From 18d9e89065d87fb4cda2c26cdd80d08e9e388098 Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Fri, 20 May 2016 16:03:53 -0700
Subject: [PATCH 1/6] Reuse the same evaluation and reblock it until there is
 no more work to do

---
 nomad/eval_endpoint.go          |  40 +++++++++++
 nomad/eval_endpoint_test.go     | 118 ++++++++++++++++++++++++++++++
 nomad/worker.go                 |  37 ++++++++++
 nomad/worker_test.go            |  48 +++++++++++++
 scheduler/generic_sched.go      |  16 ++++-
 scheduler/generic_sched_test.go | 122 +++++++++++++++++++++++++++++++-
 scheduler/scheduler.go          |   6 ++
 scheduler/testing.go            |  34 ++++++++-
 8 files changed, 415 insertions(+), 6 deletions(-)

diff --git a/nomad/eval_endpoint.go b/nomad/eval_endpoint.go
index 712111697..25895096e 100644
--- a/nomad/eval_endpoint.go
+++ b/nomad/eval_endpoint.go
@@ -202,6 +202,46 @@ func (e *Eval) Create(args *structs.EvalUpdateRequest,
 	return nil
 }
 
+// Reblock is used to reinsert an existing blocked evaluation into the blocked
+// evaluation tracker.
+func (e *Eval) Reblock(args *structs.EvalUpdateRequest, reply *structs.GenericResponse) error {
+	if done, err := e.srv.forward("Eval.Reblock", args, args, reply); done {
+		return err
+	}
+	defer metrics.MeasureSince([]string{"nomad", "eval", "reblock"}, time.Now())
+
+	// Ensure there is only a single update with token
+	if len(args.Evals) != 1 {
+		return fmt.Errorf("only a single eval can be reblocked")
+	}
+	eval := args.Evals[0]
+
+	// Verify the evaluation is outstanding, and that the tokens match.
+	if err := e.srv.evalBroker.OutstandingReset(eval.ID, args.EvalToken); err != nil {
+		return err
+	}
+
+	// Look for the eval
+	snap, err := e.srv.fsm.State().Snapshot()
+	if err != nil {
+		return err
+	}
+	out, err := snap.EvalByID(eval.ID)
+	if err != nil {
+		return err
+	}
+	if out == nil {
+		return fmt.Errorf("evaluation does not exist")
+	}
+	if out.Status != structs.EvalStatusBlocked {
+		return fmt.Errorf("evaluation not blocked")
+	}
+
+	// Reblock the eval
+	e.srv.blockedEvals.Block(eval)
+	return nil
+}
+
 // Reap is used to cleanup dead evaluations and allocations
 func (e *Eval) Reap(args *structs.EvalDeleteRequest,
 	reply *structs.GenericResponse) error {
diff --git a/nomad/eval_endpoint_test.go b/nomad/eval_endpoint_test.go
index f037c8bd3..3fa270ce8 100644
--- a/nomad/eval_endpoint_test.go
+++ b/nomad/eval_endpoint_test.go
@@ -575,3 +575,121 @@ func TestEvalEndpoint_Allocations_Blocking(t *testing.T) {
 		t.Fatalf("bad: %#v", resp.Allocations)
 	}
 }
+
+func TestEvalEndpoint_Reblock_NonExistent(t *testing.T) {
+	s1 := testServer(t, nil)
+	defer s1.Shutdown()
+	codec := rpcClient(t, s1)
+
+	testutil.WaitForResult(func() (bool, error) {
+		return s1.evalBroker.Enabled(), nil
+	}, func(err error) {
+		t.Fatalf("should enable eval broker")
+	})
+
+	// Create the register request
+	eval1 := mock.Eval()
+	s1.evalBroker.Enqueue(eval1)
+	out, token, err := s1.evalBroker.Dequeue(defaultSched, time.Second)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if out == nil {
+		t.Fatalf("missing eval")
+	}
+
+	get := &structs.EvalUpdateRequest{
+		Evals:        []*structs.Evaluation{eval1},
+		EvalToken:    token,
+		WriteRequest: structs.WriteRequest{Region: "global"},
+	}
+	var resp structs.GenericResponse
+	if err := msgpackrpc.CallWithCodec(codec, "Eval.Reblock", get, &resp); err == nil {
+		t.Fatalf("expect error since eval does not exist")
+	}
+}
+
+func TestEvalEndpoint_Reblock_NonBlocked(t *testing.T) {
+	s1 := testServer(t, nil)
+	defer s1.Shutdown()
+	codec := rpcClient(t, s1)
+
+	testutil.WaitForResult(func() (bool, error) {
+		return s1.evalBroker.Enabled(), nil
+	}, func(err error) {
+		t.Fatalf("should enable eval broker")
+	})
+
+	// Create the eval
+	eval1 := mock.Eval()
+	s1.evalBroker.Enqueue(eval1)
+
+	// Insert it into the state store
+	if err := s1.fsm.State().UpsertEvals(1000, []*structs.Evaluation{eval1}); err != nil {
+		t.Fatal(err)
+	}
+
+	out, token, err := s1.evalBroker.Dequeue(defaultSched, time.Second)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if out == nil {
+		t.Fatalf("missing eval")
+	}
+
+	get := &structs.EvalUpdateRequest{
+		Evals:        []*structs.Evaluation{eval1},
+		EvalToken:    token,
+		WriteRequest: structs.WriteRequest{Region: "global"},
+	}
+	var resp structs.GenericResponse
+	if err := msgpackrpc.CallWithCodec(codec, "Eval.Reblock", get, &resp); err == nil {
+		t.Fatalf("should error since eval was not in blocked state", err)
+	}
+}
+
+func TestEvalEndpoint_Reblock(t *testing.T) {
+	s1 := testServer(t, nil)
+	defer s1.Shutdown()
+	codec := rpcClient(t, s1)
+
+	testutil.WaitForResult(func() (bool, error) {
+		return s1.evalBroker.Enabled(), nil
+	}, func(err error) {
+		t.Fatalf("should enable eval broker")
+	})
+
+	// Create the eval
+	eval1 := mock.Eval()
+	eval1.Status = structs.EvalStatusBlocked
+	s1.evalBroker.Enqueue(eval1)
+
+	// Insert it into the state store
+	if err := s1.fsm.State().UpsertEvals(1000, []*structs.Evaluation{eval1}); err != nil {
+		t.Fatal(err)
+	}
+
+	out, token, err := s1.evalBroker.Dequeue(defaultSched, time.Second)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if out == nil {
+		t.Fatalf("missing eval")
+	}
+
+	get := &structs.EvalUpdateRequest{
+		Evals:        []*structs.Evaluation{eval1},
+		EvalToken:    token,
+		WriteRequest: structs.WriteRequest{Region: "global"},
+	}
+	var resp structs.GenericResponse
+	if err := msgpackrpc.CallWithCodec(codec, "Eval.Reblock", get, &resp); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Check that it is blocked
+	bStats := s1.blockedEvals.Stats()
+	if bStats.TotalBlocked+bStats.TotalEscaped == 0 {
+		t.Fatalf("ReblockEval didn't insert eval into the blocked eval tracker")
+	}
+}
diff --git a/nomad/worker.go b/nomad/worker.go
index 1bae409d5..9cb1a286a 100644
--- a/nomad/worker.go
+++ b/nomad/worker.go
@@ -395,6 +395,43 @@ SUBMIT:
 	return nil
 }
 
