if err = b.prepareTaskForLaunch(ctx, binding.Target.Name, task, offerId); err == nil {
		log.V(2).Infof("launching task: %q on target %q slave %q for pod \"%v/%v\", cpu %.2f, mem %.2f MB",
			task.ID, binding.Target.Name, task.Spec.SlaveID, task.Pod.Namespace, task.Pod.Name, task.Spec.CPU, task.Spec.Memory)
		if err = b.api.launchTask(task); err == nil {
			b.api.offers().Invalidate(offerId)
			task.Set(podtask.Launched)
			if err = b.api.tasks().Update(task); err != nil {
				// this should only happen if the task has been removed or has changed status,
				// which SHOULD NOT HAPPEN as long as we're synchronizing correctly
				log.Errorf("failed to update task w/ Launched status: %v", err)
			}
			return
		}
	}
	return b.rollback(task, fmt.Errorf("Failed to launch task %v: %v", task.ID, err))
}

//TODO(jdef) unit test this, ensure that task's copy of api.Pod is not modified
func (b *binder) prepareTaskForLaunch(ctx api.Context, machine string, task *podtask.T, offerId string) error {
	pod := task.Pod

	// we make an effort here to avoid making changes to the task's copy of the pod, since
	// we want that to reflect the initial user spec, and not the modified spec that we
	// build for the executor to consume.
	oemCt := pod.Spec.Containers
	pod.Spec.Containers = append([]api.Container{}, oemCt...) // (shallow) clone before mod

	if pod.Annotations == nil {
		pod.Annotations = make(map[string]string)
	}

	task.SaveRecoveryInfo(pod.Annotations)
	pod.Annotations[annotation.BindingHostKey] = task.Spec.AssignedSlave

	for _, entry := range task.Spec.PortMap {
		oemPorts := pod.Spec.Containers[entry.ContainerIdx].Ports
		ports := append([]api.ContainerPort{}, oemPorts...)
		p := &ports[entry.PortIdx]
		p.HostPort = int(entry.OfferPort)
		op := strconv.FormatUint(entry.OfferPort, 10)
		pod.Annotations[fmt.Sprintf(annotation.PortMappingKeyFormat, p.Protocol, p.ContainerPort)] = op
		if p.Name != "" {
			pod.Annotations[fmt.Sprintf(annotation.PortNameMappingKeyFormat, p.Protocol, p.Name)] = op
		}
		pod.Spec.Containers[entry.ContainerIdx].Ports = ports
	}

	// the kubelet-executor uses this to instantiate the pod
	log.V(3).Infof("prepared pod spec: %+v", pod)

	data, err := api.Codec.Encode(&pod)
	if err != nil {
		log.V(2).Infof("Failed to marshal the pod spec: %v", err)
		return err
	}
	task.Spec.Data = data
	return nil
}

type kubeScheduler struct {
	api        schedulerInterface
	podUpdates queue.FIFO
}

// recoverAssignedSlave recovers the assigned Mesos slave from a pod by searching
// the BindingHostKey. For tasks in the registry of the scheduler, the same
// value is stored in T.Spec.AssignedSlave. Before launching, the BindingHostKey
// annotation is added and the executor will eventually persist that to the
// apiserver on binding.
func recoverAssignedSlave(pod *api.Pod) string {
	return pod.Annotations[annotation.BindingHostKey]
}

// Schedule implements the Scheduler interface of Kubernetes.
// It returns the selectedMachine's name and error (if there's any).
func (k *kubeScheduler) Schedule(pod *api.Pod, unused algorithm.NodeLister) (string, error) {
	log.Infof("Try to schedule pod %v\n", pod.Name)
	ctx := api.WithNamespace(api.NewDefaultContext(), pod.Namespace)

