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ringv2.go
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ringv2.go
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package ringv2
import (
"context"
"errors"
"fmt"
"path"
"sync"
"sync/atomic"
"time"
"github.com/coreos/etcd/clientv3"
"github.com/coreos/etcd/mvcc/mvccpb"
"github.com/robfig/cron"
"github.com/sensu/sensu-go/backend/store"
"golang.org/x/time/rate"
)
// EventType is an enum that describes the type of event received by watchers.
type EventType int
const (
EventError EventType = iota
EventAdd
EventRemove
EventTrigger
EventClosing
)
func (e EventType) String() string {
switch e {
case EventAdd:
return "EventAdd"
case EventRemove:
return "EventRemove"
case EventTrigger:
return "EventTrigger"
case EventError:
return "EventError"
case EventClosing:
return "EventClosing"
default:
return "INVALID"
}
}
const MinInterval = 5
// Path returns the canonical path to a ring.
func Path(namespace, subscription string) string {
return store.NewKeyBuilder("rings").WithNamespace(namespace).Build(subscription)
}
// Event represents an event that occurred in a ring. The event can originate
// from any ring client.
type Event struct {
// Type is the type of the event.
Type EventType
// Values are the ring items associated with the event. For trigger events,
// the length of Values will be equal to the results per interval.
Values []string
// Err is any error that occurred while processing the event.
Err error
}
type Ring struct {
client *clientv3.Client
// itemPrefix is the prefix of all the ring items. The ring items are KVs
// that map from the value of the item to its sequence ID. The sequence
// IDs are updated on every ring advancement, and are used to sort the ring
// in order to find the least-valued item.
itemPrefix string
// interval is the TTL for ring triggers
interval int
// triggerPrefix is the prefix that contains ring triggers
triggerPrefix string
// watchCtr counts the number of open watchers
watchCtr int64
// cron is a cron schedule
cron cron.Schedule
// watchers is the set of active watchers
watchers map[watcherKey]*watcher
mu sync.Mutex
// limit watch restarts to one per second (defensive)
watchLimiter *rate.Limiter
}
type watcherKey struct {
name string
values int
interval int
cron string
}
func (w *watcher) triggerKey() string {
interval := w.watcherKey.cron
if interval == "" {
interval = fmt.Sprintf("%d", w.interval)
}
return path.Join(w.ring.triggerPrefix, w.name, fmt.Sprintf("%d", w.values), interval)
}
type watcher struct {
watcherKey
notifier chan struct{}
ring *Ring
cron cron.Schedule
events <-chan Event
}
func newWatcher(ring *Ring, ch <-chan Event, name string, values, interval int, schedule string) (*watcher, error) {
var sched cron.Schedule
if schedule != "" {
var err error
sched, err = cron.ParseStandard(schedule)
if err != nil {
return nil, err
}
}
return &watcher{
watcherKey: watcherKey{
name: name,
values: values,
cron: schedule,
interval: interval,
},
notifier: make(chan struct{}, 10),
ring: ring,
cron: sched,
events: ch,
}, nil
}
// New creates a new Ring.
func New(client *clientv3.Client, storePath string) *Ring {
return &Ring{
client: client,
itemPrefix: path.Join(storePath, "items"),
triggerPrefix: path.Join(storePath, "triggers"),
interval: 5,
watchers: make(map[watcherKey]*watcher),
watchLimiter: rate.NewLimiter(rate.Every(time.Second), 1),
}
}
// IsEmpty returns true if there are no items in the ring.
func (r *Ring) IsEmpty(ctx context.Context) (bool, error) {
resp, err := r.client.Get(ctx, r.itemPrefix,
clientv3.WithKeysOnly(),
clientv3.WithPrefix(),
clientv3.WithLimit(1))
if err != nil {
return false, err
}
return len(resp.Kvs) == 0, nil
}
func (w *watcher) grant(ctx context.Context) (*clientv3.LeaseGrantResponse, error) {
interval := w.getInterval()
lease, err := w.ring.client.Grant(ctx, int64(interval))
return lease, err
}
// Add adds a new value to the ring. If the value already exists, its keepalive
// will be reset. Values that are not kept alive will expire and be removed
// from the ring.