+// ReblockEval is used to reinsert a blocked evaluation into the blocked eval
+// tracker. This allows the worker to act as the planner for the scheduler.
+func (w *Worker) ReblockEval(eval *structs.Evaluation) error {
+	// Check for a shutdown before plan submission
+	if w.srv.IsShutdown() {
+		return fmt.Errorf("shutdown while planning")
+	}
+	defer metrics.MeasureSince([]string{"nomad", "worker", "reblock_eval"}, time.Now())
+
+	// TODO need to set the evaluations WorkerIndex
+
+	// Setup the request
+	req := structs.EvalUpdateRequest{
+		Evals:     []*structs.Evaluation{eval},
+		EvalToken: w.evalToken,
+		WriteRequest: structs.WriteRequest{
+			Region: w.srv.config.Region,
+		},
+	}
+	var resp structs.GenericResponse
+
+SUBMIT:
+	// Make the RPC call
+	if err := w.srv.RPC("Eval.Reblock", &req, &resp); err != nil {
+		w.logger.Printf("[ERR] worker: failed to reblock evaluation %#v: %v",
+			eval, err)
+		if w.shouldResubmit(err) && !w.backoffErr(backoffBaselineSlow, backoffLimitSlow) {
+			goto SUBMIT
+		}
+		return err
+	} else {
+		w.logger.Printf("[DEBUG] worker: reblocked evaluation %#v", eval)
+		w.backoffReset()
+	}
+	return nil
+}
+
 // shouldResubmit checks if a given error should be swallowed and the plan
 // resubmitted after a backoff. Usually these are transient errors that
 // the cluster should heal from quickly.
diff --git a/nomad/worker_test.go b/nomad/worker_test.go
index c1663592a..6b4f657a9 100644
--- a/nomad/worker_test.go
+++ b/nomad/worker_test.go
@@ -442,3 +442,51 @@ func TestWorker_CreateEval(t *testing.T) {
 		t.Fatalf("bad: %v", out)
 	}
 }
+
+func TestWorker_ReblockEval(t *testing.T) {
+	s1 := testServer(t, func(c *Config) {
+		c.NumSchedulers = 0
+		c.EnabledSchedulers = []string{structs.JobTypeService}
+	})
+	defer s1.Shutdown()
+	testutil.WaitForLeader(t, s1.RPC)
+
+	// Create the blocked eval
+	eval1 := mock.Eval()
+	eval1.Status = structs.EvalStatusBlocked
+
+	// Insert it into the state store
+	if err := s1.fsm.State().UpsertEvals(1000, []*structs.Evaluation{eval1}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Enqueue the eval
+	testutil.WaitForResult(func() (bool, error) {
+		err := s1.evalBroker.Enqueue(eval1)
+		return err == nil, err
+	}, func(err error) {
+		t.Fatalf("err: %v", err)
+	})
+	evalOut, token, err := s1.evalBroker.Dequeue([]string{eval1.Type}, time.Second)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if evalOut != eval1 {
+		t.Fatalf("Bad eval")
+	}
+
+	eval2 := evalOut.Copy()
+
+	// Attempt to reblock eval
+	w := &Worker{srv: s1, logger: s1.logger, evalToken: token}
+	err = w.ReblockEval(eval2)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Check that it is blocked
+	bStats := s1.blockedEvals.Stats()
+	if bStats.TotalBlocked+bStats.TotalEscaped == 0 {
+		t.Fatalf("ReblockEval didn't insert eval into the blocked eval tracker")
+	}
+}
diff --git a/scheduler/generic_sched.go b/scheduler/generic_sched.go
index 4f3bce9b6..8276acc97 100644
--- a/scheduler/generic_sched.go
+++ b/scheduler/generic_sched.go
@@ -113,6 +113,11 @@ func (s *GenericScheduler) Process(eval *structs.Evaluation) error {
 		if statusErr, ok := err.(*SetStatusError); ok {
 			// Scheduling was tried but made no forward progress so create a
 			// blocked eval to retry once resources become available.
+
+			// TODO: Set the trigger by reason of the blocked eval here to
+			// something like "max-attempts"
+			// We can then periodically dequeue these from the blocked_eval
+			// tracker.
 			var mErr multierror.Error
 			if err := s.createBlockedEval(); err != nil {
 				mErr.Errors = append(mErr.Errors, err)
@@ -125,6 +130,12 @@ func (s *GenericScheduler) Process(eval *structs.Evaluation) error {
 		return err
 	}
 
+	// If the current evaluation is a blocked evaluation and we didn't place
+	// everything, do not update the status to complete.
+	if s.eval.Status == structs.EvalStatusBlocked && len(s.eval.FailedTGAllocs) != 0 {
+		return s.planner.ReblockEval(s.eval)
+	}
+
 	// Update the status to complete
 	return setStatus(s.logger, s.planner, s.eval, s.nextEval, s.blocked, structs.EvalStatusComplete, "")
 }
@@ -177,8 +188,9 @@ func (s *GenericScheduler) process() (bool, error) {
 	}
 
 	// If there are failed allocations, we need to create a blocked evaluation
-	// to place the failed allocations when resources become available.
-	if len(s.eval.FailedTGAllocs) != 0 && s.blocked == nil {
+	// to place the failed allocations when resources become available. If the
+	// current evaluation is already a blocked eval, we reuse it.
+	if s.eval.Status != structs.EvalStatusBlocked && len(s.eval.FailedTGAllocs) != 0 && s.blocked == nil {
 		if err := s.createBlockedEval(); err != nil {
 			s.logger.Printf("[ERR] sched: %#v failed to make blocked eval: %v", s.eval, err)
 			return false, err
diff --git a/scheduler/generic_sched_test.go b/scheduler/generic_sched_test.go
index 52a46ff90..d44d76d38 100644
--- a/scheduler/generic_sched_test.go
+++ b/scheduler/generic_sched_test.go
@@ -275,7 +275,7 @@ func TestServiceSched_JobRegister_AllocFail(t *testing.T) {
 	h.AssertEvalStatus(t, structs.EvalStatusComplete)
 }
 
-func TestServiceSched_JobRegister_BlockedEval(t *testing.T) {
+func TestServiceSched_JobRegister_CreateBlockedEval(t *testing.T) {
 	h := NewHarness(t)
 
 	// Create a full node
@@ -454,6 +454,126 @@ func TestServiceSched_JobRegister_FeasibleAndInfeasibleTG(t *testing.T) {
 	h.AssertEvalStatus(t, structs.EvalStatusComplete)
 }
 
