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worker.go
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worker.go
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package nomad
import (
"fmt"
"log"
"strings"
"sync"
"time"
"github.com/armon/go-metrics"
"github.com/hashicorp/nomad/nomad/structs"
"github.com/hashicorp/nomad/scheduler"
)
const (
// backoffBaselineFast is the baseline time for exponential backoff
backoffBaselineFast = 20 * time.Millisecond
// backoffBaselineSlow is the baseline time for exponential backoff
// but that is much slower than backoffBaselineFast
backoffBaselineSlow = 500 * time.Millisecond
// backoffLimitFast is the limit of the exponential backoff
backoffLimitFast = time.Second
// backoffLimitSlow is the limit of the exponential backoff for
// the slower backoff
backoffLimitSlow = 10 * time.Second
// dequeueTimeout is used to timeout an evaluation dequeue so that
// we can check if there is a shutdown event
dequeueTimeout = 500 * time.Millisecond
// raftSyncLimit is the limit of time we will wait for Raft replication
// to catch up to the evaluation. This is used to fast Nack and
// allow another scheduler to pick it up.
raftSyncLimit = 5 * time.Second
// dequeueErrGrace is the grace period where we don't log about
// dequeue errors after start. This is to improve the user experience
// in dev mode where the leader isn't elected for a few seconds.
dequeueErrGrace = 10 * time.Second
)
// Worker is a single threaded scheduling worker. There may be multiple
// running per server (leader or follower). They are responsible for dequeuing
// pending evaluations, invoking schedulers, plan submission and the
// lifecycle around making task allocations. They bridge the business logic
// of the scheduler with the plumbing required to make it all work.
type Worker struct {
srv *Server
logger *log.Logger
start time.Time
paused bool
pauseLock sync.Mutex
pauseCond *sync.Cond
failures uint
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
}
// NewWorker starts a new worker associated with the given server
func NewWorker(srv *Server) (*Worker, error) {
w := &Worker{
srv: srv,
logger: srv.logger,
start: time.Now(),
}
w.pauseCond = sync.NewCond(&w.pauseLock)
go w.run()
return w, nil
}
// SetPause is used to pause or unpause a worker
func (w *Worker) SetPause(p bool) {
w.pauseLock.Lock()
w.paused = p
w.pauseLock.Unlock()
if !p {
w.pauseCond.Broadcast()
}
}
// checkPaused is used to park the worker when paused
func (w *Worker) checkPaused() {
w.pauseLock.Lock()
for w.paused {
w.pauseCond.Wait()
}
w.pauseLock.Unlock()
}
// run is the long-lived goroutine which is used to run the worker
func (w *Worker) run() {
for {
// Dequeue a pending evaluation
eval, token, shutdown := w.dequeueEvaluation(dequeueTimeout)
if shutdown {
return
}
// Check for a shutdown
if w.srv.IsShutdown() {
w.sendAck(eval.ID, token, false)
return
}
// Wait for the the raft log to catchup to the evaluation
if err := w.waitForIndex(eval.ModifyIndex, raftSyncLimit); err != nil {
w.sendAck(eval.ID, token, false)
continue
}
// Invoke the scheduler to determine placements
if err := w.invokeScheduler(eval, token); err != nil {
w.sendAck(eval.ID, token, false)
continue
}
// Complete the evaluation
w.sendAck(eval.ID, token, true)
}
}
// dequeueEvaluation is used to fetch the next ready evaluation.
// This blocks until an evaluation is available or a timeout is reached.
func (w *Worker) dequeueEvaluation(timeout time.Duration) (*structs.Evaluation, string, bool) {
// Setup the request
req := structs.EvalDequeueRequest{
Schedulers: w.srv.config.EnabledSchedulers,
Timeout: timeout,
WriteRequest: structs.WriteRequest{
Region: w.srv.config.Region,
},
}
var resp structs.EvalDequeueResponse
REQ:
// Check if we are paused
w.checkPaused()
// Make a blocking RPC
start := time.Now()
err := w.srv.RPC("Eval.Dequeue", &req, &resp)
metrics.MeasureSince([]string{"nomad", "worker", "dequeue_eval"}, start)
if err != nil {
if time.Since(w.start) > dequeueErrGrace && !w.srv.IsShutdown() {
w.logger.Printf("[ERR] worker: failed to dequeue evaluation: %v", err)
}
if w.backoffErr(backoffBaselineSlow, backoffLimitSlow) {
return nil, "", true
}
goto REQ
}
w.backoffReset()
// Check if we got a response
if resp.Eval != nil {
w.logger.Printf("[DEBUG] worker: dequeued evaluation %s", resp.Eval.ID)
return resp.Eval, resp.Token, false
}
// Check for potential shutdown
if w.srv.IsShutdown() {
return nil, "", true
}
goto REQ
}
// sendAck makes a best effort to ack or nack the evaluation.