func (r *Ring) Add(ctx context.Context, value string, keepalive int64) (rerr error) {
if keepalive < 5 {
return fmt.Errorf("couldn't add %q to ring: keepalive must be >5s", value)
}
itemKey := path.Join(r.itemPrefix, value)
getresp, err := r.client.Get(ctx, itemKey)
if err != nil {
return fmt.Errorf("couldn't add %q to ring: %s", value, err)
}
if len(getresp.Kvs) > 0 && getresp.Kvs[0].Version > 0 {
leaseID := clientv3.LeaseID(getresp.Kvs[0].Lease)
if leaseID == 0 {
goto NEWLEASE
}
// Item already exists
resp, err := r.client.KeepAliveOnce(ctx, leaseID)
if err != nil {
// error most likely due to lease not existing
goto NEWLEASE
}
if resp.TTL == keepalive {
// We can return early since the TTL is as requested.
return nil
}
// The TTL is different than the requested keepalive, so revoke the
// lease and create a new one afterwards.
_, _ = r.client.Revoke(ctx, leaseID)
}
NEWLEASE:
lease, err := r.client.Grant(ctx, keepalive)
if err != nil {
return fmt.Errorf("couldn't add %q to ring: %s", value, err)
}
defer func() {
if rerr != nil && lease != nil {
_, _ = r.client.Revoke(ctx, lease.ID)
}
}()
if _, err := r.client.Put(ctx, itemKey, "", clientv3.WithLease(lease.ID)); err != nil {
return fmt.Errorf("couldn't add %q to ring: %s", value, err)
}
r.notifyWatchers()
return nil
}
func (r *Ring) notifyWatchers() {
r.mu.Lock()
defer r.mu.Unlock()
for _, watcher := range r.watchers {
watcher.notifier <- struct{}{}
}
}
// Remove removes a value from the list. If the value does not exist, nothing
// happens.
func (r *Ring) Remove(ctx context.Context, value string) error {
_, err := r.client.Delete(ctx, path.Join(r.itemPrefix, value))
return err
}
// Watch watches the ring for events. The events are sent on the channel that
// is returned. For each interval duration in seconds, one or more values will
// be delivered, if there are any values in the ring.
//
// If the underlying etcd watcher fails, then the Event will contain a non-nil
// error.
//
// If the context is canceled, EventClosing will be sent on the channel, and it
// will be closed.
//
// The name parameter specifies the name of the watch. Watchers should use
// unique names when requesting different numbers of values.
//
// The interval parameter sets the interval at which ring items will be
// delivered, in seconds.
//
// If the cron parameter is not the empty string, the interval parameter will
// be ignored, and the watcher will deliver values according to the cron
// schedule.
//
// The values parameter controls how many ring values the event will contain.
// If the requested number of values is greater than the number of items in
// the values will contain repetitions in order to satisfy the request.
func (r *Ring) Watch(ctx context.Context, name string, values, interval int, cron string) <-chan Event {
key := watcherKey{name: name, values: values, interval: interval, cron: cron}
r.mu.Lock()
w, ok := r.watchers[key]
r.mu.Unlock()
if ok {
return w.events
}
c := make(chan Event, 1)
r.startWatchers(ctx, c, name, values, interval, cron)
atomic.AddInt64(&r.watchCtr, 1)
return c
}
func (w *watcher) getInterval() int {
if w.cron != nil {
now := time.Now()
// Add 1s to the interval to deal with the effects of truncation
interval := int(w.cron.Next(now).Sub(now)/time.Second) + 1
for interval < MinInterval {
now = now.Add(time.Second)
interval = int(w.cron.Next(now).Sub(now)/time.Second) + 1
}
return interval
}
return w.interval
}
// hasTrigger returns whether the ring has an active trigger. If the ring
// does not have an active trigger, the first lexical key in the ring will
// be returned in the string return value.