	// default upstream scheduler passes pod.Name as binding.PodID
	podKey, err := podtask.MakePodKey(ctx, pod.Name)
	if err != nil {
		return "", err
	}

	k.api.Lock()
	defer k.api.Unlock()

	switch task, state := k.api.tasks().ForPod(podKey); state {
	case podtask.StateUnknown:
		// There's a bit of a potential race here, a pod could have been yielded() and
		// then before we get *here* it could be deleted.
		// We use meta to index the pod in the store since that's what k8s reflector does.
		podName, err := cache.MetaNamespaceKeyFunc(pod)
		if err != nil {
			log.Warningf("aborting Schedule, unable to understand pod object %+v", pod)
			return "", noSuchPodErr
		}
		if deleted := k.podUpdates.Poll(podName, queue.DELETE_EVENT); deleted {
			// avoid scheduling a pod that's been deleted between yieldPod() and Schedule()
			log.Infof("aborting Schedule, pod has been deleted %+v", pod)
			return "", noSuchPodErr
		}
		return k.doSchedule(k.api.tasks().Register(k.api.createPodTask(ctx, pod)))

	//TODO(jdef) it's possible that the pod state has diverged from what
	//we knew previously, we should probably update the task.Pod state here
	//before proceeding with scheduling
	case podtask.StatePending:
		if pod.UID != task.Pod.UID {
			// we're dealing with a brand new pod spec here, so the old one must have been
			// deleted -- and so our task store is out of sync w/ respect to reality
			//TODO(jdef) reconcile task
			return "", fmt.Errorf("task %v spec is out of sync with pod %v spec, aborting schedule", task.ID, pod.Name)
		} else if task.Has(podtask.Launched) {
			// task has been marked as "launched" but the pod binding creation may have failed in k8s,
			// but we're going to let someone else handle it, probably the mesos task error handler
			return "", fmt.Errorf("task %s has already been launched, aborting schedule", task.ID)
		} else {
			return k.doSchedule(task, nil)
		}

	default:
		return "", fmt.Errorf("task %s is not pending, nothing to schedule", task.ID)
	}
}

// Call ScheduleFunc and subtract some resources, returning the name of the machine the task is scheduled on
func (k *kubeScheduler) doSchedule(task *podtask.T, err error) (string, error) {
	var offer offers.Perishable
	if task.HasAcceptedOffer() {
		// verify that the offer is still on the table
		offerId := task.GetOfferId()
		if offer, ok := k.api.offers().Get(offerId); ok && !offer.HasExpired() {
			// skip tasks that have already have assigned offers
			offer = task.Offer
		} else {
			task.Offer.Release()
			task.Reset()
			if err = k.api.tasks().Update(task); err != nil {
				return "", err
			}
		}
	}
	if err == nil && offer == nil {
		offer, err = k.api.algorithm().SchedulePod(k.api.offers(), k.api, task)
	}
	if err != nil {
		return "", err
	}
	details := offer.Details()
	if details == nil {
		return "", fmt.Errorf("offer already invalid/expired for task %v", task.ID)
	}
	slaveId := details.GetSlaveId().GetValue()
	if slaveHostName := k.api.slaveHostNameFor(slaveId); slaveHostName == "" {
		// not much sense in Release()ing the offer here since its owner died
		offer.Release()
		k.api.offers().Invalidate(details.Id.GetValue())
		return "", fmt.Errorf("Slave disappeared (%v) while scheduling task %v", slaveId, task.ID)
	} else {
		if task.Offer != nil && task.Offer != offer {
			return "", fmt.Errorf("task.offer assignment must be idempotent, task %+v: offer %+v", task, offer)
		}

		task.Offer = offer
		k.api.algorithm().Procurement()(task, details) // TODO(jdef) why is nothing checking the error returned here?

		if err := k.api.tasks().Update(task); err != nil {
			offer.Release()
			return "", err
		}
		return slaveHostName, nil
	}
}

type queuer struct {
	lock            sync.Mutex       // shared by condition variables of this struct
	podUpdates      queue.FIFO       // queue of pod updates to be processed
	podQueue        *queue.DelayFIFO // queue of pods to be scheduled
	deltaCond       sync.Cond        // pod changes are available for processing
	unscheduledCond sync.Cond        // there are unscheduled pods for processing
}

func newQueuer(store queue.FIFO) *queuer {
	q := &queuer{
		podQueue:   queue.NewDelayFIFO(),
		podUpdates: store,
	}
	q.deltaCond.L = &q.lock
	q.unscheduledCond.L = &q.lock
	return q
}

