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75142be Mar 23, 2015
@zeebo @fd @patrickxb @edsrzf
268 lines (235 sloc) 6.08 KB
// Copyright (C) 2012 Numerotron Inc.
// Use of this source code is governed by an MIT-style license
// that can be found in the LICENSE file.
// Package consistent provides a consistent hashing function.
//
// Consistent hashing is often used to distribute requests to a changing set of servers. For example,
// say you have some cache servers cacheA, cacheB, and cacheC. You want to decide which cache server
// to use to look up information on a user.
//
// You could use a typical hash table and hash the user id
// to one of cacheA, cacheB, or cacheC. But with a typical hash table, if you add or remove a server,
// almost all keys will get remapped to different results, which basically could bring your service
// to a grinding halt while the caches get rebuilt.
//
// With a consistent hash, adding or removing a server drastically reduces the number of keys that
// get remapped.
//
// Read more about consistent hashing on wikipedia: http://en.wikipedia.org/wiki/Consistent_hashing
//
package consistent // import "stathat.com/c/consistent"
import (
"errors"
"hash/crc32"
"sort"
"strconv"
"sync"
)
type uints []uint32
// Len returns the length of the uints array.
func (x uints) Len() int { return len(x) }
// Less returns true if element i is less than element j.
func (x uints) Less(i, j int) bool { return x[i] < x[j] }
// Swap exchanges elements i and j.
func (x uints) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
// ErrEmptyCircle is the error returned when trying to get an element when nothing has been added to hash.
var ErrEmptyCircle = errors.New("empty circle")
// Consistent holds the information about the members of the consistent hash circle.
type Consistent struct {
circle map[uint32]string
members map[string]bool
sortedHashes uints
NumberOfReplicas int
count int64
scratch [64]byte
sync.RWMutex
}
// New creates a new Consistent object with a default setting of 20 replicas for each entry.
//
// To change the number of replicas, set NumberOfReplicas before adding entries.
func New() *Consistent {
c := new(Consistent)
c.NumberOfReplicas = 20
c.circle = make(map[uint32]string)
c.members = make(map[string]bool)
return c
}
// eltKey generates a string key for an element with an index.
func (c *Consistent) eltKey(elt string, idx int) string {
// return elt + "|" + strconv.Itoa(idx)
return strconv.Itoa(idx) + elt
}
// Add inserts a string element in the consistent hash.
func (c *Consistent) Add(elt string) {
c.Lock()
defer c.Unlock()
c.add(elt)
}
// need c.Lock() before calling
func (c *Consistent) add(elt string) {
for i := 0; i < c.NumberOfReplicas; i++ {
c.circle[c.hashKey(c.eltKey(elt, i))] = elt
}
c.members[elt] = true
c.updateSortedHashes()
c.count++
}
// Remove removes an element from the hash.
func (c *Consistent) Remove(elt string) {
c.Lock()
defer c.Unlock()
c.remove(elt)
}
// need c.Lock() before calling
func (c *Consistent) remove(elt string) {
for i := 0; i < c.NumberOfReplicas; i++ {
delete(c.circle, c.hashKey(c.eltKey(elt, i)))
}
delete(c.members, elt)
c.updateSortedHashes()
c.count--
}
// Set sets all the elements in the hash. If there are existing elements not
// present in elts, they will be removed.
func (c *Consistent) Set(elts []string) {
c.Lock()
defer c.Unlock()
for k := range c.members {
found := false
for _, v := range elts {
if k == v {
found = true
break
}
}
if !found {
c.remove(k)
}
}
for _, v := range elts {
_, exists := c.members[v]
if exists {
continue
}
c.add(v)
}
}
func (c *Consistent) Members() []string {
c.RLock()
defer c.RUnlock()
var m []string
for k := range c.members {
m = append(m, k)
}
return m
}
// Get returns an element close to where name hashes to in the circle.
func (c *Consistent) Get(name string) (string, error) {
c.RLock()
defer c.RUnlock()
if len(c.circle) == 0 {
return "", ErrEmptyCircle
}
key := c.hashKey(name)
i := c.search(key)
return c.circle[c.sortedHashes[i]], nil
}
func (c *Consistent) search(key uint32) (i int) {
f := func(x int) bool {
return c.sortedHashes[x] > key
}
i = sort.Search(len(c.sortedHashes), f)
if i >= len(c.sortedHashes) {
i = 0
}
return
}
// GetTwo returns the two closest distinct elements to the name input in the circle.
func (c *Consistent) GetTwo(name string) (string, string, error) {
c.RLock()
defer c.RUnlock()
if len(c.circle) == 0 {
return "", "", ErrEmptyCircle
}
key := c.hashKey(name)
i := c.search(key)
a := c.circle[c.sortedHashes[i]]
if c.count == 1 {
return a, "", nil
}
start := i
var b string
for i = start + 1; i != start; i++ {
if i >= len(c.sortedHashes) {
i = 0
}
b = c.circle[c.sortedHashes[i]]
if b != a {
break
}
}
return a, b, nil
}
// GetN returns the N closest distinct elements to the name input in the circle.
func (c *Consistent) GetN(name string, n int) ([]string, error) {
c.RLock()
defer c.RUnlock()
if len(c.circle) == 0 {
return nil, ErrEmptyCircle
}
if c.count < int64(n) {
n = int(c.count)
}
var (
key = c.hashKey(name)
i = c.search(key)
start = i
res = make([]string, 0, n)
elem = c.circle[c.sortedHashes[i]]
)
res = append(res, elem)
if len(res) == n {
return res, nil
}
for i = start + 1; i != start; i++ {
if i >= len(c.sortedHashes) {
i = 0
}
elem = c.circle[c.sortedHashes[i]]
if !sliceContainsMember(res, elem) {
res = append(res, elem)
}
if len(res) == n {
break
}
}
return res, nil
}
func (c *Consistent) hashKey(key string) uint32 {
if len(key) < 64 {
var scratch [64]byte
copy(scratch[:], key)
return crc32.ChecksumIEEE(scratch[:len(key)])
}
return crc32.ChecksumIEEE([]byte(key))
}
func (c *Consistent) updateSortedHashes() {
hashes := c.sortedHashes[:0]
//reallocate if we're holding on to too much (1/4th)
if cap(c.sortedHashes)/(c.NumberOfReplicas*4) > len(c.circle) {
hashes = nil
}
for k := range c.circle {
hashes = append(hashes, k)
}
sort.Sort(hashes)
c.sortedHashes = hashes
}
func sliceContainsMember(set []string, member string) bool {
for _, m := range set {
if m == member {
return true
}
}
return false
}