+func TestServiceSched_EvaluateBlockedEval(t *testing.T) {
+	h := NewHarness(t)
+
+	// Create a job and set the task group count to zero.
+	job := mock.Job()
+	noErr(t, h.State.UpsertJob(h.NextIndex(), job))
+
+	// Create a mock blocked evaluation
+	eval := &structs.Evaluation{
+		ID:          structs.GenerateUUID(),
+		Status:      structs.EvalStatusBlocked,
+		Priority:    job.Priority,
+		TriggeredBy: structs.EvalTriggerJobRegister,
+		JobID:       job.ID,
+	}
+
+	// Insert it into the state store
+	noErr(t, h.State.UpsertEvals(h.NextIndex(), []*structs.Evaluation{eval}))
+
+	// Process the evaluation
+	err := h.Process(NewServiceScheduler, eval)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Ensure there was no plan
+	if len(h.Plans) != 0 {
+		t.Fatalf("bad: %#v", h.Plans)
+	}
+
+	// Ensure that the eval was reblocked
+	if len(h.ReblockEvals) != 1 {
+		t.Fatalf("bad: %#v", h.ReblockEvals)
+	}
+	if h.ReblockEvals[0].ID != eval.ID {
+		t.Fatalf("expect same eval to be reblocked; got %q; want %q", h.ReblockEvals[0].ID, eval.ID)
+	}
+
+	// Ensure the eval status was not updated
+	if len(h.Evals) != 0 {
+		t.Fatalf("Existing eval should not have status set")
+	}
+}
+
+func TestServiceSched_EvaluateBlockedEval_Finished(t *testing.T) {
+	h := NewHarness(t)
+
+	// Create some nodes
+	for i := 0; i < 10; i++ {
+		node := mock.Node()
+		noErr(t, h.State.UpsertNode(h.NextIndex(), node))
+	}
+
+	// Create a job and set the task group count to zero.
+	job := mock.Job()
+	noErr(t, h.State.UpsertJob(h.NextIndex(), job))
+
+	// Create a mock blocked evaluation
+	eval := &structs.Evaluation{
+		ID:          structs.GenerateUUID(),
+		Status:      structs.EvalStatusBlocked,
+		Priority:    job.Priority,
+		TriggeredBy: structs.EvalTriggerJobRegister,
+		JobID:       job.ID,
+	}
+
+	// Insert it into the state store
+	noErr(t, h.State.UpsertEvals(h.NextIndex(), []*structs.Evaluation{eval}))
+
+	// Process the evaluation
+	err := h.Process(NewServiceScheduler, eval)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Ensure a single plan
+	if len(h.Plans) != 1 {
+		t.Fatalf("bad: %#v", h.Plans)
+	}
+	plan := h.Plans[0]
+
+	// Ensure the plan doesn't have annotations.
+	if plan.Annotations != nil {
+		t.Fatalf("expected no annotations")
+	}
+
+	// Ensure the eval has no spawned blocked eval
+	if len(h.Evals) != 1 {
+		t.Fatalf("bad: %#v", h.Evals)
+		if h.Evals[0].SpawnedBlockedEval != "" {
+			t.Fatalf("bad: %#v", h.Evals[0])
+		}
+	}
+
+	// Ensure the plan allocated
+	var planned []*structs.Allocation
+	for _, allocList := range plan.NodeAllocation {
+		planned = append(planned, allocList...)
+	}
+	if len(planned) != 10 {
+		t.Fatalf("bad: %#v", plan)
+	}
+
+	// Lookup the allocations by JobID
+	out, err := h.State.AllocsByJob(job.ID)
+	noErr(t, err)
+
+	// Ensure all allocations placed
+	if len(out) != 10 {
+		t.Fatalf("bad: %#v", out)
+	}
+
+	// Ensure the eval was not reblocked
+	if len(h.ReblockEvals) != 0 {
+		t.Fatalf("Existing eval should not have been reblocked as it placed all allocations")
+	}
+
+	h.AssertEvalStatus(t, structs.EvalStatusComplete)
+}
+
 func TestServiceSched_JobModify(t *testing.T) {
 	h := NewHarness(t)
 
diff --git a/scheduler/scheduler.go b/scheduler/scheduler.go
index 518280d07..d96c6aed7 100644
--- a/scheduler/scheduler.go
+++ b/scheduler/scheduler.go
@@ -87,4 +87,10 @@ type Planner interface {
 	// CreateEval is used to create an evaluation. This should set the
 	// PreviousEval to that of the current evaluation.
 	CreateEval(*structs.Evaluation) error
+
+	// ReblockEval takes a blocked evaluation and re-inserts it into the blocked
+	// evaluation tracker. This update occurs only in-memory on the leader. The
+	// evaluation must exist in a blocked state prior to this being called such
+	// that on leader changes, the evaluation will be reblocked properly.
+	ReblockEval(*structs.Evaluation) error
 }
diff --git a/scheduler/testing.go b/scheduler/testing.go
index b7a4d7da1..19cc0221f 100644
--- a/scheduler/testing.go
+++ b/scheduler/testing.go
@@ -1,6 +1,7 @@
 package scheduler
 
 import (
+	"fmt"
 	"log"
 	"os"
 	"sync"
@@ -29,6 +30,10 @@ func (r *RejectPlan) CreateEval(*structs.Evaluation) error {
 	return nil
 }
 
+func (r *RejectPlan) ReblockEval(*structs.Evaluation) error {
+	return nil
+}
+
 // Harness is a lightweight testing harness for schedulers. It manages a state
 // store copy and provides the planner interface. It can be extended for various
 // testing uses or for invoking the scheduler without side effects.
@@ -38,9 +43,10 @@ type Harness struct {
 	Planner  Planner
 	planLock sync.Mutex
 
-	Plans       []*structs.Plan
-	Evals       []*structs.Evaluation
-	CreateEvals []*structs.Evaluation
+	Plans        []*structs.Plan
+	Evals        []*structs.Evaluation
+	CreateEvals  []*structs.Evaluation
+	ReblockEvals []*structs.Evaluation
 
 	nextIndex     uint64
 	nextIndexLock sync.Mutex
@@ -138,6 +144,28 @@ func (h *Harness) CreateEval(eval *structs.Evaluation) error {
 	return nil
 }
 
+func (h *Harness) ReblockEval(eval *structs.Evaluation) error {
+	// Ensure sequential plan application
+	h.planLock.Lock()
+	defer h.planLock.Unlock()
+
+	// Check that the evaluation was already blocked.
+	old, err := h.State.EvalByID(eval.ID)
+	if err != nil {
+		return err
+	}
+
+	if old == nil {
+		return fmt.Errorf("evaluation does not exist to be reblocked")
+	}
+	if old.Status != structs.EvalStatusBlocked {
+		return fmt.Errorf("evaluation %q is not already in a blocked state", old.ID)
+	}
+
+	h.ReblockEvals = append(h.ReblockEvals, eval)
+	return nil
+}
+
 // NextIndex returns the next index
 func (h *Harness) NextIndex() uint64 {
 	h.nextIndexLock.Lock()

From 15936822a4343c6ab99edabd33fa345eab284e5f Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Fri, 20 May 2016 18:07:10 -0700
Subject: [PATCH 2/6] Worker annotates evals with their snapshot index

---
 nomad/state/state_store.go      | 26 +++++++++++++++++++++++++
 nomad/state/state_store_test.go | 22 +++++++++++++++++++++
 nomad/structs/structs.go        |  5 +++++
 nomad/worker.go                 | 21 ++++++++++++++++++--
 nomad/worker_test.go            | 34 +++++++++++++++++++++++++++++++--
 5 files changed, 104 insertions(+), 4 deletions(-)

diff --git a/nomad/state/state_store.go b/nomad/state/state_store.go
index 42365225f..63fae8e5a 100644
--- a/nomad/state/state_store.go
+++ b/nomad/state/state_store.go
@@ -972,6 +972,32 @@ func (s *StateStore) Allocs() (memdb.ResultIterator, error) {
 	return iter, nil
 }
 
