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eval_broker.go
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eval_broker.go
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package nomad
import (
"container/heap"
"errors"
"fmt"
"math/rand"
"sync"
"time"
"github.com/armon/go-metrics"
"github.com/hashicorp/nomad/nomad/structs"
)
const (
// failedQueue is the queue we add Evaluations to once
// they've reached the deliveryLimit. This allows the leader to
// set the status to failed.
failedQueue = "_failed"
)
var (
// ErrNotOutstanding is returned if an evaluation is not outstanding
ErrNotOutstanding = errors.New("evaluation is not outstanding")
// ErrTokenMismatch is the outstanding eval has a different token
ErrTokenMismatch = errors.New("evaluation token does not match")
// ErrNackTimeoutReached is returned if an expired evaluation is reset
ErrNackTimeoutReached = errors.New("evaluation nack timeout reached")
)
// EvalBroker is used to manage brokering of evaluations. When an evaluation is
// created, due to a change in a job specification or a node, we put it into the
// broker. The broker sorts by evaluations by priority and scheduler type. This
// allows us to dequeue the highest priority work first, while also allowing sub-schedulers
// to only dequeue work they know how to handle. The broker is designed to be entirely
// in-memory and is managed by the leader node.
//
// The broker must provide at-least-once delivery semantics. It relies on explicit
// Ack/Nack messages to handle this. If a delivery is not Ack'd in a sufficient time
// span, it will be assumed Nack'd.
type EvalBroker struct {
nackTimeout time.Duration
deliveryLimit int
enabled bool
stats *BrokerStats
// evals tracks queued evaluations by ID to de-duplicate enqueue.
// The counter is the number of times we've attempted delivery,
// and is used to eventually fail an evaluation.
evals map[string]int
// jobEvals tracks queued evaluations by JobID to serialize them
jobEvals map[string]string
// blocked tracks the blocked evaluations by JobID in a priority queue
blocked map[string]PendingEvaluations
// ready tracks the ready jobs by scheduler in a priority queue
ready map[string]PendingEvaluations
// unack is a map of evalID to an un-acknowledged evaluation
unack map[string]*unackEval
// waiting is used to notify on a per-scheduler basis of ready work
waiting map[string]chan struct{}
// timeWait has evaluations that are waiting for time to elapse
timeWait map[string]*time.Timer
l sync.RWMutex
}
// unackEval tracks an unacknowledged evaluation along with the Nack timer
type unackEval struct {
Eval *structs.Evaluation
Token string
NackTimer *time.Timer
}
// PendingEvaluations is a list of waiting evaluations.
// We implement the container/heap interface so that this is a
// priority queue
type PendingEvaluations []*structs.Evaluation
// NewEvalBroker creates a new evaluation broker. This is parameterized
// with the timeout used for messages that are not acknowledged before we
// assume a Nack and attempt to redeliver as well as the deliveryLimit
// which prevents a failing eval from being endlessly delivered.
func NewEvalBroker(timeout time.Duration, deliveryLimit int) (*EvalBroker, error) {
if timeout < 0 {
return nil, fmt.Errorf("timeout cannot be negative")
}
b := &EvalBroker{
nackTimeout: timeout,
deliveryLimit: deliveryLimit,
enabled: false,
stats: new(BrokerStats),
evals: make(map[string]int),
jobEvals: make(map[string]string),
blocked: make(map[string]PendingEvaluations),
ready: make(map[string]PendingEvaluations),
unack: make(map[string]*unackEval),
waiting: make(map[string]chan struct{}),
timeWait: make(map[string]*time.Timer),
}
b.stats.ByScheduler = make(map[string]*SchedulerStats)
return b, nil
}
// Enabled is used to check if the broker is enabled.
func (b *EvalBroker) Enabled() bool {
b.l.RLock()
defer b.l.RUnlock()
return b.enabled
}
// SetEnabled is used to control if the broker is enabled. The broker
// should only be enabled on the active leader.
