1// Copyright 2016 The etcd Authors 2// 3// Licensed under the Apache License, Version 2.0 (the "License"); 4// you may not use this file except in compliance with the License. 5// You may obtain a copy of the License at 6// 7// http://www.apache.org/licenses/LICENSE-2.0 8// 9// Unless required by applicable law or agreed to in writing, software 10// distributed under the License is distributed on an "AS IS" BASIS, 11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12// See the License for the specific language governing permissions and 13// limitations under the License. 14 15package recipe 16 17import ( 18 "context" 19 20 v3 "github.com/coreos/etcd/clientv3" 21 "github.com/coreos/etcd/mvcc/mvccpb" 22) 23 24// Queue implements a multi-reader, multi-writer distributed queue. 25type Queue struct { 26 client *v3.Client 27 ctx context.Context 28 29 keyPrefix string 30} 31 32func NewQueue(client *v3.Client, keyPrefix string) *Queue { 33 return &Queue{client, context.TODO(), keyPrefix} 34} 35 36func (q *Queue) Enqueue(val string) error { 37 _, err := newUniqueKV(q.client, q.keyPrefix, val) 38 return err 39} 40 41// Dequeue returns Enqueue()'d elements in FIFO order. If the 42// queue is empty, Dequeue blocks until elements are available. 43func (q *Queue) Dequeue() (string, error) { 44 // TODO: fewer round trips by fetching more than one key 45 resp, err := q.client.Get(q.ctx, q.keyPrefix, v3.WithFirstRev()...) 46 if err != nil { 47 return "", err 48 } 49 50 kv, err := claimFirstKey(q.client, resp.Kvs) 51 if err != nil { 52 return "", err 53 } else if kv != nil { 54 return string(kv.Value), nil 55 } else if resp.More { 56 // missed some items, retry to read in more 57 return q.Dequeue() 58 } 59 60 // nothing yet; wait on elements 61 ev, err := WaitPrefixEvents( 62 q.client, 63 q.keyPrefix, 64 resp.Header.Revision, 65 []mvccpb.Event_EventType{mvccpb.PUT}) 66 if err != nil { 67 return "", err 68 } 69 70 ok, err := deleteRevKey(q.client, string(ev.Kv.Key), ev.Kv.ModRevision) 71 if err != nil { 72 return "", err 73 } else if !ok { 74 return q.Dequeue() 75 } 76 return string(ev.Kv.Value), err 77} 78