+// LastIndex returns the greatest index value for all indexes
+func (s *StateStore) LatestIndex() (uint64, error) {
+	indexes, err := s.Indexes()
+	if err != nil {
+		return 0, err
+	}
+
+	var max uint64 = 0
+	for {
+		raw := indexes.Next()
+		if raw == nil {
+			break
+		}
+
+		// Prepare the request struct
+		idx := raw.(*IndexEntry)
+
+		// Determine the max
+		if idx.Value > max {
+			max = idx.Value
+		}
+	}
+
+	return max, nil
+}
+
 // Index finds the matching index value
 func (s *StateStore) Index(name string) (uint64, error) {
 	txn := s.db.Txn(false)
diff --git a/nomad/state/state_store_test.go b/nomad/state/state_store_test.go
index 22166ef1c..dcdd13571 100644
--- a/nomad/state/state_store_test.go
+++ b/nomad/state/state_store_test.go
@@ -1019,6 +1019,28 @@ func TestStateStore_Indexes(t *testing.T) {
 	}
 }
 
+func TestStateStore_LatestIndex(t *testing.T) {
+	state := testStateStore(t)
+
+	if err := state.UpsertNode(1000, mock.Node()); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	exp := uint64(2000)
+	if err := state.UpsertJob(exp, mock.Job()); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	latest, err := state.LatestIndex()
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	if latest != exp {
+		t.Fatalf("LatestIndex() returned %d; want %d", latest, exp)
+	}
+}
+
 func TestStateStore_RestoreIndex(t *testing.T) {
 	state := testStateStore(t)
 
diff --git a/nomad/structs/structs.go b/nomad/structs/structs.go
index bf885229b..9b1aa6696 100644
--- a/nomad/structs/structs.go
+++ b/nomad/structs/structs.go
@@ -2671,6 +2671,11 @@ type Evaluation struct {
 	// during the evaluation. This should not be set during normal operations.
 	AnnotatePlan bool
 
+	// SnapshotIndex is the Raft index of the snapshot used to process the
+	// evaluation. As such it will only be set once it has gone through the
+	// scheduler.
+	SnapshotIndex uint64
+
 	// Raft Indexes
 	CreateIndex uint64
 	ModifyIndex uint64
diff --git a/nomad/worker.go b/nomad/worker.go
index 9cb1a286a..0e02377e2 100644
--- a/nomad/worker.go
+++ b/nomad/worker.go
@@ -58,7 +58,8 @@ type Worker struct {
 
 	failures uint
 
-	evalToken string
+	evalToken     string
+	snapshotIndex uint64
 }
 
 // NewWorker starts a new worker associated with the given server
@@ -241,6 +242,12 @@ func (w *Worker) invokeScheduler(eval *structs.Evaluation, token string) error {
 		return fmt.Errorf("failed to snapshot state: %v", err)
 	}
 
+	// Store the snapshot's index
+	w.snapshotIndex, err = snap.LatestIndex()
+	if err != nil {
+		return fmt.Errorf("failed to determine snapshot's index: %v", err)
+	}
+
 	// Create the scheduler, or use the special system scheduler
 	var sched scheduler.Scheduler
 	if eval.Type == structs.JobTypeCore {
@@ -319,6 +326,9 @@ SUBMIT:
 			return nil, nil, fmt.Errorf("failed to snapshot state: %v", err)
 		}
 		state = snap
+
+		// Store the snapshot's index
+		w.snapshotIndex = result.RefreshIndex
 	}
 
 	// Return the result and potential state update
@@ -334,6 +344,9 @@ func (w *Worker) UpdateEval(eval *structs.Evaluation) error {
 	}
 	defer metrics.MeasureSince([]string{"nomad", "worker", "update_eval"}, time.Now())
 
+	// Store the snapshot index in the eval
+	eval.SnapshotIndex = w.snapshotIndex
+
 	// Setup the request
 	req := structs.EvalUpdateRequest{
 		Evals:     []*structs.Evaluation{eval},
@@ -369,6 +382,9 @@ func (w *Worker) CreateEval(eval *structs.Evaluation) error {
 	}
 	defer metrics.MeasureSince([]string{"nomad", "worker", "create_eval"}, time.Now())
 
+	// Store the snapshot index in the eval
+	eval.SnapshotIndex = w.snapshotIndex
+
 	// Setup the request
 	req := structs.EvalUpdateRequest{
 		Evals:     []*structs.Evaluation{eval},
@@ -404,7 +420,8 @@ func (w *Worker) ReblockEval(eval *structs.Evaluation) error {
 	}
 	defer metrics.MeasureSince([]string{"nomad", "worker", "reblock_eval"}, time.Now())
 
-	// TODO need to set the evaluations WorkerIndex
+	// Store the snapshot index in the eval
+	eval.SnapshotIndex = w.snapshotIndex
 
 	// Setup the request
 	req := structs.EvalUpdateRequest{
diff --git a/nomad/worker_test.go b/nomad/worker_test.go
index 6b4f657a9..fa92ef0a4 100644
--- a/nomad/worker_test.go
+++ b/nomad/worker_test.go
@@ -398,6 +398,9 @@ func TestWorker_UpdateEval(t *testing.T) {
 	if out.Status != structs.EvalStatusComplete {
 		t.Fatalf("bad: %v", out)
 	}
+	if out.SnapshotIndex != w.snapshotIndex {
+		t.Fatalf("bad: %v", out)
+	}
 }
 
 func TestWorker_CreateEval(t *testing.T) {
@@ -441,6 +444,9 @@ func TestWorker_CreateEval(t *testing.T) {
 	if out.PreviousEval != eval1.ID {
 		t.Fatalf("bad: %v", out)
 	}
+	if out.SnapshotIndex != w.snapshotIndex {
+		t.Fatalf("bad: %v", out)
+	}
 }
 
 func TestWorker_ReblockEval(t *testing.T) {
@@ -484,9 +490,33 @@ func TestWorker_ReblockEval(t *testing.T) {
 		t.Fatalf("err: %v", err)
 	}
 
+	// Ack the eval
+	w.sendAck(evalOut.ID, token, true)
+
 	// Check that it is blocked
 	bStats := s1.blockedEvals.Stats()
-	if bStats.TotalBlocked+bStats.TotalEscaped == 0 {
-		t.Fatalf("ReblockEval didn't insert eval into the blocked eval tracker")
+	if bStats.TotalBlocked+bStats.TotalEscaped != 1 {
+		t.Fatalf("ReblockEval didn't insert eval into the blocked eval tracker: %#v", bStats)
+	}
+
+	// Check that the snapshot index was set properly by unblocking the eval and
+	// then dequeuing.
+	s1.blockedEvals.Unblock("foobar")
+
+	reblockedEval, _, err := s1.evalBroker.Dequeue([]string{eval1.Type}, 1*time.Second)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if reblockedEval == nil {
+		t.Fatalf("Nil eval")
+	}
+	if reblockedEval.ID != eval1.ID {
+		t.Fatalf("Bad eval")
+	}
+
+	// Check that the SnapshotIndex is set
+	if reblockedEval.SnapshotIndex != w.snapshotIndex {
+		t.Fatalf("incorrect snapshot index; got %d; want %d",
+			reblockedEval.SnapshotIndex, w.snapshotIndex)
 	}
 }

From bfdd5846e14623ed0903fbdb0d0936c6378ab553 Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Mon, 23 May 2016 15:24:31 -0700
Subject: [PATCH 3/6] Track unblock indexes and check evals on block to see if
 they missed an update while in the scheduler

---
 nomad/blocked_evals.go      |  93 ++++++++++++++++++++++---
 nomad/blocked_evals_test.go | 134 ++++++++++++++++++++++++++++++++++--
 nomad/fsm.go                |   6 +-
 nomad/worker.go             |   9 +--
 nomad/worker_test.go        |   2 +-
 5 files changed, 223 insertions(+), 21 deletions(-)

diff --git a/nomad/blocked_evals.go b/nomad/blocked_evals.go
index ecb845c12..558826937 100644
--- a/nomad/blocked_evals.go
+++ b/nomad/blocked_evals.go
@@ -35,15 +35,21 @@ type BlockedEvals struct {
 	escaped map[string]*structs.Evaluation
 