func (b *EvalBroker) SetEnabled(enabled bool) {
b.l.Lock()
b.enabled = enabled
b.l.Unlock()
if !enabled {
b.Flush()
}
}
// Enqueue is used to enqueue an evaluation
func (b *EvalBroker) Enqueue(eval *structs.Evaluation) error {
b.l.Lock()
defer b.l.Unlock()
// Check if already enqueued
if _, ok := b.evals[eval.ID]; ok {
return nil
} else if b.enabled {
b.evals[eval.ID] = 0
}
// Check if we need to enforce a wait
if eval.Wait > 0 {
timer := time.AfterFunc(eval.Wait, func() {
b.enqueueWaiting(eval)
})
b.timeWait[eval.ID] = timer
b.stats.TotalWaiting += 1
return nil
}
b.enqueueLocked(eval, eval.Type)
return nil
}
// enqueueWaiting is used to enqueue a waiting evaluation
func (b *EvalBroker) enqueueWaiting(eval *structs.Evaluation) {
b.l.Lock()
defer b.l.Unlock()
delete(b.timeWait, eval.ID)
b.stats.TotalWaiting -= 1
b.enqueueLocked(eval, eval.Type)
}
// enqueueLocked is used to enqueue with the lock held
func (b *EvalBroker) enqueueLocked(eval *structs.Evaluation, queue string) {
// Do nothing if not enabled
if !b.enabled {
return
}
// Check if there is an evaluation for this JobID pending
pendingEval := b.jobEvals[eval.JobID]
if pendingEval == "" {
b.jobEvals[eval.JobID] = eval.ID
} else if pendingEval != eval.ID {
blocked := b.blocked[eval.JobID]
heap.Push(&blocked, eval)
b.blocked[eval.JobID] = blocked
b.stats.TotalBlocked += 1
return
}
// Find the pending by scheduler class
pending, ok := b.ready[queue]
if !ok {
pending = make([]*structs.Evaluation, 0, 16)
if _, ok := b.waiting[queue]; !ok {
b.waiting[queue] = make(chan struct{}, 1)
}
}
// Push onto the heap
heap.Push(&pending, eval)
b.ready[queue] = pending
// Update the stats
b.stats.TotalReady += 1
bySched, ok := b.stats.ByScheduler[queue]
if !ok {
bySched = &SchedulerStats{}
b.stats.ByScheduler[queue] = bySched
}
bySched.Ready += 1
// Unblock any blocked dequeues
select {
case b.waiting[queue] <- struct{}{}:
default:
}
}
// Dequeue is used to perform a blocking dequeue
func (b *EvalBroker) Dequeue(schedulers []string, timeout time.Duration) (*structs.Evaluation, string, error) {
var timeoutTimer *time.Timer
var timeoutCh <-chan time.Time
SCAN:
// Scan for work
eval, token, err := b.scanForSchedulers(schedulers)
if err != nil {
if timeoutTimer != nil {
timeoutTimer.Stop()
}
return nil, "", err
}
// Check if we have something
if eval != nil {
if timeoutTimer != nil {
timeoutTimer.Stop()
}
return eval, token, nil
}
// Setup the timeout channel the first time around
if timeoutTimer == nil && timeout != 0 {
timeoutTimer = time.NewTimer(timeout)
timeoutCh = timeoutTimer.C
}
// Block until we get work
scan := b.waitForSchedulers(schedulers, timeoutCh)
if scan {
goto SCAN
}
return nil, "", nil
}
// scanForSchedulers scans for work on any of the schedulers. The highest priority work
// is dequeued first. This may return nothing if there is no work waiting.
func (b *EvalBroker) scanForSchedulers(schedulers []string) (*structs.Evaluation, string, error) {
b.l.Lock()
defer b.l.Unlock()
// Do nothing if not enabled
if !b.enabled {
return nil, "", fmt.Errorf("eval broker disabled")
}
// Scan for eligible work
var eligibleSched []string
var eligiblePriority int
for _, sched := range schedulers {
// Get the pending queue
pending, ok := b.ready[sched]
if !ok {
continue
}
// Peek at the next item
ready := pending.Peek()
if ready == nil {
continue
}
// Add to eligible if equal or greater priority
if len(eligibleSched) == 0 || ready.Priority > eligiblePriority {
eligibleSched = []string{sched}
eligiblePriority = ready.Priority
} else if eligiblePriority > ready.Priority {
continue
} else if eligiblePriority == ready.Priority {
eligibleSched = append(eligibleSched, sched)
}
}
// Determine behavior based on eligible work
switch n := len(eligibleSched); n {
case 0:
// No work to do!
return nil, "", nil
case 1:
// Only a single task, dequeue
return b.dequeueForSched(eligibleSched[0])
default:
// Multiple tasks. We pick a random task so that we fairly
// distribute work.
offset := rand.Int63() % int64(n)
return b.dequeueForSched(eligibleSched[offset])
}
}
// dequeueForSched is used to dequeue the next work item for a given scheduler.
