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runner.go
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runner.go
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// Copyright 2017 Google Inc. 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 election
import (
"context"
"math/rand"
"time"
"github.com/golang/glog"
"github.com/google/trillian/util"
)
// Minimum values for configuration intervals.
const (
MinPreElectionPause = 10 * time.Millisecond
MinMasterCheckInterval = 50 * time.Millisecond
MinMasterHoldInterval = 10 * time.Second
)
// RunnerConfig describes the parameters for an election Runner.
type RunnerConfig struct {
// PreElectionPause is the maximum interval to wait before starting a
// mastership election for a particular log.
PreElectionPause time.Duration
// MasterCheckInterval is the interval between checks that we still
// hold mastership for a log.
MasterCheckInterval time.Duration
// MasterHoldInterval is the minimum interval to hold mastership for.
MasterHoldInterval time.Duration
// ResignOdds gives the chance of resigning mastership after each
// check interval, as the N for 1-in-N.
ResignOdds int
TimeSource util.TimeSource
}
// fixupRunnerConfig ensures operation parameters have required minimum values.
func fixupRunnerConfig(cfg *RunnerConfig) {
if cfg.PreElectionPause < MinPreElectionPause {
cfg.PreElectionPause = MinPreElectionPause
}
if cfg.MasterCheckInterval < MinMasterCheckInterval {
cfg.MasterCheckInterval = MinMasterCheckInterval
}
if cfg.MasterHoldInterval < MinMasterHoldInterval {
cfg.MasterHoldInterval = MinMasterHoldInterval
}
if cfg.ResignOdds < 1 {
cfg.ResignOdds = 1
}
if cfg.TimeSource == nil {
cfg.TimeSource = util.SystemTimeSource{}
}
}
// Runner controls a continuous election process.
type Runner struct {
// Allow the user to store a Cancel function with the runner for convenience.
Cancel context.CancelFunc
id string
cfg *RunnerConfig
tracker *MasterTracker
election MasterElection
}
// NewRunner builds a new election Runner instance with the given configuration. On calling
// Run(), the provided election will be continuously monitored and mastership changes will
// be notified to the provided MasterTracker instance.
func NewRunner(id string, cfg *RunnerConfig, tracker *MasterTracker, cancel context.CancelFunc, el MasterElection) *Runner {
fixupRunnerConfig(cfg)
return &Runner{
Cancel: cancel,
id: id,
cfg: cfg,
tracker: tracker,
election: el,
}
}
// Run performs a continuous election process. It runs continuously until the
// context is canceled or an internal error is encountered.
func (er *Runner) Run(ctx context.Context, pending chan<- Resignation) {
// Pause for a random interval so that if multiple instances start at the same
// time there is less of a thundering herd.
pause := rand.Int63n(er.cfg.PreElectionPause.Nanoseconds())
if err := util.SleepContext(ctx, time.Duration(pause)); err != nil {
return
}
glog.V(1).Infof("%s: start election-monitoring loop ", er.id)
if err := er.election.Start(ctx); err != nil {
glog.Errorf("%s: election.Start() failed: %v", er.id, err)
return
}
defer func(ctx context.Context, er *Runner) {
glog.Infof("%s: shutdown election-monitoring loop", er.id)
er.election.Close(ctx)
}(ctx, er)
for {
glog.V(1).Infof("%s: When I left you, I was but the learner", er.id)
if err := er.election.WaitForMastership(ctx); err != nil {
glog.Errorf("%s: er.election.WaitForMastership() failed: %v", er.id, err)
return
}
glog.V(1).Infof("%s: Now, I am the master", er.id)
er.tracker.Set(er.id, true)
masterSince := er.cfg.TimeSource.Now()
// While-master loop
for {
if err := util.SleepContext(ctx, er.cfg.MasterCheckInterval); err != nil {
glog.Infof("%s: termination requested", er.id)
return
}
master, err := er.election.IsMaster(ctx)
if err != nil {
glog.Errorf("%s: failed to check mastership status", er.id)
break
}
if !master {
glog.Errorf("%s: no longer the master!", er.id)
er.tracker.Set(er.id, false)
break
}
if er.ShouldResign(masterSince) {
glog.Infof("%s: queue up resignation of mastership", er.id)
er.tracker.Set(er.id, false)
done := make(chan bool)
r := Resignation{ID: er.id, er: er, done: done}
pending <- r
<-done // block until acted on
break // no longer master
}
}
}
}
// ShouldResign randomly decides whether this runner should resign mastership.
func (er *Runner) ShouldResign(masterSince time.Time) bool {
now := er.cfg.TimeSource.Now()
duration := now.Sub(masterSince)
if duration < er.cfg.MasterHoldInterval {
// Always hold onto mastership for a minimum interval to prevent churn.
return false
}
// Roll the bones.
odds := er.cfg.ResignOdds
if odds <= 0 {
return true
}
return rand.Intn(er.cfg.ResignOdds) == 0
}
// Resignation indicates that a master should explicitly resign mastership, by invoking
// the Execute() method at a point where no master-related activity is ongoing.
type Resignation struct {
ID string
er *Runner
done chan<- bool
}
// Execute performs the pending deliberate resignation for an election runner.
func (r *Resignation) Execute(ctx context.Context) {
glog.Infof("%s: deliberately resigning mastership", r.er.id)
if err := r.er.election.Resign(ctx); err != nil {
glog.Errorf("%s: failed to resign mastership: %v", r.er.id, err)
}
if err := r.er.election.Start(ctx); err != nil {
glog.Errorf("%s: failed to restart election: %v", r.er.id, err)
}
r.done <- true
}