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Adds consistent hashing with bound loads sharding algorithm
Signed-off-by: Akram Ben Aissi <akram.benaissi@gmail.com>
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// An implementation of Consistent Hashing and | ||
// Consistent Hashing With Bounded Loads. | ||
// | ||
// https://en.wikipedia.org/wiki/Consistent_hashing | ||
// | ||
// https://research.googleblog.com/2017/04/consistent-hashing-with-bounded-loads.html | ||
package consistent | ||
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import ( | ||
"encoding/binary" | ||
"errors" | ||
"fmt" | ||
"math" | ||
"sync" | ||
"sync/atomic" | ||
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"github.com/google/btree" | ||
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blake2b "github.com/minio/blake2b-simd" | ||
) | ||
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var ErrNoHosts = errors.New("no hosts added") | ||
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type Host struct { | ||
Name string | ||
Load int64 | ||
} | ||
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type Consistent struct { | ||
servers map[uint64]string | ||
clients *btree.BTree | ||
loadMap map[string]*Host | ||
totalLoad int64 | ||
replicationFactor int | ||
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sync.RWMutex | ||
} | ||
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type item struct { | ||
value uint64 | ||
} | ||
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func (i item) Less(than btree.Item) bool { | ||
return i.value < than.(item).value | ||
} | ||
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func New() *Consistent { | ||
return &Consistent{ | ||
servers: map[uint64]string{}, | ||
clients: btree.New(2), | ||
loadMap: map[string]*Host{}, | ||
replicationFactor: 1000, | ||
} | ||
} | ||
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func NewWithReplicationFactor(replicationFactor int) *Consistent { | ||
return &Consistent{ | ||
servers: map[uint64]string{}, | ||
clients: btree.New(2), | ||
loadMap: map[string]*Host{}, | ||
replicationFactor: replicationFactor, | ||
} | ||
} | ||
func (c *Consistent) Add(server string) { | ||
c.Lock() | ||
defer c.Unlock() | ||
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if _, ok := c.loadMap[server]; ok { | ||
return | ||
} | ||
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c.loadMap[server] = &Host{Name: server, Load: 0} | ||
for i := 0; i < c.replicationFactor; i++ { | ||
h := c.hash(fmt.Sprintf("%s%d", server, i)) | ||
c.servers[h] = server | ||
c.clients.ReplaceOrInsert(item{h}) | ||
} | ||
} | ||
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// Get returns the server that owns the given client. | ||
// As described in https://en.wikipedia.org/wiki/Consistent_hashing | ||
// It returns ErrNoHosts if the ring has no servers in it. | ||
func (c *Consistent) Get(client string) (string, error) { | ||
c.RLock() | ||
defer c.RUnlock() | ||
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if c.clients.Len() == 0 { | ||
return "", ErrNoHosts | ||
} | ||
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h := c.hash(client) | ||
var foundItem btree.Item | ||
c.clients.AscendGreaterOrEqual(item{h}, func(i btree.Item) bool { | ||
foundItem = i | ||
return false // stop the iteration | ||
}) | ||
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if foundItem == nil { | ||
// If no host found, wrap around to the first one. | ||
foundItem = c.clients.Min() | ||
} | ||
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host := c.servers[foundItem.(item).value] | ||
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return host, nil | ||
} | ||
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// GetLeast returns the least loaded host that can serve the key. | ||
// It uses Consistent Hashing With Bounded loads. | ||
// https://research.googleblog.com/2017/04/consistent-hashing-with-bounded-loads.html | ||
// It returns ErrNoHosts if the ring has no hosts in it. | ||
func (c *Consistent) GetLeast(client string) (string, error) { | ||
c.RLock() | ||
defer c.RUnlock() | ||
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if c.clients.Len() == 0 { | ||
return "", ErrNoHosts | ||
} | ||
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h := c.hash(client) | ||
idx := c.search(h) | ||
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i := idx | ||
for { | ||
x := item{uint64(i)} | ||
key := c.clients.Get(x) | ||
if key != nil { | ||
host := c.servers[key.(*item).value] | ||
if c.loadOK(host) { | ||
return host, nil | ||
} | ||
i++ | ||
if i >= c.clients.Len() { | ||
i = 0 | ||
} | ||
} else { | ||
return client, nil | ||
} | ||
} | ||
} | ||
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func (c *Consistent) search(key uint64) int { | ||