func (q *queuer) installDebugHandlers(mux *http.ServeMux) {
	mux.HandleFunc("/debug/scheduler/podqueue", func(w http.ResponseWriter, r *http.Request) {
		for _, x := range q.podQueue.List() {
			if _, err := io.WriteString(w, fmt.Sprintf("%+v\n", x)); err != nil {
				break
			}
		}
	})
	mux.HandleFunc("/debug/scheduler/podstore", func(w http.ResponseWriter, r *http.Request) {
		for _, x := range q.podUpdates.List() {
			if _, err := io.WriteString(w, fmt.Sprintf("%+v\n", x)); err != nil {
				break
			}
		}
	})
}

// signal that there are probably pod updates waiting to be processed
func (q *queuer) updatesAvailable() {
	q.deltaCond.Broadcast()
}

// delete a pod from the to-be-scheduled queue
func (q *queuer) dequeue(id string) {
	q.podQueue.Delete(id)
}

// re-add a pod to the to-be-scheduled queue, will not overwrite existing pod data (that
// may have already changed).
func (q *queuer) requeue(pod *Pod) {
	// use KeepExisting in case the pod has already been updated (can happen if binding fails
	// due to constraint voilations); we don't want to overwrite a newer entry with stale data.
	q.podQueue.Add(pod, queue.KeepExisting)
	q.unscheduledCond.Broadcast()
}

// same as requeue but calls podQueue.Offer instead of podQueue.Add
func (q *queuer) reoffer(pod *Pod) {
	// use KeepExisting in case the pod has already been updated (can happen if binding fails
	// due to constraint voilations); we don't want to overwrite a newer entry with stale data.
	if q.podQueue.Offer(pod, queue.KeepExisting) {
		q.unscheduledCond.Broadcast()
	}
}

// spawns a go-routine to watch for unscheduled pods and queue them up
// for scheduling. returns immediately.
func (q *queuer) Run(done <-chan struct{}) {
	go runtime.Until(func() {
		log.Info("Watching for newly created pods")
		q.lock.Lock()
		defer q.lock.Unlock()

		for {
			// limit blocking here for short intervals so that scheduling
			// may proceed even if there have been no recent pod changes
			p := q.podUpdates.Await(enqueuePopTimeout)
			if p == nil {
				signalled := runtime.After(q.deltaCond.Wait)
				// we've yielded the lock
				select {
				case <-time.After(enqueueWaitTimeout):
					q.deltaCond.Broadcast() // abort Wait()
					<-signalled             // wait for lock re-acquisition
					log.V(4).Infoln("timed out waiting for a pod update")
				case <-signalled:
					// we've acquired the lock and there may be
					// changes for us to process now
				}
				continue
			}

			pod := p.(*Pod)
			if recoverAssignedSlave(pod.Pod) != "" {
				log.V(3).Infof("dequeuing assigned pod for scheduling: %v", pod.Pod.Name)
				q.dequeue(pod.GetUID())
			} else {
				// use ReplaceExisting because we are always pushing the latest state
				now := time.Now()
				pod.deadline = &now
				if q.podQueue.Offer(pod, queue.ReplaceExisting) {
					q.unscheduledCond.Broadcast()
					log.V(3).Infof("queued pod for scheduling: %v", pod.Pod.Name)
				} else {
					log.Warningf("failed to queue pod for scheduling: %v", pod.Pod.Name)
				}
			}
		}
	}, 1*time.Second, done)
}

// implementation of scheduling plugin's NextPod func; see k8s plugin/pkg/scheduler
func (q *queuer) yield() *api.Pod {
	log.V(2).Info("attempting to yield a pod")
	q.lock.Lock()
	defer q.lock.Unlock()

	for {
		// limit blocking here to short intervals so that we don't block the
		// enqueuer Run() routine for very long
		kpod := q.podQueue.Await(yieldPopTimeout)
		if kpod == nil {
			signalled := runtime.After(q.unscheduledCond.Wait)
			// lock is yielded at this point and we're going to wait for either
			// a timeout, or a signal that there's data
			select {
			case <-time.After(yieldWaitTimeout):
				q.unscheduledCond.Broadcast() // abort Wait()
				<-signalled                   // wait for the go-routine, and the lock
				log.V(4).Infoln("timed out waiting for a pod to yield")
			case <-signalled:
				// we have acquired the lock, and there
				// may be a pod for us to pop now
			}
			continue
		}

