go.etcd.io/etcd@v3.3.27+incompatible/contrib/recipes/double_barrier.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  
    20  	"github.com/coreos/etcd/clientv3"
    21  	"github.com/coreos/etcd/clientv3/concurrency"
    22  	"github.com/coreos/etcd/mvcc/mvccpb"
    23  )
    24  
    25  // DoubleBarrier blocks processes on Enter until an expected count enters, then
    26  // blocks again on Leave until all processes have left.
    27  type DoubleBarrier struct {
    28  	s   *concurrency.Session
    29  	ctx context.Context
    30  
    31  	key   string // key for the collective barrier
    32  	count int
    33  	myKey *EphemeralKV // current key for this process on the barrier
    34  }
    35  
    36  func NewDoubleBarrier(s *concurrency.Session, key string, count int) *DoubleBarrier {
    37  	return &DoubleBarrier{
    38  		s:     s,
    39  		ctx:   context.TODO(),
    40  		key:   key,
    41  		count: count,
    42  	}
    43  }
    44  
    45  // Enter waits for "count" processes to enter the barrier then returns
    46  func (b *DoubleBarrier) Enter() error {
    47  	client := b.s.Client()
    48  	ek, err := newUniqueEphemeralKey(b.s, b.key+"/waiters")
    49  	if err != nil {
    50  		return err
    51  	}
    52  	b.myKey = ek
    53  
    54  	resp, err := client.Get(b.ctx, b.key+"/waiters", clientv3.WithPrefix())
    55  	if err != nil {
    56  		return err
    57  	}
    58  
    59  	if len(resp.Kvs) > b.count {
    60  		return ErrTooManyClients
    61  	}
    62  
    63  	if len(resp.Kvs) == b.count {
    64  		// unblock waiters
    65  		_, err = client.Put(b.ctx, b.key+"/ready", "")
    66  		return err
    67  	}
    68  
    69  	_, err = WaitEvents(
    70  		client,
    71  		b.key+"/ready",
    72  		ek.Revision(),
    73  		[]mvccpb.Event_EventType{mvccpb.PUT})
    74  	return err
    75  }
    76  
    77  // Leave waits for "count" processes to leave the barrier then returns
    78  func (b *DoubleBarrier) Leave() error {
    79  	client := b.s.Client()
    80  	resp, err := client.Get(b.ctx, b.key+"/waiters", clientv3.WithPrefix())
    81  	if err != nil {
    82  		return err
    83  	}
    84  	if len(resp.Kvs) == 0 {
    85  		return nil
    86  	}
    87  
    88  	lowest, highest := resp.Kvs[0], resp.Kvs[0]
    89  	for _, k := range resp.Kvs {
    90  		if k.ModRevision < lowest.ModRevision {
    91  			lowest = k
    92  		}
    93  		if k.ModRevision > highest.ModRevision {
    94  			highest = k
    95  		}
    96  	}
    97  	isLowest := string(lowest.Key) == b.myKey.Key()
    98  
    99  	if len(resp.Kvs) == 1 {
   100  		// this is the only node in the barrier; finish up
   101  		if _, err = client.Delete(b.ctx, b.key+"/ready"); err != nil {
   102  			return err
   103  		}
   104  		return b.myKey.Delete()
   105  	}
   106  
   107  	// this ensures that if a process fails, the ephemeral lease will be
   108  	// revoked, its barrier key is removed, and the barrier can resume
   109  
   110  	// lowest process in node => wait on highest process
   111  	if isLowest {
   112  		_, err = WaitEvents(
   113  			client,
   114  			string(highest.Key),
   115  			highest.ModRevision,
   116  			[]mvccpb.Event_EventType{mvccpb.DELETE})
   117  		if err != nil {
   118  			return err
   119  		}
   120  		return b.Leave()
   121  	}
   122  
   123  	// delete self and wait on lowest process
   124  	if err = b.myKey.Delete(); err != nil {
   125  		return err
   126  	}
   127  
   128  	key := string(lowest.Key)
   129  	_, err = WaitEvents(
   130  		client,
   131  		key,
   132  		lowest.ModRevision,
   133  		[]mvccpb.Event_EventType{mvccpb.DELETE})
   134  	if err != nil {
   135  		return err
   136  	}
   137  	return b.Leave()
   138  }