func (w *watcher) hasTrigger(ctx context.Context) (bool, string, error) {
getTrigger := clientv3.OpGet(w.triggerKey())
getFirst := clientv3.OpGet(w.ring.itemPrefix, clientv3.WithPrefix(), clientv3.WithLimit(1))
resp, err := w.ring.client.Txn(ctx).Then(getTrigger, getFirst).Commit()
if err != nil {
return false, "", err
}
if got := len(resp.Responses); got != 2 {
return false, "", fmt.Errorf("bad response length: got %d, want 2", got)
}
triggerResp := resp.Responses[0].GetResponseRange()
valueResp := resp.Responses[1].GetResponseRange()
if len(triggerResp.Kvs) == 0 || len(triggerResp.Kvs[0].Value) == 0 {
value := ""
if len(valueResp.Kvs) > 0 {
value = string(valueResp.Kvs[0].Key)
}
return false, value, nil
}
return true, "", nil
}
func (w *watcher) ensureActiveTrigger(ctx context.Context) error {
has, next, err := w.hasTrigger(ctx)
if err != nil {
return err
}
if has || next == "" {
// if next == "", there are no ring items
return nil
}
lease, err := w.grant(ctx)
if err != nil {
return err
}
nextValue := path.Base(next)
triggerOp := clientv3.OpPut(w.triggerKey(), nextValue, clientv3.WithLease(lease.ID))
triggerCmp := clientv3.Compare(clientv3.Version(w.triggerKey()), "=", 0)
resp, err := w.ring.client.Txn(ctx).If(triggerCmp).Then(triggerOp).Commit()
if !resp.Succeeded {
_, _ = w.ring.client.Revoke(ctx, lease.ID)
}
return err
}
func (r *Ring) startWatchers(ctx context.Context, ch chan Event, name string, values, interval int, cron string) {
_ = r.watchLimiter.Wait(ctx)
watcher, err := newWatcher(r, ch, name, values, interval, cron)
if err != nil {
notifyError(ch, err)
notifyClosing(ch)
return
}
cancelCtx, cancel := context.WithCancel(clientv3.WithRequireLeader(ctx))
itemsC := r.client.Watch(cancelCtx, r.itemPrefix, clientv3.WithPrefix())
nextC := r.client.Watch(cancelCtx, watcher.triggerKey(), clientv3.WithFilterPut(), clientv3.WithPrevKV())
r.mu.Lock()
r.watchers[watcher.watcherKey] = watcher
r.mu.Unlock()
if err := watcher.ensureActiveTrigger(ctx); err != nil {
notifyError(ch, fmt.Errorf("error while starting ring watcher: %s", err))
notifyClosing(ch)
cancel()
return
}
go func() {
defer cancel()
for {
select {
case <-ctx.Done():
r.mu.Lock()
delete(r.watchers, watcher.watcherKey)
r.mu.Unlock()
notifyClosing(ch)
return
case response := <-itemsC:
if err := response.Err(); err != nil {
notifyError(ch, err)
}
if response.Canceled {
// The watcher needs to be reinstated
r.startWatchers(ctx, ch, name, values, interval, cron)
return
}
notifyAddRemove(ch, response)
case response := <-nextC:
if err := response.Err(); err != nil {
notifyError(ch, err)
}
if response.Canceled {
// The watcher needs to be reinstated
r.startWatchers(ctx, ch, name, values, interval, cron)
return
}
watcher.handleRingTrigger(ctx, ch, response)
case <-watcher.notifier:
if err := watcher.ensureActiveTrigger(ctx); err != nil {
notifyError(ch, err)
}
}
}
}()
}
func notifyClosing(ch chan<- Event) {
ch <- Event{Type: EventClosing}
close(ch)
}
func (r *Ring) nextInRing(ctx context.Context, prevKv *mvccpb.KeyValue, n int64) ([]*mvccpb.KeyValue, error) {
opts := []clientv3.OpOption{clientv3.WithLimit(n)}
var key string
if prevKv == nil {
key = r.itemPrefix
opts = append(opts, clientv3.WithPrefix())
} else {
value := string(prevKv.Value)
key = path.Join(r.itemPrefix, value)
end := path.Join(r.itemPrefix, string([]byte{0xFF}))
opts = append(opts, clientv3.WithFromKey())
opts = append(opts, clientv3.WithRange(end))
}
resp, err := r.client.Get(ctx, key, opts...)