// This assumes locks are held and that this scheduler has work
func (b *EvalBroker) dequeueForSched(sched string) (*structs.Evaluation, string, error) {
// Get the pending queue
pending := b.ready[sched]
raw := heap.Pop(&pending)
b.ready[sched] = pending
eval := raw.(*structs.Evaluation)
// Generate a UUID for the token
token := structs.GenerateUUID()
// Setup Nack timer
nackTimer := time.AfterFunc(b.nackTimeout, func() {
b.Nack(eval.ID, token)
})
// Add to the unack queue
b.unack[eval.ID] = &unackEval{
Eval: eval,
Token: token,
NackTimer: nackTimer,
}
// Increment the dequeue count
b.evals[eval.ID] += 1
// Update the stats
b.stats.TotalReady -= 1
b.stats.TotalUnacked += 1
bySched := b.stats.ByScheduler[sched]
bySched.Ready -= 1
bySched.Unacked += 1
return eval, token, nil
}
// waitForSchedulers is used to wait for work on any of the scheduler or until a timeout.
// Returns if there is work waiting potentially.
func (b *EvalBroker) waitForSchedulers(schedulers []string, timeoutCh <-chan time.Time) bool {
doneCh := make(chan struct{})
readyCh := make(chan struct{}, 1)
defer close(doneCh)
// Start all the watchers
b.l.Lock()
for _, sched := range schedulers {
waitCh, ok := b.waiting[sched]
if !ok {
waitCh = make(chan struct{}, 1)
b.waiting[sched] = waitCh
}
// Start a goroutine that either waits for the waitCh on this scheduler
// to unblock or for this waitForSchedulers call to return
go func() {
select {
case <-waitCh:
select {
case readyCh <- struct{}{}:
default:
}
case <-doneCh:
}
}()
}
b.l.Unlock()
// Block until we have ready work and should scan, or until we timeout
// and should not make an attempt to scan for work
select {
case <-readyCh:
return true
case <-timeoutCh:
return false
}
}
// Outstanding checks if an EvalID has been delivered but not acknowledged
// and returns the associated token for the evaluation.
func (b *EvalBroker) Outstanding(evalID string) (string, bool) {
b.l.RLock()
defer b.l.RUnlock()
unack, ok := b.unack[evalID]
if !ok {
return "", false
}
return unack.Token, true
}
// OutstandingReset resets the Nack timer for the EvalID if the
// token matches and the eval is outstanding
func (b *EvalBroker) OutstandingReset(evalID, token string) error {
b.l.RLock()
defer b.l.RUnlock()
unack, ok := b.unack[evalID]
if !ok {
return ErrNotOutstanding
}
if unack.Token != token {
return ErrTokenMismatch
}
if !unack.NackTimer.Reset(b.nackTimeout) {
return ErrNackTimeoutReached
}
return nil
}
// Ack is used to positively acknowledge handling an evaluation
func (b *EvalBroker) Ack(evalID, token string) error {
b.l.Lock()
defer b.l.Unlock()
// Lookup the unack'd eval
unack, ok := b.unack[evalID]
if !ok {
return fmt.Errorf("Evaluation ID not found")
}
if unack.Token != token {
return fmt.Errorf("Token does not match for Evaluation ID")
}
jobID := unack.Eval.JobID
// Ensure we were able to stop the timer
if !unack.NackTimer.Stop() {
return fmt.Errorf("Evaluation ID Ack'd after Nack timer expiration")
}
// Update the stats
b.stats.TotalUnacked -= 1
queue := unack.Eval.Type
if b.evals[evalID] >= b.deliveryLimit {
queue = failedQueue
}
bySched := b.stats.ByScheduler[queue]
bySched.Unacked -= 1
// Cleanup
delete(b.unack, evalID)
delete(b.evals, evalID)
delete(b.jobEvals, jobID)
// Check if there are any blocked evaluations
if blocked := b.blocked[jobID]; len(blocked) != 0 {
raw := heap.Pop(&blocked)
if len(blocked) > 0 {
b.blocked[jobID] = blocked
} else {
delete(b.blocked, jobID)
}
eval := raw.(*structs.Evaluation)
b.stats.TotalBlocked -= 1
b.enqueueLocked(eval, eval.Type)
return nil
}
return nil
}
// Nack is used to negatively acknowledge handling an evaluation
func (b *EvalBroker) Nack(evalID, token string) error {
b.l.Lock()
defer b.l.Unlock()
// Lookup the unack'd eval
unack, ok := b.unack[evalID]
if !ok {
return fmt.Errorf("Evaluation ID not found")
}
if unack.Token != token {
return fmt.Errorf("Token does not match for Evaluation ID")
}
// Stop the timer, doesn't matter if we've missed it