// Any errors are logged but swallowed.
func (w *Worker) sendAck(evalID, token string, ack bool) {
defer metrics.MeasureSince([]string{"nomad", "worker", "send_ack"}, time.Now())
// Setup the request
req := structs.EvalAckRequest{
EvalID: evalID,
Token: token,
WriteRequest: structs.WriteRequest{
Region: w.srv.config.Region,
},
}
var resp structs.GenericResponse
// Determine if this is an Ack or Nack
verb := "ack"
endpoint := "Eval.Ack"
if !ack {
verb = "nack"
endpoint = "Eval.Nack"
}
// Make the RPC call
err := w.srv.RPC(endpoint, &req, &resp)
if err != nil {
w.logger.Printf("[ERR] worker: failed to %s evaluation '%s': %v",
verb, evalID, err)
} else {
w.logger.Printf("[DEBUG] worker: %s for evaluation %s", verb, evalID)
}
}
// waitForIndex ensures that the local state is at least as fresh
// as the given index. This is used before starting an evaluation,
// but also potentially mid-stream. If a Plan fails because of stale
// state (attempt to allocate to a failed/dead node), we may need
// to sync our state again and do the planning with more recent data.
func (w *Worker) waitForIndex(index uint64, timeout time.Duration) error {
// XXX: Potential optimization is to set up a watch on the state stores
// index table and only unblock via a trigger rather than timing out and
// checking.
start := time.Now()
defer metrics.MeasureSince([]string{"nomad", "worker", "wait_for_index"}, start)
CHECK:
// Get the states current index
snapshotIndex, err := w.srv.fsm.State().LatestIndex()
if err != nil {
return fmt.Errorf("failed to determine state store's index: %v", err)
}
// We only need the FSM state to be as recent as the given index
if index <= snapshotIndex {
w.backoffReset()
return nil
}
// Check if we've reached our limit
if time.Now().Sub(start) > timeout {
return fmt.Errorf("sync wait timeout reached")
}
// Exponential back off if we haven't yet reached it
if w.backoffErr(backoffBaselineFast, backoffLimitFast) {
return fmt.Errorf("shutdown while waiting for state sync")
}
goto CHECK
}
// invokeScheduler is used to invoke the business logic of the scheduler
func (w *Worker) invokeScheduler(eval *structs.Evaluation, token string) error {
defer metrics.MeasureSince([]string{"nomad", "worker", "invoke_scheduler", eval.Type}, time.Now())
// Store the evaluation token
w.evalToken = token
// Snapshot the current state
snap, err := w.srv.fsm.State().Snapshot()
if err != nil {
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 {
sched = NewCoreScheduler(w.srv, snap)
} else {
sched, err = scheduler.NewScheduler(eval.Type, w.logger, snap, w)
if err != nil {
return fmt.Errorf("failed to instantiate scheduler: %v", err)
}
}
// Process the evaluation
err = sched.Process(eval)
if err != nil {
return fmt.Errorf("failed to process evaluation: %v", err)
}
return nil
}
// SubmitPlan is used to submit a plan for consideration. This allows
// the worker to act as the planner for the scheduler.