		pod := kpod.(*Pod).Pod
		if podName, err := cache.MetaNamespaceKeyFunc(pod); err != nil {
			log.Warningf("yield unable to understand pod object %+v, will skip: %v", pod, err)
		} else if !q.podUpdates.Poll(podName, queue.POP_EVENT) {
			log.V(1).Infof("yield popped a transitioning pod, skipping: %+v", pod)
		} else if recoverAssignedSlave(pod) != "" {
			// should never happen if enqueuePods is filtering properly
			log.Warningf("yield popped an already-scheduled pod, skipping: %+v", pod)
		} else {
			return pod
		}
	}
}

type errorHandler struct {
	api     schedulerInterface
	backoff *backoff.Backoff
	qr      *queuer
}

// implementation of scheduling plugin's Error func; see plugin/pkg/scheduler
func (k *errorHandler) handleSchedulingError(pod *api.Pod, schedulingErr error) {

	if schedulingErr == noSuchPodErr {
		log.V(2).Infof("Not rescheduling non-existent pod %v", pod.Name)
		return
	}

	log.Infof("Error scheduling %v: %v; retrying", pod.Name, schedulingErr)
	defer util.HandleCrash()

	// default upstream scheduler passes pod.Name as binding.PodID
	ctx := api.WithNamespace(api.NewDefaultContext(), pod.Namespace)
	podKey, err := podtask.MakePodKey(ctx, pod.Name)
	if err != nil {
		log.Errorf("Failed to construct pod key, aborting scheduling for pod %v: %v", pod.Name, err)
		return
	}

	k.backoff.GC()
	k.api.Lock()
	defer k.api.Unlock()

	switch task, state := k.api.tasks().ForPod(podKey); state {
	case podtask.StateUnknown:
		// if we don't have a mapping here any more then someone deleted the pod
		log.V(2).Infof("Could not resolve pod to task, aborting pod reschdule: %s", podKey)
		return

	case podtask.StatePending:
		if task.Has(podtask.Launched) {
			log.V(2).Infof("Skipping re-scheduling for already-launched pod %v", podKey)
			return
		}
		breakoutEarly := queue.BreakChan(nil)
		if schedulingErr == noSuitableOffersErr {
			log.V(3).Infof("adding backoff breakout handler for pod %v", podKey)
			breakoutEarly = queue.BreakChan(k.api.offers().Listen(podKey, func(offer *mesos.Offer) bool {
				k.api.Lock()
				defer k.api.Unlock()
				switch task, state := k.api.tasks().Get(task.ID); state {
				case podtask.StatePending:
					// Assess fitness of pod with the current offer. The scheduler normally
					// "backs off" when it can't find an offer that matches up with a pod.
					// The backoff period for a pod can terminate sooner if an offer becomes
					// available that matches up.
					return !task.Has(podtask.Launched) && k.api.algorithm().FitPredicate()(task, offer, nil)
				default:
					// no point in continuing to check for matching offers
					return true
				}
			}))
		}
		delay := k.backoff.Get(podKey)
		log.V(3).Infof("requeuing pod %v with delay %v", podKey, delay)
		k.qr.requeue(&Pod{Pod: pod, delay: &delay, notify: breakoutEarly})

	default:
		log.V(2).Infof("Task is no longer pending, aborting reschedule for pod %v", podKey)
	}
}

type deleter struct {
	api schedulerInterface
	qr  *queuer
}

// currently monitors for "pod deleted" events, upon which handle()
// is invoked.
func (k *deleter) Run(updates <-chan queue.Entry, done <-chan struct{}) {
	go runtime.Until(func() {
		for {
			entry := <-updates
			pod := entry.Value().(*Pod)
			if entry.Is(queue.DELETE_EVENT) {
				if err := k.deleteOne(pod); err != nil {
					log.Error(err)
				}
			} else if !entry.Is(queue.POP_EVENT) {
				k.qr.updatesAvailable()
			}
		}
	}, 1*time.Second, done)
}

func (k *deleter) deleteOne(pod *Pod) error {
	ctx := api.WithNamespace(api.NewDefaultContext(), pod.Namespace)
	podKey, err := podtask.MakePodKey(ctx, pod.Name)
	if err != nil {
		return err
	}

	log.V(2).Infof("pod deleted: %v", podKey)

