/
election.go
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/
election.go
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/*
* Copyright (C) 2016 Red Hat, Inc.
*
* 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 ofthe 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 specificlanguage governing permissions and
* limitations under the License.
*
*/
package client
import (
"sync"
"time"
"github.com/safchain/insanelock"
etcd "go.etcd.io/etcd/client/v2"
"golang.org/x/net/context"
"github.com/skydive-project/skydive/graffiti/logging"
"github.com/skydive-project/skydive/graffiti/service"
)
const (
timeout = time.Second * 30
)
// MasterElector describes an ETCD master elector
type MasterElector struct {
insanelock.RWMutex
EtcdKeyAPI etcd.KeysAPI
HolderID string
path string
listeners []MasterElectionListener
cancel context.CancelFunc
master bool
state service.State
wg sync.WaitGroup
logger logging.Logger
}
// TTL time to live
func (le *MasterElector) TTL() time.Duration {
return timeout
}
func (le *MasterElector) holdLock(quit chan bool) {
defer close(quit)
tick := time.NewTicker(timeout / 2)
defer tick.Stop()
setOptions := &etcd.SetOptions{
TTL: timeout,
PrevExist: etcd.PrevExist,
PrevValue: le.HolderID,
}
ch := tick.C
for {
select {
case <-ch:
if _, err := le.EtcdKeyAPI.Set(context.Background(), le.path, le.HolderID, setOptions); err != nil {
return
}
case <-quit:
return
}
}
}
// IsMaster returns true if the current instance is master
func (le *MasterElector) IsMaster() bool {
le.RLock()
defer le.RUnlock()
return le.master
}
// start starts the election process and send something to the chan when the first
// election is done
func (le *MasterElector) start(first chan struct{}) {
// delete previous Lock
le.EtcdKeyAPI.Delete(context.Background(), le.path, &etcd.DeleteOptions{PrevValue: le.HolderID})
quit := make(chan bool)
// try to get the lock
setOptions := &etcd.SetOptions{
TTL: timeout,
PrevExist: etcd.PrevNoExist,
}
if _, err := le.EtcdKeyAPI.Set(context.Background(), le.path, le.HolderID, setOptions); err == nil {
le.logger.Infof("starting as the master for %s: %s", le.path, le.HolderID)
le.Lock()
le.master = true
le.Unlock()
go le.holdLock(quit)
for _, listener := range le.listeners {
listener.OnStartAsMaster()
}
} else {
le.logger.Infof("starting as a follower for %s: %s", le.path, le.HolderID)
for _, listener := range le.listeners {
listener.OnStartAsSlave()
}
}
if first != nil {
first <- struct{}{}
}
// now watch for changes
watcher := le.EtcdKeyAPI.Watcher(le.path, &etcd.WatcherOptions{})
ctx, cancel := context.WithCancel(context.Background())
le.cancel = cancel
le.wg.Add(1)
defer le.wg.Done()
le.state.Store(service.RunningState)
for le.state.Load() == service.RunningState {
resp, err := watcher.Next(ctx)
if err != nil {
if err == context.Canceled {
break
}
le.logger.Errorf("Error while watching etcd: %s", err.Error())
time.Sleep(1 * time.Second)
continue
}
switch resp.Action {
case "expire", "delete", "compareAndDelete":
_, err = le.EtcdKeyAPI.Set(context.Background(), le.path, le.HolderID, setOptions)
if err == nil && !le.master {
le.Lock()
le.master = true
le.Unlock()
go le.holdLock(quit)
le.logger.Infof("I'm now the master: %s", le.HolderID)
for _, listener := range le.listeners {
listener.OnSwitchToMaster()
}
}
case "create", "update":
le.RLock()
master := le.master
le.RUnlock()
if !master {
le.logger.Infof("The master is now: %s", resp.Node.Value)
for _, listener := range le.listeners {
listener.OnSwitchToSlave()
}
}
}
}
// unlock before leaving so that another can take the lead
le.EtcdKeyAPI.Delete(context.Background(), le.path, &etcd.DeleteOptions{PrevValue: le.HolderID})
}
// Start the master election mechanism
func (le *MasterElector) Start() {
go le.start(nil)
}
// StartAndWait starts the election mechanism and wait for the first election
// before returning
func (le *MasterElector) StartAndWait() {
first := make(chan struct{})
defer close(first)
go le.start(first)
<-first
}
// Stop the election mechanism
func (le *MasterElector) Stop() {
if le.state.CompareAndSwap(service.RunningState, service.StoppingState) {
le.cancel()
le.wg.Wait()
}
}
// AddEventListener registers a new listener
func (le *MasterElector) AddEventListener(listener MasterElectionListener) {
le.listeners = append(le.listeners, listener)
}
// NewMasterElector creates a new ETCD master elector
func NewMasterElector(etcdClient *Client, path string) *MasterElector {
return &MasterElector{
EtcdKeyAPI: etcdClient.KeysAPI,
HolderID: etcdClient.id,
path: path,
master: false,
logger: etcdClient.logger,
}
}