func (w *Worker) SubmitPlan(plan *structs.Plan) (*structs.PlanResult, scheduler.State, error) {
// Check for a shutdown before plan submission
if w.srv.IsShutdown() {
return nil, nil, fmt.Errorf("shutdown while planning")
}
defer metrics.MeasureSince([]string{"nomad", "worker", "submit_plan"}, time.Now())
// Add the evaluation token to the plan
plan.EvalToken = w.evalToken
// Setup the request
req := structs.PlanRequest{
Plan: plan,
WriteRequest: structs.WriteRequest{
Region: w.srv.config.Region,
},
}
var resp structs.PlanResponse
SUBMIT:
// Make the RPC call
if err := w.srv.RPC("Plan.Submit", &req, &resp); err != nil {
w.logger.Printf("[ERR] worker: failed to submit plan for evaluation %s: %v",
plan.EvalID, err)
if w.shouldResubmit(err) && !w.backoffErr(backoffBaselineSlow, backoffLimitSlow) {
goto SUBMIT
}
return nil, nil, err
} else {
w.logger.Printf("[DEBUG] worker: submitted plan for evaluation %s", plan.EvalID)
w.backoffReset()
}
// Look for a result
result := resp.Result
if result == nil {
return nil, nil, fmt.Errorf("missing result")
}
// Check if a state update is required. This could be required if we
// planning based on stale data, which is causing issues. For example, a
// node failure since the time we've started planning or conflicting task
// allocations.
var state scheduler.State
if result.RefreshIndex != 0 {
// Wait for the the raft log to catchup to the evaluation
w.logger.Printf("[DEBUG] worker: refreshing state to index %d for %q", result.RefreshIndex, plan.EvalID)
if err := w.waitForIndex(result.RefreshIndex, raftSyncLimit); err != nil {
return nil, nil, err
}
// Snapshot the current state
snap, err := w.srv.fsm.State().Snapshot()
if err != nil {
return nil, nil, fmt.Errorf("failed to snapshot state: %v", err)
}
state = snap
}
// Return the result and potential state update
return result, state, nil
}
// UpdateEval is used to submit an updated evaluation. This allows
// the worker to act as the planner for the scheduler.
func (w *Worker) UpdateEval(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", "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},
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.Update", &req, &resp); err != nil {
w.logger.Printf("[ERR] worker: failed to update evaluation %#v: %v",
eval, err)
if w.shouldResubmit(err) && !w.backoffErr(backoffBaselineSlow, backoffLimitSlow) {
goto SUBMIT
}
return err
} else {
w.logger.Printf("[DEBUG] worker: updated evaluation %#v", eval)
w.backoffReset()
}
return nil
}
// CreateEval is used to create a new evaluation. This allows
// the worker to act as the planner for the scheduler.
func (w *Worker) CreateEval(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", "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},
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.Create", &req, &resp); err != nil {
w.logger.Printf("[ERR] worker: failed to create evaluation %#v: %v",
eval, err)
if w.shouldResubmit(err) && !w.backoffErr(backoffBaselineSlow, backoffLimitSlow) {
goto SUBMIT
}
return err
} else {
w.logger.Printf("[DEBUG] worker: created evaluation %#v", eval)
w.backoffReset()
}
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())
// Update the evaluation if the queued jobs is not same as what is
// recorded in the job summary
summary, err := w.srv.fsm.state.JobSummaryByID(eval.JobID)
if err != nil {
return fmt.Errorf("couldn't retreive job summary: %v", err)
}
if summary != nil {
var hasChanged bool
for tg, summary := range summary.Summary {
if queued, ok := eval.QueuedAllocations[tg]; ok {
if queued != summary.Queued {
hasChanged = true
break
}
}
}
if hasChanged {
if err := w.UpdateEval(eval); err != nil {
return err
}
}
}
// Store the snapshot index in the eval
eval.SnapshotIndex = w.snapshotIndex
// 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.
func (w *Worker) shouldResubmit(err error) bool {
s := err.Error()
switch {
case strings.Contains(s, "No cluster leader"):
return true
case strings.Contains(s, "plan queue is disabled"):
return true
default:
return false
}
}
// backoffErr is used to do an exponential back off on error. This is
// maintained statefully for the worker. Returns if attempts should be
// abandoneded due to shutdown.
func (w *Worker) backoffErr(base, limit time.Duration) bool {
backoff := (1 << (2 * w.failures)) * base
if backoff > limit {
backoff = limit
} else {
w.failures++
}
select {
case <-time.After(backoff):
return false
case <-w.srv.shutdownCh:
return true
}
}
// backoffReset is used to reset the failure count for
// exponential backoff
func (w *Worker) backoffReset() {
w.failures = 0
}