	// order is important here: we want to make sure we have the lock before
	// removing the pod from the scheduling queue. this makes the concurrent
	// execution of scheduler-error-handling and delete-handling easier to
	// reason about.
	k.api.Lock()
	defer k.api.Unlock()

	// prevent the scheduler from attempting to pop this; it's also possible that
	// it's concurrently being scheduled (somewhere between pod scheduling and
	// binding) - if so, then we'll end up removing it from taskRegistry which
	// will abort Bind()ing
	k.qr.dequeue(pod.GetUID())

	switch task, state := k.api.tasks().ForPod(podKey); state {
	case podtask.StateUnknown:
		log.V(2).Infof("Could not resolve pod '%s' to task id", podKey)
		return noSuchPodErr

	// determine if the task has already been launched to mesos, if not then
	// cleanup is easier (unregister) since there's no state to sync
	case podtask.StatePending:
		if !task.Has(podtask.Launched) {
			// we've been invoked in between Schedule() and Bind()
			if task.HasAcceptedOffer() {
				task.Offer.Release()
				task.Reset()
				task.Set(podtask.Deleted)
				//TODO(jdef) probably want better handling here
				if err := k.api.tasks().Update(task); err != nil {
					return err
				}
			}
			k.api.tasks().Unregister(task)
			return nil
		}
		fallthrough

	case podtask.StateRunning:
		// signal to watchers that the related pod is going down
		task.Set(podtask.Deleted)
		if err := k.api.tasks().Update(task); err != nil {
			log.Errorf("failed to update task w/ Deleted status: %v", err)
		}
		return k.api.killTask(task.ID)

	default:
		log.Infof("cannot kill pod '%s': non-terminal task not found %v", podKey, task.ID)
		return noSuchTaskErr
	}
}

// Create creates a scheduler plugin and all supporting background functions.
func (k *KubernetesScheduler) NewDefaultPluginConfig(terminate <-chan struct{}, mux *http.ServeMux) *PluginConfig {
	// use ListWatch watching pods using the client by default
	return k.NewPluginConfig(terminate, mux, createAllPodsLW(k.client))
}

func (k *KubernetesScheduler) NewPluginConfig(terminate <-chan struct{}, mux *http.ServeMux,
	podsWatcher *cache.ListWatch) *PluginConfig {

	// Watch and queue pods that need scheduling.
	updates := make(chan queue.Entry, k.schedcfg.UpdatesBacklog)
	podUpdates := &podStoreAdapter{queue.NewHistorical(updates)}
	reflector := cache.NewReflector(podsWatcher, &api.Pod{}, podUpdates, 0)

	// lock that guards critial sections that involve transferring pods from
	// the store (cache) to the scheduling queue; its purpose is to maintain
	// an ordering (vs interleaving) of operations that's easier to reason about.
	kapi := &k8smScheduler{internal: k}
	q := newQueuer(podUpdates)
	podDeleter := &deleter{
		api: kapi,
		qr:  q,
	}
	eh := &errorHandler{
		api:     kapi,
		backoff: backoff.New(k.schedcfg.InitialPodBackoff.Duration, k.schedcfg.MaxPodBackoff.Duration),
		qr:      q,
	}
	startLatch := make(chan struct{})
	eventBroadcaster := record.NewBroadcaster()
	runtime.On(startLatch, func() {
		eventBroadcaster.StartRecordingToSink(k.client.Events(""))
		reflector.Run() // TODO(jdef) should listen for termination
		podDeleter.Run(updates, terminate)
		q.Run(terminate)

