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dex/vendor/go.etcd.io/etcd/raft/quorum/majority.go
2020-01-31 10:32:00 +01:00

211 lines
5.5 KiB
Go

// 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()
}
// Slice returns the MajorityConfig as a sorted slice.
func (c MajorityConfig) Slice() []uint64 {
var sl []uint64
for id := range c {
sl = append(sl, id)
}
sort.Slice(sl, func(i, j int) bool { return sl[i] < sl[j] })
return sl
}
func insertionSort(sl []uint64) {
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
var srt []uint64
if len(stk) >= n {
srt = stk[:n]
} else {
srt = make([]uint64, 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
}