go.etcd.io/etcd@v3.3.27+incompatible/pkg/idutil/id.go (about)

     1  // Copyright 2015 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 idutil implements utility functions for generating unique,
    16  // randomized ids.
    17  package idutil
    18  
    19  import (
    20  	"math"
    21  	"sync"
    22  	"time"
    23  )
    24  
    25  const (
    26  	tsLen     = 5 * 8
    27  	cntLen    = 8
    28  	suffixLen = tsLen + cntLen
    29  )
    30  
    31  // Generator generates unique identifiers based on counters, timestamps, and
    32  // a node member ID.
    33  //
    34  // The initial id is in this format:
    35  // High order 2 bytes are from memberID, next 5 bytes are from timestamp,
    36  // and low order one byte is a counter.
    37  // | prefix   | suffix              |
    38  // | 2 bytes  | 5 bytes   | 1 byte  |
    39  // | memberID | timestamp | cnt     |
    40  //
    41  // The timestamp 5 bytes is different when the machine is restart
    42  // after 1 ms and before 35 years.
    43  //
    44  // It increases suffix to generate the next id.
    45  // The count field may overflow to timestamp field, which is intentional.
    46  // It helps to extend the event window to 2^56. This doesn't break that
    47  // id generated after restart is unique because etcd throughput is <<
    48  // 256req/ms(250k reqs/second).
    49  type Generator struct {
    50  	mu sync.Mutex
    51  	// high order 2 bytes
    52  	prefix uint64
    53  	// low order 6 bytes
    54  	suffix uint64
    55  }
    56  
    57  func NewGenerator(memberID uint16, now time.Time) *Generator {
    58  	prefix := uint64(memberID) << suffixLen
    59  	unixMilli := uint64(now.UnixNano()) / uint64(time.Millisecond/time.Nanosecond)
    60  	suffix := lowbit(unixMilli, tsLen) << cntLen
    61  	return &Generator{
    62  		prefix: prefix,
    63  		suffix: suffix,
    64  	}
    65  }
    66  
    67  // Next generates a id that is unique.
    68  func (g *Generator) Next() uint64 {
    69  	g.mu.Lock()
    70  	defer g.mu.Unlock()
    71  	g.suffix++
    72  	id := g.prefix | lowbit(g.suffix, suffixLen)
    73  	return id
    74  }
    75  
    76  func lowbit(x uint64, n uint) uint64 {
    77  	return x & (math.MaxUint64 >> (64 - n))
    78  }