 	// unblockCh is used to buffer unblocking of evaluations.
-	capacityChangeCh chan string
+	capacityChangeCh chan *capacityUpdate
 
 	// jobs is the map of blocked job and is used to ensure that only one
 	// blocked eval exists for each job.
 	jobs map[string]struct{}
 
+	// unblockIndexes maps computed node classes to the index in which they were
+	// unblocked. This is used to check if an evaluation could have been
+	// unblocked between the time they were in the scheduler and the time they
+	// are being blocked.
+	unblockIndexes map[string]uint64
+
 	// duplicates is the set of evaluations for jobs that had pre-existing
 	// blocked evaluations. These should be marked as cancelled since only one
-	// blocked eval is neeeded bper job.
+	// blocked eval is neeeded per job.
 	duplicates []*structs.Evaluation
 
 	// duplicateCh is used to signal that a duplicate eval was added to the
@@ -55,6 +61,12 @@ type BlockedEvals struct {
 	stopCh chan struct{}
 }
 
+// capacityUpdate stores unblock data.
+type capacityUpdate struct {
+	computedClass string
+	index         uint64
+}
+
 // BlockedStats returns all the stats about the blocked eval tracker.
 type BlockedStats struct {
 	// TotalEscaped is the total number of blocked evaluations that have escaped
@@ -73,7 +85,8 @@ func NewBlockedEvals(evalBroker *EvalBroker) *BlockedEvals {
 		captured:         make(map[string]*structs.Evaluation),
 		escaped:          make(map[string]*structs.Evaluation),
 		jobs:             make(map[string]struct{}),
-		capacityChangeCh: make(chan string, unblockBuffer),
+		unblockIndexes:   make(map[string]uint64),
+		capacityChangeCh: make(chan *capacityUpdate, unblockBuffer),
 		duplicateCh:      make(chan struct{}, 1),
 		stopCh:           make(chan struct{}),
 		stats:            new(BlockedStats),
@@ -133,6 +146,14 @@ func (b *BlockedEvals) Block(eval *structs.Evaluation) {
 		return
 	}
 
+	// Check if the eval missed an unblock while it was in the scheduler at an
+	// older index.
+	if b.missedUnblock(eval) {
+		// Just re-enqueue the eval immediately
+		b.evalBroker.Enqueue(eval)
+		return
+	}
+
 	// Mark the job as tracked.
 	b.stats.TotalBlocked++
 	b.jobs[eval.JobID] = struct{}{}
@@ -152,16 +173,65 @@ func (b *BlockedEvals) Block(eval *structs.Evaluation) {
 	b.captured[eval.ID] = eval
 }
 
+// missedUnblock returns whether an evaluation missed an unblock while it was in
+// the scheduler. Since the scheduler can operate at an index in the past, the
+// evaluation may have been processed missing data that would allow it to
+// complete. This method returns if that is the case and should be called with
+// the lock held.
+func (b *BlockedEvals) missedUnblock(eval *structs.Evaluation) bool {
+	var max uint64 = 0
+	for class, index := range b.unblockIndexes {
+		// Calculate the max unblock index
+		if max < index {
+			max = index
+		}
+
+		elig, ok := eval.ClassEligibility[class]
+		if !ok {
+			// The evaluation was processed and did not encounter this class.
+			// Thus for correctness we need to unblock it.
+			return true
+		}
+
+		// The evaluation could use the computed node class and the eval was
+		// processed before the last unblock.
+		if elig && eval.SnapshotIndex < index {
+			return true
+		}
+	}
+
+	// If the evaluation has escaped, and the map contains an index older than
+	// the evaluations, it should be unblocked.
+	if eval.EscapedComputedClass && eval.SnapshotIndex < max {
+		return true
+	}
+
+	// The evaluation is ahead of all recent unblocks.
+	return false
+}
+
 // Unblock causes any evaluation that could potentially make progress on a
 // capacity change on the passed computed node class to be enqueued into the
 // eval broker.
-func (b *BlockedEvals) Unblock(computedClass string) {
+func (b *BlockedEvals) Unblock(computedClass string, index uint64) {
+	b.l.Lock()
+
 	// Do nothing if not enabled
 	if !b.enabled {
+		b.l.Unlock()
 		return
 	}
 
-	b.capacityChangeCh <- computedClass
+	// Store the index in which the unblock happened. We use this on subsequent
+	// block calls in case the evaluation was in the scheduler when a trigger
+	// occured.
+	b.unblockIndexes[computedClass] = index
+	b.l.Unlock()
+
+	b.capacityChangeCh <- &capacityUpdate{
+		computedClass: computedClass,
+		index:         index,
+	}
 }
 
 // watchCapacity is a long lived function that watches for capacity changes in
@@ -171,15 +241,15 @@ func (b *BlockedEvals) watchCapacity() {
 		select {
 		case <-b.stopCh:
 			return
-		case computedClass := <-b.capacityChangeCh:
-			b.unblock(computedClass)
+		case update := <-b.capacityChangeCh:
+			b.unblock(update.computedClass, update.index)
 		}
 	}
 }
 
 // unblock unblocks all blocked evals that could run on the passed computed node
 // class.
-func (b *BlockedEvals) unblock(computedClass string) {
+func (b *BlockedEvals) unblock(computedClass string, index uint64) {
 	b.l.Lock()
 	defer b.l.Unlock()
 
@@ -188,6 +258,11 @@ func (b *BlockedEvals) unblock(computedClass string) {
 		return
 	}
 
+	// Store the index in which the unblock happened. We use this on subsequent
+	// block calls in case the evaluation was in the scheduler when a trigger
+	// occured.
+	b.unblockIndexes[computedClass] = index
+
 	// Every eval that has escaped computed node class has to be unblocked
 	// because any node could potentially be feasible.
 	var unblocked []*structs.Evaluation
@@ -273,7 +348,7 @@ func (b *BlockedEvals) Flush() {
 	b.escaped = make(map[string]*structs.Evaluation)
 	b.jobs = make(map[string]struct{})
 	b.duplicates = nil
-	b.capacityChangeCh = make(chan string, unblockBuffer)
+	b.capacityChangeCh = make(chan *capacityUpdate, unblockBuffer)
 	b.stopCh = make(chan struct{})
 	b.duplicateCh = make(chan struct{}, 1)
 }
diff --git a/nomad/blocked_evals_test.go b/nomad/blocked_evals_test.go
index cf725a7a1..32487d9ee 100644
--- a/nomad/blocked_evals_test.go
+++ b/nomad/blocked_evals_test.go
@@ -53,6 +53,27 @@ func TestBlockedEvals_Block_SameJob(t *testing.T) {
 	}
 }
 
+func TestBlockedEvals_Block_PriorUnblocks(t *testing.T) {
+	blocked, _ := testBlockedEvals(t)
+
+	// Do unblocks prior to blocking
+	blocked.Unblock("v1:123", 1000)
+	blocked.Unblock("v1:123", 1001)
+
+	// Create two blocked evals and add them to the blocked tracker.
+	e := mock.Eval()
+	e.Status = structs.EvalStatusBlocked
+	e.ClassEligibility = map[string]bool{"v1:123": false, "v1:456": false}
+	e.SnapshotIndex = 999
+	blocked.Block(e)
+
+	// Verify block did track both
+	bStats := blocked.Stats()
+	if bStats.TotalBlocked != 1 || bStats.TotalEscaped != 0 {
+		t.Fatalf("bad: %#v", bStats)
+	}
+}
+
 func TestBlockedEvals_GetDuplicates(t *testing.T) {
 	blocked, _ := testBlockedEvals(t)
 
