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