idx := 0 | ||
found := false | ||
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c.clients.Ascend(func(i btree.Item) bool { | ||
if i.(item).value >= key { | ||
found = true | ||
return false // stop the iteration | ||
} | ||
idx++ | ||
return true | ||
}) | ||
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if !found { | ||
idx = 0 | ||
} | ||
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return idx | ||
} | ||
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// Sets the load of `server` to the given `load` | ||
func (c *Consistent) UpdateLoad(server string, load int64) { | ||
c.Lock() | ||
defer c.Unlock() | ||
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if _, ok := c.loadMap[server]; !ok { | ||
return | ||
} | ||
c.totalLoad -= c.loadMap[server].Load | ||
c.loadMap[server].Load = load | ||
c.totalLoad += load | ||
} | ||
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// Increments the load of host by 1 | ||
// | ||
// should only be used with if you obtained a host with GetLeast | ||
func (c *Consistent) Inc(server string) { | ||
c.Lock() | ||
defer c.Unlock() | ||
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if _, ok := c.loadMap[server]; !ok { | ||
return | ||
} | ||
atomic.AddInt64(&c.loadMap[server].Load, 1) | ||
atomic.AddInt64(&c.totalLoad, 1) | ||
} | ||
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// Decrements the load of host by 1 | ||
// | ||
// should only be used with if you obtained a host with GetLeast | ||
func (c *Consistent) Done(server string) { | ||
c.Lock() | ||
defer c.Unlock() | ||
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if _, ok := c.loadMap[server]; !ok { | ||
return | ||
} | ||
atomic.AddInt64(&c.loadMap[server].Load, -1) | ||
atomic.AddInt64(&c.totalLoad, -1) | ||
} | ||
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// Deletes host from the ring | ||
func (c *Consistent) Remove(server string) bool { | ||
c.Lock() | ||
defer c.Unlock() | ||
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for i := 0; i < c.replicationFactor; i++ { | ||
h := c.hash(fmt.Sprintf("%s%d", server, i)) | ||
delete(c.servers, h) | ||
c.delSlice(h) | ||
} | ||
delete(c.loadMap, server) | ||
return true | ||
} | ||
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// Return the list of servers in the ring | ||
func (c *Consistent) Servers() (servers []string) { | ||
c.RLock() | ||
defer c.RUnlock() | ||
for k := range c.loadMap { | ||
servers = append(servers, k) | ||
} | ||
return servers | ||
} | ||
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// Returns the loads of all the hosts | ||
func (c *Consistent) GetLoads() map[string]int64 { | ||
loads := map[string]int64{} | ||
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for k, v := range c.loadMap { | ||
loads[k] = v.Load | ||
} | ||
return loads | ||
} | ||
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// Returns the maximum load of the single host | ||
// which is: | ||
// (total_load/number_of_hosts)*1.25 | ||
// total_load = is the total number of active requests served by hosts | ||
// for more info: | ||
// https://research.googleblog.com/2017/04/consistent-hashing-with-bounded-loads.html | ||
func (c *Consistent) MaxLoad() int64 { | ||
if c.totalLoad == 0 { | ||
c.totalLoad = 1 | ||
} | ||
var avgLoadPerNode float64 | ||
avgLoadPerNode = float64(c.totalLoad / int64(len(c.loadMap))) | ||
if avgLoadPerNode == 0 { | ||
avgLoadPerNode = 1 | ||
} | ||
avgLoadPerNode = math.Ceil(avgLoadPerNode * 1.25) | ||
return int64(avgLoadPerNode) | ||
} | ||
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func (c *Consistent) loadOK(server string) bool { | ||
// a safety check if someone performed c.Done more than needed | ||
if c.totalLoad < 0 { | ||
c.totalLoad = 0 | ||
} | ||
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var avgLoadPerNode float64 | ||
avgLoadPerNode = float64((c.totalLoad + 1) / int64(len(c.loadMap))) | ||
if avgLoadPerNode == 0 { | ||
avgLoadPerNode = 1 | ||
} | ||
avgLoadPerNode = math.Ceil(avgLoadPerNode * 1.25) | ||
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bserver, ok := c.loadMap[server] | ||
if !ok { | ||
panic(fmt.Sprintf("given host(%s) not in loadsMap", bserver.Name)) | ||
} | ||
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if float64(bserver.Load)+1 <= avgLoadPerNode { | ||
return true | ||
} | ||
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return false | ||
} | ||
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func (c *Consistent) delSlice(val uint64) { | ||
c.clients.Delete(item{val}) | ||
} | ||
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func (c *Consistent) hash(key string) uint64 { | ||
out := blake2b.Sum512([]byte(key)) | ||
return binary.LittleEndian.Uint64(out[:]) | ||
} |
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