@@ -105,7 +126,7 @@ func TestBlockedEvals_UnblockEscaped(t *testing.T) {
 		t.Fatalf("bad: %#v", bStats)
 	}
 
-	blocked.Unblock("v1:123")
+	blocked.Unblock("v1:123", 1000)
 
 	testutil.WaitForResult(func() (bool, error) {
 		// Verify Unblock caused an enqueue
@@ -141,7 +162,7 @@ func TestBlockedEvals_UnblockEligible(t *testing.T) {
 		t.Fatalf("bad: %#v", blockedStats)
 	}
 
-	blocked.Unblock("v1:123")
+	blocked.Unblock("v1:123", 1000)
 
 	testutil.WaitForResult(func() (bool, error) {
 		// Verify Unblock caused an enqueue
@@ -178,7 +199,7 @@ func TestBlockedEvals_UnblockIneligible(t *testing.T) {
 	}
 
 	// Should do nothing
-	blocked.Unblock("v1:123")
+	blocked.Unblock("v1:123", 1000)
 
 	testutil.WaitForResult(func() (bool, error) {
 		// Verify Unblock didn't cause an enqueue
@@ -214,7 +235,7 @@ func TestBlockedEvals_UnblockUnknown(t *testing.T) {
 	}
 
 	// Should unblock because the eval hasn't seen this node class.
-	blocked.Unblock("v1:789")
+	blocked.Unblock("v1:789", 1000)
 
 	testutil.WaitForResult(func() (bool, error) {
 		// Verify Unblock causes an enqueue
@@ -233,3 +254,108 @@ func TestBlockedEvals_UnblockUnknown(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	})
 }
+
+// Test the block case in which the eval should be immediately unblocked since
+// it is escaped and old
+func TestBlockedEvals_Block_ImmediateUnblock_Escaped(t *testing.T) {
+	blocked, broker := testBlockedEvals(t)
+
+	// Do an unblock prior to blocking
+	blocked.Unblock("v1:123", 1000)
+
+	// Create a blocked eval that is eligible on a specific node class and add
+	// it to the blocked tracker.
+	e := mock.Eval()
+	e.Status = structs.EvalStatusBlocked
+	e.EscapedComputedClass = true
+	e.SnapshotIndex = 900
+	blocked.Block(e)
+
+	// Verify block caused the eval to be immediately unblocked
+	blockedStats := blocked.Stats()
+	if blockedStats.TotalBlocked != 0 && blockedStats.TotalEscaped != 0 {
+		t.Fatalf("bad: %#v", blockedStats)
+	}
+
+	testutil.WaitForResult(func() (bool, error) {
+		// Verify Unblock caused an enqueue
+		brokerStats := broker.Stats()
+		if brokerStats.TotalReady != 1 {
+			return false, fmt.Errorf("bad: %#v", brokerStats)
+		}
+
+		return true, nil
+	}, func(err error) {
+		t.Fatalf("err: %s", err)
+	})
+}
+
+// Test the block case in which the eval should be immediately unblocked since
+// it there is an unblock on an unseen class
+func TestBlockedEvals_Block_ImmediateUnblock_UnseenClass(t *testing.T) {
+	blocked, broker := testBlockedEvals(t)
+
+	// Do an unblock prior to blocking
+	blocked.Unblock("v1:123", 1000)
+
+	// Create a blocked eval that is eligible on a specific node class and add
+	// it to the blocked tracker.
+	e := mock.Eval()
+	e.Status = structs.EvalStatusBlocked
+	e.EscapedComputedClass = false
+	e.SnapshotIndex = 900
+	blocked.Block(e)
+
+	// Verify block caused the eval to be immediately unblocked
+	blockedStats := blocked.Stats()
+	if blockedStats.TotalBlocked != 0 && blockedStats.TotalEscaped != 0 {
+		t.Fatalf("bad: %#v", blockedStats)
+	}
+
+	testutil.WaitForResult(func() (bool, error) {
+		// Verify Unblock caused an enqueue
+		brokerStats := broker.Stats()
+		if brokerStats.TotalReady != 1 {
+			return false, fmt.Errorf("bad: %#v", brokerStats)
+		}
+
+		return true, nil
+	}, func(err error) {
+		t.Fatalf("err: %s", err)
+	})
+}
+
+// Test the block case in which the eval should be immediately unblocked since
+// it a class it is eligible for has been unblocked
+func TestBlockedEvals_Block_ImmediateUnblock_SeenClass(t *testing.T) {
+	blocked, broker := testBlockedEvals(t)
+
+	// Do an unblock prior to blocking
+	blocked.Unblock("v1:123", 1000)
+
+	// Create a blocked eval that is eligible on a specific node class and add
+	// it to the blocked tracker.
+	e := mock.Eval()
+	e.Status = structs.EvalStatusBlocked
+	e.ClassEligibility = map[string]bool{"v1:123": true, "v1:456": false}
+	e.SnapshotIndex = 900
+	blocked.Block(e)
+
+	// Verify block caused the eval to be immediately unblocked
+	blockedStats := blocked.Stats()
+	if blockedStats.TotalBlocked != 0 && blockedStats.TotalEscaped != 0 {
+		t.Fatalf("bad: %#v", blockedStats)
+	}
+
+	testutil.WaitForResult(func() (bool, error) {
+		// Verify Unblock caused an enqueue
+		brokerStats := broker.Stats()
+		if brokerStats.TotalReady != 1 {
+			return false, fmt.Errorf("bad: %#v", brokerStats)
+		}
+
+		return true, nil
+	}, func(err error) {
+		t.Fatalf("err: %s", err)
+	})
+}
diff --git a/nomad/fsm.go b/nomad/fsm.go
index 6237e9760..a220138f8 100644
--- a/nomad/fsm.go
+++ b/nomad/fsm.go
@@ -158,7 +158,7 @@ func (n *nomadFSM) applyUpsertNode(buf []byte, index uint64) interface{} {
 	// Unblock evals for the nodes computed node class if it is in a ready
 	// state.
 	if req.Node.Status == structs.NodeStatusReady {
-		n.blockedEvals.Unblock(req.Node.ComputedClass)
+		n.blockedEvals.Unblock(req.Node.ComputedClass, index)
 	}
 
 	return nil
@@ -199,7 +199,7 @@ func (n *nomadFSM) applyStatusUpdate(buf []byte, index uint64) interface{} {
 			return err
 
 		}
-		n.blockedEvals.Unblock(node.ComputedClass)
+		n.blockedEvals.Unblock(node.ComputedClass, index)
 	}
 
 	return nil
@@ -420,7 +420,7 @@ func (n *nomadFSM) applyAllocClientUpdate(buf []byte, index uint64) interface{}
 				return err
 
 			}
-			n.blockedEvals.Unblock(node.ComputedClass)
+			n.blockedEvals.Unblock(node.ComputedClass, index)
 		}
 	}
 
diff --git a/nomad/worker.go b/nomad/worker.go
index 0e02377e2..9326f112e 100644
--- a/nomad/worker.go
+++ b/nomad/worker.go
@@ -58,7 +58,11 @@ type Worker struct {
 
 	failures uint
 
-	evalToken     string
+	evalToken string
+
+	// snapshotIndex is the index of the snapshot in which the scheduler was
+	// first envoked. It is used to mark the SnapshotIndex of evaluations
+	// Created, Updated or Reblocked.
 	snapshotIndex uint64
 }
 