unack.NackTimer.Stop()
// Cleanup
delete(b.unack, evalID)
// Update the stats
b.stats.TotalUnacked -= 1
bySched := b.stats.ByScheduler[unack.Eval.Type]
bySched.Unacked -= 1
// Check if we've hit the delivery limit, and re-enqueue
// in the failedQueue
if b.evals[evalID] >= b.deliveryLimit {
b.enqueueLocked(unack.Eval, failedQueue)
} else {
b.enqueueLocked(unack.Eval, unack.Eval.Type)
}
return nil
}
// Flush is used to clear the state of the broker
func (b *EvalBroker) Flush() {
b.l.Lock()
defer b.l.Unlock()
// Unblock any waiters
for _, waitCh := range b.waiting {
close(waitCh)
}
b.waiting = make(map[string]chan struct{})
// Cancel any Nack timers
for _, unack := range b.unack {
unack.NackTimer.Stop()
}
// Cancel any time wait evals
for _, wait := range b.timeWait {
wait.Stop()
}
// Reset the broker
b.stats.TotalReady = 0
b.stats.TotalUnacked = 0
b.stats.TotalBlocked = 0
b.stats.TotalWaiting = 0
b.stats.ByScheduler = make(map[string]*SchedulerStats)
b.evals = make(map[string]int)
b.jobEvals = make(map[string]string)
b.blocked = make(map[string]PendingEvaluations)
b.ready = make(map[string]PendingEvaluations)
b.unack = make(map[string]*unackEval)
b.timeWait = make(map[string]*time.Timer)
}
// Stats is used to query the state of the broker
func (b *EvalBroker) Stats() *BrokerStats {
// Allocate a new stats struct
stats := new(BrokerStats)
stats.ByScheduler = make(map[string]*SchedulerStats)
b.l.RLock()
defer b.l.RUnlock()
// Copy all the stats
stats.TotalReady = b.stats.TotalReady
stats.TotalUnacked = b.stats.TotalUnacked
stats.TotalBlocked = b.stats.TotalBlocked
stats.TotalWaiting = b.stats.TotalWaiting
for sched, subStat := range b.stats.ByScheduler {
subStatCopy := new(SchedulerStats)
*subStatCopy = *subStat
stats.ByScheduler[sched] = subStatCopy
}
return stats
}
// EmitStats is used to export metrics about the broker while enabled
func (b *EvalBroker) EmitStats(period time.Duration, stopCh chan struct{}) {
for {
select {
case <-time.After(period):
stats := b.Stats()
metrics.SetGauge([]string{"nomad", "broker", "total_ready"}, float32(stats.TotalReady))
metrics.SetGauge([]string{"nomad", "broker", "total_unacked"}, float32(stats.TotalUnacked))
metrics.SetGauge([]string{"nomad", "broker", "total_blocked"}, float32(stats.TotalBlocked))
metrics.SetGauge([]string{"nomad", "broker", "total_waiting"}, float32(stats.TotalWaiting))
for sched, schedStats := range stats.ByScheduler {
metrics.SetGauge([]string{"nomad", "broker", sched, "ready"}, float32(schedStats.Ready))
metrics.SetGauge([]string{"nomad", "broker", sched, "unacked"}, float32(schedStats.Unacked))
}
case <-stopCh:
return
}
}
}
// BrokerStats returns all the stats about the broker
type BrokerStats struct {
TotalReady int
TotalUnacked int
TotalBlocked int
TotalWaiting int
ByScheduler map[string]*SchedulerStats
}
// SchedulerStats returns the stats per scheduler
type SchedulerStats struct {
Ready int
Unacked int
}
// Len is for the sorting interface
func (p PendingEvaluations) Len() int {
return len(p)
}
// Less is for the sorting interface. We flip the check
// so that the "min" in the min-heap is the element with the
// highest priority
func (p PendingEvaluations) Less(i, j int) bool {
if p[i].JobID != p[j].JobID && p[i].Priority != p[j].Priority {
return !(p[i].Priority < p[j].Priority)
}
return p[i].CreateIndex < p[j].CreateIndex
}
// Swap is for the sorting interface
func (p PendingEvaluations) Swap(i, j int) {
p[i], p[j] = p[j], p[i]
}
// Push is used to add a new evalution to the slice
func (p *PendingEvaluations) Push(e interface{}) {
*p = append(*p, e.(*structs.Evaluation))
}
// Pop is used to remove an evaluation from the slice
func (p *PendingEvaluations) Pop() interface{} {
n := len(*p)
e := (*p)[n-1]
(*p)[n-1] = nil
*p = (*p)[:n-1]
return e
}
// Peek is used to peek at the next element that would be popped
func (p PendingEvaluations) Peek() *structs.Evaluation {
n := len(p)
if n == 0 {
return nil
}
return p[n-1]
}