		q.installDebugHandlers(mux)
		podtask.InstallDebugHandlers(k.taskRegistry, mux)
	})
	return &PluginConfig{
		Config: &plugin.Config{
			NodeLister: nil,
			Algorithm: &kubeScheduler{
				api:        kapi,
				podUpdates: podUpdates,
			},
			Binder:   &binder{api: kapi},
			NextPod:  q.yield,
			Error:    eh.handleSchedulingError,
			Recorder: eventBroadcaster.NewRecorder(api.EventSource{Component: "scheduler"}),
		},
		api:      kapi,
		client:   k.client,
		qr:       q,
		deleter:  podDeleter,
		starting: startLatch,
	}
}

type PluginConfig struct {
	*plugin.Config
	api      schedulerInterface
	client   *client.Client
	qr       *queuer
	deleter  *deleter
	starting chan struct{} // startup latch
}

func NewPlugin(c *PluginConfig) PluginInterface {
	return &schedulingPlugin{
		config:   c.Config,
		api:      c.api,
		client:   c.client,
		qr:       c.qr,
		deleter:  c.deleter,
		starting: c.starting,
	}
}

type schedulingPlugin struct {
	config   *plugin.Config
	api      schedulerInterface
	client   *client.Client
	qr       *queuer
	deleter  *deleter
	starting chan struct{}
}

func (s *schedulingPlugin) Run(done <-chan struct{}) {
	defer close(s.starting)
	go runtime.Until(s.scheduleOne, pluginRecoveryDelay, done)
}

// hacked from GoogleCloudPlatform/kubernetes/plugin/pkg/scheduler/scheduler.go,
// with the Modeler stuff removed since we don't use it because we have mesos.
func (s *schedulingPlugin) scheduleOne() {
	pod := s.config.NextPod()

	// pods which are pre-scheduled (i.e. NodeName is set) are deleted by the kubelet
	// in upstream. Not so in Mesos because the kubelet hasn't see that pod yet. Hence,
	// the scheduler has to take care of this:
	if pod.Spec.NodeName != "" && pod.DeletionTimestamp != nil {
		log.V(3).Infof("deleting pre-scheduled, not yet running pod: %s/%s", pod.Namespace, pod.Name)
		s.client.Pods(pod.Namespace).Delete(pod.Name, api.NewDeleteOptions(0))
		return
	}

	log.V(3).Infof("Attempting to schedule: %+v", pod)
	dest, err := s.config.Algorithm.Schedule(pod, s.config.NodeLister) // call kubeScheduler.Schedule
	if err != nil {
		log.V(1).Infof("Failed to schedule: %+v", pod)
		s.config.Recorder.Eventf(pod, FailedScheduling, "Error scheduling: %v", err)
		s.config.Error(pod, err)
		return
	}
	b := &api.Binding{
		ObjectMeta: api.ObjectMeta{Namespace: pod.Namespace, Name: pod.Name},
		Target: api.ObjectReference{
			Kind: "Node",
			Name: dest,
		},
	}
	if err := s.config.Binder.Bind(b); err != nil {
		log.V(1).Infof("Failed to bind pod: %+v", err)
		s.config.Recorder.Eventf(pod, FailedScheduling, "Binding rejected: %v", err)
		s.config.Error(pod, err)
		return
	}
	s.config.Recorder.Eventf(pod, Scheduled, "Successfully assigned %v to %v", pod.Name, dest)
}

// this pod may be out of sync with respect to the API server registry:
//      this pod   |  apiserver registry
//    -------------|----------------------
//      host=.*    |  404           ; pod was deleted
//      host=.*    |  5xx           ; failed to sync, try again later?
//      host=""    |  host=""       ; perhaps no updates to process?
//      host=""    |  host="..."    ; pod has been scheduled and assigned, is there a task assigned? (check TaskIdKey in binding?)
//      host="..." |  host=""       ; pod is no longer scheduled, does it need to be re-queued?
//      host="..." |  host="..."    ; perhaps no updates to process?
//
// TODO(jdef) this needs an integration test
func (s *schedulingPlugin) reconcileTask(t *podtask.T) {
	log.V(1).Infof("reconcile pod %v, assigned to slave %q", t.Pod.Name, t.Spec.AssignedSlave)
	ctx := api.WithNamespace(api.NewDefaultContext(), t.Pod.Namespace)
	pod, err := s.client.Pods(api.NamespaceValue(ctx)).Get(t.Pod.Name)
	if err != nil {
		if errors.IsNotFound(err) {
			// attempt to delete
			if err = s.deleter.deleteOne(&Pod{Pod: &t.Pod}); err != nil && err != noSuchPodErr && err != noSuchTaskErr {
				log.Errorf("failed to delete pod: %v: %v", t.Pod.Name, err)
			}
		} else {
			//TODO(jdef) other errors should probably trigger a retry (w/ backoff).
			//For now, drop the pod on the floor
			log.Warning("aborting reconciliation for pod %v: %v", t.Pod.Name, err)
		}
		return
	}