@@ -326,9 +330,6 @@ SUBMIT:
 			return nil, nil, fmt.Errorf("failed to snapshot state: %v", err)
 		}
 		state = snap
-
-		// Store the snapshot's index
-		w.snapshotIndex = result.RefreshIndex
 	}
 
 	// Return the result and potential state update
diff --git a/nomad/worker_test.go b/nomad/worker_test.go
index fa92ef0a4..840ea1a28 100644
--- a/nomad/worker_test.go
+++ b/nomad/worker_test.go
@@ -501,7 +501,7 @@ func TestWorker_ReblockEval(t *testing.T) {
 
 	// Check that the snapshot index was set properly by unblocking the eval and
 	// then dequeuing.
-	s1.blockedEvals.Unblock("foobar")
+	s1.blockedEvals.Unblock("foobar", 1000)
 
 	reblockedEval, _, err := s1.evalBroker.Dequeue([]string{eval1.Type}, 1*time.Second)
 	if err != nil {

From 3fd51ecece575213d569e1f884f85db11f912729 Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Mon, 23 May 2016 16:27:26 -0700
Subject: [PATCH 4/6] Periodically unblock failed evaluations

---
 nomad/blocked_evals.go      | 27 +++++++++++++++++++++++++++
 nomad/blocked_evals_test.go | 31 +++++++++++++++++++++++++++++++
 nomad/leader.go             | 18 ++++++++++++++++++
 nomad/structs/structs.go    |  1 +
 scheduler/generic_sched.go  | 18 +++++++++---------
 5 files changed, 86 insertions(+), 9 deletions(-)

diff --git a/nomad/blocked_evals.go b/nomad/blocked_evals.go
index 558826937..1209c548c 100644
--- a/nomad/blocked_evals.go
+++ b/nomad/blocked_evals.go
@@ -304,6 +304,33 @@ func (b *BlockedEvals) unblock(computedClass string, index uint64) {
 	}
 }
 
+// UnblockFailed unblocks all blocked evaluation that were due to scheduler
+// failure.
+func (b *BlockedEvals) UnblockFailed() {
+	b.l.Lock()
+	defer b.l.Unlock()
+
+	// Do nothing if not enabled
+	if !b.enabled {
+		return
+	}
+
+	var unblock []*structs.Evaluation
+	for _, eval := range b.captured {
+		if eval.TriggeredBy == structs.EvalTriggerMaxPlans {
+			unblock = append(unblock, eval)
+		}
+	}
+
+	for _, eval := range b.escaped {
+		if eval.TriggeredBy == structs.EvalTriggerMaxPlans {
+			unblock = append(unblock, eval)
+		}
+	}
+
+	b.evalBroker.EnqueueAll(unblock)
+}
+
 // GetDuplicates returns all the duplicate evaluations and blocks until the
 // passed timeout.
 func (b *BlockedEvals) GetDuplicates(timeout time.Duration) []*structs.Evaluation {
diff --git a/nomad/blocked_evals_test.go b/nomad/blocked_evals_test.go
index 32487d9ee..9b963feff 100644
--- a/nomad/blocked_evals_test.go
+++ b/nomad/blocked_evals_test.go
@@ -359,3 +359,34 @@ func TestBlockedEvals_Block_ImmediateUnblock_SeenClass(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	})
 }
+
+func TestBlockedEvals_UnblockFailed(t *testing.T) {
+	blocked, broker := testBlockedEvals(t)
+
+	// Create blocked evals that are due to failures
+	e := mock.Eval()
+	e.Status = structs.EvalStatusBlocked
+	e.TriggeredBy = structs.EvalTriggerMaxPlans
+	e.EscapedComputedClass = true
+	blocked.Block(e)
+
+	e2 := mock.Eval()
+	e2.Status = structs.EvalStatusBlocked
+	e2.TriggeredBy = structs.EvalTriggerMaxPlans
+	e2.ClassEligibility = map[string]bool{"v1:123": true, "v1:456": false}
+	blocked.Block(e2)
+
+	// Trigger an unblock fail
+	blocked.UnblockFailed()
+
+	testutil.WaitForResult(func() (bool, error) {
+		// Verify Unblock caused an enqueue
+		brokerStats := broker.Stats()
+		if brokerStats.TotalReady != 2 {
+			return false, fmt.Errorf("bad: %#v", brokerStats)
+		}
+		return true, nil
+	}, func(err error) {
+		t.Fatalf("err: %s", err)
+	})
+}
diff --git a/nomad/leader.go b/nomad/leader.go
index 04f5f1432..58e94c588 100644
--- a/nomad/leader.go
+++ b/nomad/leader.go
@@ -143,6 +143,9 @@ func (s *Server) establishLeadership(stopCh chan struct{}) error {
 	// Reap any duplicate blocked evaluations
 	go s.reapDupBlockedEvaluations(stopCh)
 
+	// Periodically unblock failed allocations
+	go s.periodicUnblockFailedEvals(stopCh)
+
 	// Setup the heartbeat timers. This is done both when starting up or when
 	// a leader fail over happens. Since the timers are maintained by the leader
 	// node, effectively this means all the timers are renewed at the time of failover.
@@ -341,6 +344,21 @@ func (s *Server) reapDupBlockedEvaluations(stopCh chan struct{}) {
 	}
 }
 
+// periodicUnblockFailedEvals periodically unblocks failed, blocked evaluations.
+func (s *Server) periodicUnblockFailedEvals(stopCh chan struct{}) {
+	ticker := time.NewTimer(1 * time.Minute)
+	for {
+		select {
+		case <-stopCh:
+			ticker.Stop()
+			return
+		case <-ticker.C:
+			// Unblock the failed allocations
+			s.blockedEvals.UnblockFailed()
+		}
+	}
+}
+
 // revokeLeadership is invoked once we step down as leader.
 // This is used to cleanup any state that may be specific to a leader.
 func (s *Server) revokeLeadership() error {
diff --git a/nomad/structs/structs.go b/nomad/structs/structs.go
index 9b1aa6696..64039cf51 100644
--- a/nomad/structs/structs.go
+++ b/nomad/structs/structs.go
@@ -2570,6 +2570,7 @@ const (
 	EvalTriggerNodeUpdate    = "node-update"
 	EvalTriggerScheduled     = "scheduled"
 	EvalTriggerRollingUpdate = "rolling-update"
+	EvalTriggerMaxPlans      = "max-plan-attempts"
 )
 
 const (
diff --git a/scheduler/generic_sched.go b/scheduler/generic_sched.go
index 8276acc97..9db02a813 100644
--- a/scheduler/generic_sched.go
+++ b/scheduler/generic_sched.go
@@ -113,13 +113,8 @@ func (s *GenericScheduler) Process(eval *structs.Evaluation) error {
 		if statusErr, ok := err.(*SetStatusError); ok {
 			// Scheduling was tried but made no forward progress so create a
 			// blocked eval to retry once resources become available.
-
-			// TODO: Set the trigger by reason of the blocked eval here to
-			// something like "max-attempts"
-			// We can then periodically dequeue these from the blocked_eval
-			// tracker.
 			var mErr multierror.Error
-			if err := s.createBlockedEval(); err != nil {
+			if err := s.createBlockedEval(true); err != nil {
 				mErr.Errors = append(mErr.Errors, err)
 			}
 			if err := setStatus(s.logger, s.planner, s.eval, s.nextEval, s.blocked, statusErr.EvalStatus, err.Error()); err != nil {
@@ -140,8 +135,9 @@ func (s *GenericScheduler) Process(eval *structs.Evaluation) error {
 	return setStatus(s.logger, s.planner, s.eval, s.nextEval, s.blocked, structs.EvalStatusComplete, "")
 }
 
