go.etcd.io/etcd@v3.3.27+incompatible/contrib/recipes/priority_queue.go (about)

     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  
    15  package recipe
    16  
    17  import (
    18  	"context"
    19  	"fmt"
    20  
    21  	v3 "github.com/coreos/etcd/clientv3"
    22  	"github.com/coreos/etcd/mvcc/mvccpb"
    23  )
    24  
    25  // PriorityQueue implements a multi-reader, multi-writer distributed queue.
    26  type PriorityQueue struct {
    27  	client *v3.Client
    28  	ctx    context.Context
    29  	key    string
    30  }
    31  
    32  // NewPriorityQueue creates an etcd priority queue.
    33  func NewPriorityQueue(client *v3.Client, key string) *PriorityQueue {
    34  	return &PriorityQueue{client, context.TODO(), key + "/"}
    35  }
    36  
    37  // Enqueue puts a value into a queue with a given priority.
    38  func (q *PriorityQueue) Enqueue(val string, pr uint16) error {
    39  	prefix := fmt.Sprintf("%s%05d", q.key, pr)
    40  	_, err := newSequentialKV(q.client, prefix, val)
    41  	return err
    42  }
    43  
    44  // Dequeue returns Enqueue()'d items in FIFO order. If the
    45  // queue is empty, Dequeue blocks until items are available.
    46  func (q *PriorityQueue) Dequeue() (string, error) {
    47  	// TODO: fewer round trips by fetching more than one key
    48  	resp, err := q.client.Get(q.ctx, q.key, v3.WithFirstKey()...)
    49  	if err != nil {
    50  		return "", err
    51  	}
    52  
    53  	kv, err := claimFirstKey(q.client, resp.Kvs)
    54  	if err != nil {
    55  		return "", err
    56  	} else if kv != nil {
    57  		return string(kv.Value), nil
    58  	} else if resp.More {
    59  		// missed some items, retry to read in more
    60  		return q.Dequeue()
    61  	}
    62  
    63  	// nothing to dequeue; wait on items
    64  	ev, err := WaitPrefixEvents(
    65  		q.client,
    66  		q.key,
    67  		resp.Header.Revision,
    68  		[]mvccpb.Event_EventType{mvccpb.PUT})
    69  	if err != nil {
    70  		return "", err
    71  	}
    72  
    73  	ok, err := deleteRevKey(q.client, string(ev.Kv.Key), ev.Kv.ModRevision)
    74  	if err != nil {
    75  		return "", err
    76  	} else if !ok {
    77  		return q.Dequeue()
    78  	}
    79  	return string(ev.Kv.Value), err
    80  }