if err != nil {
return nil, fmt.Errorf("couldn't get next item(s) in ring: %s", err)
}
result := resp.Kvs
if len(result) == 0 && prevKv == nil {
return nil, nil
} else if len(result) == 0 {
// If a delete occurred and it corresponded to the trigger, need to try
// again with nil prevKv
return r.nextInRing(ctx, nil, n)
}
if int64(len(result)) < n {
m := n - int64(len(result))
resp, err := r.client.Get(ctx, r.itemPrefix, clientv3.WithPrefix(), clientv3.WithLimit(m))
if err != nil {
return nil, fmt.Errorf("couldn't get next item(s) in ring: %s", err)
}
result = append(result, resp.Kvs...)
}
return result, nil
}
func repeatKVs(kvs []*mvccpb.KeyValue, items int) []*mvccpb.KeyValue {
result := make([]*mvccpb.KeyValue, 0, items)
for i := 0; i < (items / len(kvs)); i++ {
result = append(result, kvs...)
}
for i := 0; i < (items % len(kvs)); i++ {
result = append(result, kvs[i])
}
return result
}
func (w *watcher) advanceRing(ctx context.Context, prevKv *mvccpb.KeyValue) ([]*mvccpb.KeyValue, error) {
items, err := w.ring.nextInRing(ctx, prevKv, int64(w.values)+1)
if err != nil {
return nil, fmt.Errorf("couldn't advance ring: %s", err)
}
if len(items) == 0 {
// The ring is empty
return nil, nil
}
nextItem := items[len(items)-1]
repeatItems := repeatKVs(items, w.values)
if len(items) < w.values+1 {
// There are fewer items than requested values
nextItem = items[0]
}
lease, err := w.grant(ctx)
if err != nil {
return nil, fmt.Errorf("couldn't advance ring: %s", err)
}
txnSuccess := false
defer func() {
if !txnSuccess {
_, _ = w.ring.client.Revoke(ctx, lease.ID)
}
}()
nextValue := path.Base(string(nextItem.Key))
triggerOp := clientv3.OpPut(w.triggerKey(), nextValue, clientv3.WithLease(lease.ID))
triggerCmp := clientv3.Compare(clientv3.Version(w.triggerKey()), "=", 0)
resp, err := w.ring.client.Txn(ctx).If(triggerCmp).Then(triggerOp).Commit()
if err != nil {
return nil, fmt.Errorf("couldn't advance ring: %s", err)
}
// Captured by the deferred function
txnSuccess = resp.Succeeded
return repeatItems, nil
}
func (w *watcher) handleRingTrigger(ctx context.Context, ch chan<- Event, response clientv3.WatchResponse) {
for _, event := range response.Events {
items, err := w.advanceRing(ctx, event.PrevKv)
if err != nil {
notifyError(ch, err)
}
if len(items) > 0 {
// When the ring trigger was deleted by the Remove() method, the
// items will be empty.
notifyTrigger(ch, items)
}
}
}
// notifyAddRemove sends EventAdd or EventRemove events to the channel
func notifyAddRemove(ch chan<- Event, response clientv3.WatchResponse) {
for _, event := range response.Events {
if event.Kv == nil {
ch <- Event{
Err: errors.New("nil Kv from ring watcher"),
}
continue
}
if event.Kv.Version > 1 {
// The item was put, and already existed
continue
}
eventType := EventRemove
if event.Type == mvccpb.PUT {
eventType = EventAdd
}
ch <- Event{
Type: eventType,
Values: []string{path.Base(string(event.Kv.Key))},
}
}
}
// notifyTrigger sents EventTrigger events to the channel
func notifyTrigger(ch chan<- Event, items []*mvccpb.KeyValue) {
if len(items) == 0 {
ch <- Event{
Err: errors.New("trigger without items"),
}
return
}
values := make([]string, len(items))
for i := range values {
values[i] = path.Base(string(items[i].Key))
}
event := Event{
Type: EventTrigger,
Values: values,
}
ch <- event
}
// notifyError sends EventError events to the channel
func notifyError(ch chan<- Event, err error) {
ch <- Event{Err: err, Type: EventError}
}