	log.Infof("pod %v scheduled on %q according to apiserver", pod.Name, pod.Spec.NodeName)
	if t.Spec.AssignedSlave != pod.Spec.NodeName {
		if pod.Spec.NodeName == "" {
			// pod is unscheduled.
			// it's possible that we dropped the pod in the scheduler error handler
			// because of task misalignment with the pod (task.Has(podtask.Launched) == true)

			podKey, err := podtask.MakePodKey(ctx, pod.Name)
			if err != nil {
				log.Error(err)
				return
			}

			s.api.Lock()
			defer s.api.Unlock()

			if _, state := s.api.tasks().ForPod(podKey); state != podtask.StateUnknown {
				//TODO(jdef) reconcile the task
				log.Errorf("task already registered for pod %v", pod.Name)
				return
			}

			now := time.Now()
			log.V(3).Infof("reoffering pod %v", podKey)
			s.qr.reoffer(&Pod{
				Pod:      pod,
				deadline: &now,
			})
		} else {
			// pod is scheduled.
			// not sure how this happened behind our backs. attempt to reconstruct
			// at least a partial podtask.T record.
			//TODO(jdef) reconcile the task
			log.Errorf("pod already scheduled: %v", pod.Name)
		}
	} else {
		//TODO(jdef) for now, ignore the fact that the rest of the spec may be different
		//and assume that our knowledge of the pod aligns with that of the apiserver
		log.Error("pod reconciliation does not support updates; not yet implemented")
	}
}

func parseSelectorOrDie(s string) fields.Selector {
	selector, err := fields.ParseSelector(s)
	if err != nil {
		panic(err)
	}
	return selector
}

// createAllPodsLW returns a listWatch that finds all pods
func createAllPodsLW(cl *client.Client) *cache.ListWatch {
	return cache.NewListWatchFromClient(cl, "pods", api.NamespaceAll, parseSelectorOrDie(""))
}

// Consumes *api.Pod, produces *Pod; the k8s reflector wants to push *api.Pod
// objects at us, but we want to store more flexible (Pod) type defined in
// this package. The adapter implementation facilitates this. It's a little
// hackish since the object type going in is different than the object type
// coming out -- you've been warned.
type podStoreAdapter struct {
	queue.FIFO
}

func (psa *podStoreAdapter) Add(obj interface{}) error {
	pod := obj.(*api.Pod)
	return psa.FIFO.Add(&Pod{Pod: pod})
}

func (psa *podStoreAdapter) Update(obj interface{}) error {
	pod := obj.(*api.Pod)
	return psa.FIFO.Update(&Pod{Pod: pod})
}

func (psa *podStoreAdapter) Delete(obj interface{}) error {
	pod := obj.(*api.Pod)
	return psa.FIFO.Delete(&Pod{Pod: pod})
}

func (psa *podStoreAdapter) Get(obj interface{}) (interface{}, bool, error) {
	pod := obj.(*api.Pod)
	return psa.FIFO.Get(&Pod{Pod: pod})
}

// Replace will delete the contents of the store, using instead the
// given map. This store implementation does NOT take ownership of the map.
func (psa *podStoreAdapter) Replace(objs []interface{}, resourceVersion string) error {
	newobjs := make([]interface{}, len(objs))
	for i, v := range objs {
		pod := v.(*api.Pod)
		newobjs[i] = &Pod{Pod: pod}
	}
	return psa.FIFO.Replace(newobjs, resourceVersion)
}