-
Notifications
You must be signed in to change notification settings - Fork 9.7k
/
majority.go
202 lines (178 loc) · 5.3 KB
/
majority.go
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
// Copyright 2019 The etcd Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package quorum
import (
"fmt"
"math"
"sort"
"strings"
)
// MajorityConfig is a set of IDs that uses majority quorums to make decisions.
type MajorityConfig map[uint64]struct{}
func (c MajorityConfig) String() string {
sl := make([]uint64, 0, len(c))
for id := range c {
sl = append(sl, id)
}
sort.Slice(sl, func(i, j int) bool { return sl[i] < sl[j] })
var buf strings.Builder
buf.WriteByte('(')
for i := range sl {
if i > 0 {
buf.WriteByte(' ')
}
fmt.Fprint(&buf, sl[i])
}
buf.WriteByte(')')
return buf.String()
}
// Describe returns a (multi-line) representation of the commit indexes for the
// given lookuper.
func (c MajorityConfig) Describe(l AckedIndexer) string {
if len(c) == 0 {
return "<empty majority quorum>"
}
type tup struct {
id uint64
idx Index
ok bool // idx found?
bar int // length of bar displayed for this tup
}
// Below, populate .bar so that the i-th largest commit index has bar i (we
// plot this as sort of a progress bar). The actual code is a bit more
// complicated and also makes sure that equal index => equal bar.
n := len(c)
info := make([]tup, 0, n)
for id := range c {
idx, ok := l.AckedIndex(id)
info = append(info, tup{id: id, idx: idx, ok: ok})
}
// Sort by index
sort.Slice(info, func(i, j int) bool {
if info[i].idx == info[j].idx {
return info[i].id < info[j].id
}
return info[i].idx < info[j].idx
})
// Populate .bar.
for i := range info {
if i > 0 && info[i-1].idx < info[i].idx {
info[i].bar = i
}
}
// Sort by ID.
sort.Slice(info, func(i, j int) bool {
return info[i].id < info[j].id
})
var buf strings.Builder
// Print.
fmt.Fprint(&buf, strings.Repeat(" ", n)+" idx\n")
for i := range info {
bar := info[i].bar
if !info[i].ok {
fmt.Fprint(&buf, "?"+strings.Repeat(" ", n))
} else {
fmt.Fprint(&buf, strings.Repeat("x", bar)+">"+strings.Repeat(" ", n-bar))
}
fmt.Fprintf(&buf, " %5d (id=%d)\n", info[i].idx, info[i].id)
}
return buf.String()
}
type uint64Slice []uint64
func insertionSort(sl uint64Slice) {
a, b := 0, len(sl)
for i := a + 1; i < b; i++ {
for j := i; j > a && sl[j] < sl[j-1]; j-- {
sl[j], sl[j-1] = sl[j-1], sl[j]
}
}
}
// CommittedIndex computes the committed index from those supplied via the
// provided AckedIndexer (for the active config).
func (c MajorityConfig) CommittedIndex(l AckedIndexer) Index {
n := len(c)
if n == 0 {
// This plays well with joint quorums which, when one half is the zero
// MajorityConfig, should behave like the other half.
return math.MaxUint64
}
// Use an on-stack slice to collect the committed indexes when n <= 7
// (otherwise we alloc). The alternative is to stash a slice on
// MajorityConfig, but this impairs usability (as is, MajorityConfig is just
// a map, and that's nice). The assumption is that running with a
// replication factor of >7 is rare, and in cases in which it happens
// performance is a lesser concern (additionally the performance
// implications of an allocation here are far from drastic).
var stk [7]uint64
srt := uint64Slice(stk[:])
if cap(srt) < n {
srt = make([]uint64, n)
}
srt = srt[:n]
{
// Fill the slice with the indexes observed. Any unused slots will be
// left as zero; these correspond to voters that may report in, but
// haven't yet. We fill from the right (since the zeroes will end up on
// the left after sorting below anyway).
i := n - 1
for id := range c {
if idx, ok := l.AckedIndex(id); ok {
srt[i] = uint64(idx)
i--
}
}
}
// Sort by index. Use a bespoke algorithm (copied from the stdlib's sort
// package) to keep srt on the stack.
insertionSort(srt)
// The smallest index into the array for which the value is acked by a
// quorum. In other words, from the end of the slice, move n/2+1 to the
// left (accounting for zero-indexing).
pos := n - (n/2 + 1)
return Index(srt[pos])
}
// VoteResult takes a mapping of voters to yes/no (true/false) votes and returns
// a result indicating whether the vote is pending (i.e. neither a quorum of
// yes/no has been reached), won (a quorum of yes has been reached), or lost (a
// quorum of no has been reached).
func (c MajorityConfig) VoteResult(votes map[uint64]bool) VoteResult {
if len(c) == 0 {
// By convention, the elections on an empty config win. This comes in
// handy with joint quorums because it'll make a half-populated joint
// quorum behave like a majority quorum.
return VoteWon
}
ny := [2]int{} // vote counts for no and yes, respectively
var missing int
for id := range c {
v, ok := votes[id]
if !ok {
missing++
continue
}
if v {
ny[1]++
} else {
ny[0]++
}
}
q := len(c)/2 + 1
if ny[1] >= q {
return VoteWon
}
if ny[1]+missing >= q {
return VotePending
}
return VoteLost
}