traefik/cluster/leadership.go

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package cluster
import (
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"context"
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"net/http"
"time"
"github.com/cenk/backoff"
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"github.com/containous/mux"
"github.com/containous/traefik/log"
"github.com/containous/traefik/safe"
"github.com/containous/traefik/types"
"github.com/docker/leadership"
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"github.com/unrolled/render"
)
const clusterLeaderKeySuffix = "/leader"
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var templatesRenderer = render.New(render.Options{
Directory: "nowhere",
})
// Leadership allows leadership election using a KV store
type Leadership struct {
*safe.Pool
*types.Cluster
candidate *leadership.Candidate
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leader *safe.Safe
listeners []LeaderListener
}
// NewLeadership creates a leadership
func NewLeadership(ctx context.Context, cluster *types.Cluster) *Leadership {
return &Leadership{
Pool: safe.NewPool(ctx),
Cluster: cluster,
candidate: leadership.NewCandidate(cluster.Store, cluster.Store.Prefix+clusterLeaderKeySuffix, cluster.Node, 20*time.Second),
listeners: []LeaderListener{},
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leader: safe.New(false),
}
}
// LeaderListener is called when leadership has changed
type LeaderListener func(elected bool) error
// Participate tries to be a leader
func (l *Leadership) Participate(pool *safe.Pool) {
pool.GoCtx(func(ctx context.Context) {
log.Debugf("Node %s running for election", l.Cluster.Node)
defer log.Debugf("Node %s no more running for election", l.Cluster.Node)
backOff := backoff.NewExponentialBackOff()
operation := func() error {
return l.run(ctx, l.candidate)
}
notify := func(err error, time time.Duration) {
log.Errorf("Leadership election error %+v, retrying in %s", err, time)
}
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err := backoff.RetryNotify(safe.OperationWithRecover(operation), backOff, notify)
if err != nil {
log.Errorf("Cannot elect leadership %+v", err)
}
})
}
// AddListener adds a leadership listener
func (l *Leadership) AddListener(listener LeaderListener) {
l.listeners = append(l.listeners, listener)
}
// Resign resigns from being a leader
func (l *Leadership) Resign() {
l.candidate.Resign()
log.Infof("Node %s resigned", l.Cluster.Node)
}
func (l *Leadership) run(ctx context.Context, candidate *leadership.Candidate) error {
electedCh, errCh := candidate.RunForElection()
for {
select {
case elected := <-electedCh:
l.onElection(elected)
case err := <-errCh:
return err
case <-ctx.Done():
l.candidate.Resign()
return nil
}
}
}
func (l *Leadership) onElection(elected bool) {
if elected {
log.Infof("Node %s elected leader ♚", l.Cluster.Node)
l.leader.Set(true)
l.Start()
} else {
log.Infof("Node %s elected worker ♝", l.Cluster.Node)
l.leader.Set(false)
l.Stop()
}
for _, listener := range l.listeners {
err := listener(elected)
if err != nil {
log.Errorf("Error calling Leadership listener: %s", err)
}
}
}
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type leaderResponse struct {
Leader bool `json:"leader"`
LeaderNode string `json:"leader_node"`
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}
func (l *Leadership) getLeaderHandler(response http.ResponseWriter, request *http.Request) {
leaderNode := ""
leaderKv, err := l.Cluster.Store.Get(l.Cluster.Store.Prefix+clusterLeaderKeySuffix, nil)
if err != nil {
log.Error(err)
} else {
leaderNode = string(leaderKv.Value)
}
leader := &leaderResponse{Leader: l.IsLeader(), LeaderNode: leaderNode}
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status := http.StatusOK
if !leader.Leader {
// Set status to be `429`, as this will typically cause load balancers to stop sending requests to the instance without removing them from rotation.
status = http.StatusTooManyRequests
}
err = templatesRenderer.JSON(response, status, leader)
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if err != nil {
log.Error(err)
}
}
// IsLeader returns true if current node is leader
func (l *Leadership) IsLeader() bool {
return l.leader.Get().(bool)
}
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// AddRoutes add dashboard routes on a router
func (l *Leadership) AddRoutes(router *mux.Router) {
// Expose cluster leader
router.Methods(http.MethodGet).Path("/api/cluster/leader").HandlerFunc(l.getLeaderHandler)
}