-// createBlockedEval creates a blocked eval and stores it.
-func (s *GenericScheduler) createBlockedEval() error {
+// createBlockedEval creates a blocked eval and submits it to the planner. If
+// failure is set to true, the eval's trigger reason reflects that.
+func (s *GenericScheduler) createBlockedEval(planFailure bool) error {
 	e := s.ctx.Eligibility()
 	escaped := e.HasEscaped()
 
@@ -152,6 +148,10 @@ func (s *GenericScheduler) createBlockedEval() error {
 	}
 
 	s.blocked = s.eval.CreateBlockedEval(classEligibility, escaped)
+	if planFailure {
+		s.blocked.TriggeredBy = structs.EvalTriggerMaxPlans
+	}
+
 	return s.planner.CreateEval(s.blocked)
 }
 
@@ -191,7 +191,7 @@ func (s *GenericScheduler) process() (bool, error) {
 	// to place the failed allocations when resources become available. If the
 	// current evaluation is already a blocked eval, we reuse it.
 	if s.eval.Status != structs.EvalStatusBlocked && len(s.eval.FailedTGAllocs) != 0 && s.blocked == nil {
-		if err := s.createBlockedEval(); err != nil {
+		if err := s.createBlockedEval(false); err != nil {
 			s.logger.Printf("[ERR] sched: %#v failed to make blocked eval: %v", s.eval, err)
 			return false, err
 		}

From a3336b7761a49821be4edbaef7de9a480e7163d4 Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Tue, 24 May 2016 20:20:06 -0700
Subject: [PATCH 5/6] test fixes and delete

---
 nomad/blocked_evals.go | 6 ++++--
 nomad/worker_test.go   | 9 ++-------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/nomad/blocked_evals.go b/nomad/blocked_evals.go
index 1209c548c..8f5a6f5c4 100644
--- a/nomad/blocked_evals.go
+++ b/nomad/blocked_evals.go
@@ -316,15 +316,17 @@ func (b *BlockedEvals) UnblockFailed() {
 	}
 
 	var unblock []*structs.Evaluation
-	for _, eval := range b.captured {
+	for id, eval := range b.captured {
 		if eval.TriggeredBy == structs.EvalTriggerMaxPlans {
 			unblock = append(unblock, eval)
+			delete(b.captured, id)
 		}
 	}
 
-	for _, eval := range b.escaped {
+	for id, eval := range b.escaped {
 		if eval.TriggeredBy == structs.EvalTriggerMaxPlans {
 			unblock = append(unblock, eval)
+			delete(b.escaped, id)
 		}
 	}
 
diff --git a/nomad/worker_test.go b/nomad/worker_test.go
index 840ea1a28..52b557bba 100644
--- a/nomad/worker_test.go
+++ b/nomad/worker_test.go
@@ -466,13 +466,8 @@ func TestWorker_ReblockEval(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// Enqueue the eval
-	testutil.WaitForResult(func() (bool, error) {
-		err := s1.evalBroker.Enqueue(eval1)
-		return err == nil, err
-	}, func(err error) {
-		t.Fatalf("err: %v", err)
-	})
+	// Enqueue the eval and then dequeue
+	s1.evalBroker.Enqueue(eval1)
 	evalOut, token, err := s1.evalBroker.Dequeue([]string{eval1.Type}, time.Second)
 	if err != nil {
 		t.Fatalf("err: %v", err)

From 6a236872b42dc13521b61d6c85867bea6e7a29ab Mon Sep 17 00:00:00 2001
From: Alex Dadgar <alex.dadgar@gmail.com>
Date: Wed, 25 May 2016 10:28:25 -0700
Subject: [PATCH 6/6] address comment

---
 nomad/blocked_evals.go          |  9 +++------
 nomad/leader.go                 | 11 +++++++++--
 scheduler/generic_sched_test.go |  2 +-
 3 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/nomad/blocked_evals.go b/nomad/blocked_evals.go
index 8f5a6f5c4..06e7410ac 100644
--- a/nomad/blocked_evals.go
+++ b/nomad/blocked_evals.go
@@ -147,7 +147,9 @@ func (b *BlockedEvals) Block(eval *structs.Evaluation) {
 	}
 
 	// Check if the eval missed an unblock while it was in the scheduler at an
-	// older index.
+	// older index. The scheduler could have been invoked with a snapshot of
+	// state that was prior to additional capacity being added or allocations
+	// becoming terminal.
 	if b.missedUnblock(eval) {
 		// Just re-enqueue the eval immediately
 		b.evalBroker.Enqueue(eval)
@@ -258,11 +260,6 @@ func (b *BlockedEvals) unblock(computedClass string, index uint64) {
 		return
 	}
 
-	// Store the index in which the unblock happened. We use this on subsequent
-	// block calls in case the evaluation was in the scheduler when a trigger
-	// occured.
-	b.unblockIndexes[computedClass] = index
-
 	// Every eval that has escaped computed node class has to be unblocked
 	// because any node could potentially be feasible.
 	var unblocked []*structs.Evaluation
diff --git a/nomad/leader.go b/nomad/leader.go
index 58e94c588..f1e10b71d 100644
--- a/nomad/leader.go
+++ b/nomad/leader.go
@@ -11,6 +11,13 @@ import (
 	"github.com/hashicorp/serf/serf"
 )
 
+const (
+	// failedEvalUnblockInterval is the interval at which failed evaluations are
+	// unblocked to re-enter the scheduler. A failed evaluation occurs under
+	// high contention when the schedulers plan does not make progress.
+	failedEvalUnblockInterval = 1 * time.Minute
+)
+
 // monitorLeadership is used to monitor if we acquire or lose our role
 // as the leader in the Raft cluster. There is some work the leader is
 // expected to do, so we must react to changes
@@ -346,11 +353,11 @@ func (s *Server) reapDupBlockedEvaluations(stopCh chan struct{}) {
 
 // periodicUnblockFailedEvals periodically unblocks failed, blocked evaluations.
 func (s *Server) periodicUnblockFailedEvals(stopCh chan struct{}) {
-	ticker := time.NewTimer(1 * time.Minute)
+	ticker := time.NewTimer(failedEvalUnblockInterval)
+	defer ticker.Stop()
 	for {
 		select {
 		case <-stopCh:
-			ticker.Stop()
 			return
 		case <-ticker.C:
 			// Unblock the failed allocations
diff --git a/scheduler/generic_sched_test.go b/scheduler/generic_sched_test.go
index d44d76d38..4b9f9f402 100644
--- a/scheduler/generic_sched_test.go
+++ b/scheduler/generic_sched_test.go
@@ -543,7 +543,7 @@ func TestServiceSched_EvaluateBlockedEval_Finished(t *testing.T) {
 	// Ensure the eval has no spawned blocked eval
 	if len(h.Evals) != 1 {
 		t.Fatalf("bad: %#v", h.Evals)
-		if h.Evals[0].SpawnedBlockedEval != "" {
+		if h.Evals[0].BlockedEval != "" {
 			t.Fatalf("bad: %#v", h.Evals[0])
 		}
 	}