serialx / hashring Goto Github PK

could you tell me the reason?

1, why use 40 there?

factor := math.Floor(float64(40*len(h.nodes)*weight) / float64(totalWeight))

2, why use 3 there?

for i := 0; i < 3; i++ {
	key := hashVal(bKey[i*4 : i*4+4])
        h.ring[key] = node
	h.sortedKeys = append(h.sortedKeys, key)
}

thanks

Hashring.generateCircle() is using sort.Sort(data Interface) to sort sortedkeys, but this function is not guaranteed to be stable.
Is this a potential risk that, with some very edge cases, (for example, Hashring.AddNode or Hashring.RemoveNode is called, which invokes Hashring.generateCircle,) a range of keys will be mapped to a different node?

It seems from the code that the ring is not thread safe to modify and use at the same time, right? Like one goroutine add/remove node, while other goroutines call GetNode. Is it true that I need my own locking?

Hi everyone,

the last commit broke our assumption that a randomly ordered slice of nodes will consistently return the same node for the same hash. The following test code fails on the current HEAD of master and works fine with the previous commit:

func TestStableHashring(t *testing.T) {
	nodes := []string{"NODE_1", "NODE_2", "NODE_3", "NODE_4", "NODE_5"}

	hash := "HASH"
	var constantNode string
	for i := 0; i < 100; i++ {

		rand.Shuffle(len(nodes), func(i, j int) { nodes[i], nodes[j] = nodes[j], nodes[i] })

		hr := New(nodes)
		n, ok := hr.GetNode(hash)
		if !ok {
			t.Error("error getting node")
		}

		if constantNode == "" {
			constantNode = n
		}

		if n != constantNode {
			t.Errorf("nodes are not equalt %s != %s", n, constantNode)
		}
	}
}

Is this a falsey assumption or did the latest changes introduce an unintended behaviour?

Cheers,
Dennis

Hi, does it possible to control placement (near, far) in case of using this package?
For example i have rack with servers and ring contains 40 nodes and i have 3 replica count, how can i specify that i need to place data on far servers?
Or another use-case - i have 3 racks and replica count 2, how can i place data on rack that far from another?

unbalanced example:

$ cat main.go
package main

import (
	"fmt"
	"github.com/google/uuid"
	"github.com/serialx/hashring"
)

func main() {
	schedulers := []string{
		"1.1.1.1",
		"2.2.2.2",
	}
	ring := hashring.New(schedulers)
	serverCount := make(map[string]int)

	for i := 0; i < 1000; i++ {
		etName := uuid.New().String()
		server, _ := ring.GetNode(etName)
		serverCount[server]++
	}
	fmt.Println(serverCount)
}

$ go run main.go
map[1.1.1.1:966 2.2.2.2:34]

In the latest commit which “Allow passing a hash.Hash at creation time”, it allows to use custom hashers to calculate the hash value, not only crypto/md5.Sum().

However, the code doesn't run as it should be when using the default hasher md5.New().

Things are wrong

If you run following code listed at README.md, you would find that only two servers are inserted to the ring, when you print some info of the ring.

serversInRing := []string{"192.168.0.246:11212",
                          "192.168.0.247:11212",
                          "192.168.0.248:11212",
                          "192.168.0.249:11212",
                          "192.168.0.250:11212",
                          "192.168.0.251:11212",
                          "192.168.0.252:11212"}

replicaCount := 3
ring := hashring.New(serversInRing)
server, _ := ring.GetNodes("my_key", replicaCount)

There is an open issue reporting similar problems.

Inspect the code

In earlier version, it uses md5.Sum([]byte(key)) to calculate hash values.

// crypto/md5
func Sum(data []byte) [Size]byte {
	var d digest
	d.Reset()
	d.Write(data)
	return d.checkSum()
}

For now, it creates a new digest when you use a default hasher. And it calculates hash values by calling digest.Sum(in []byte). Here comes the problem. They are not the same function at all!

// crypto/md5
func (d *digest) Sum(in []byte) []byte {
	// Make a copy of d so that caller can keep writing and summing.
	d0 := *d
	hash := d0.checkSum()
	return append(in, hash[:]...)
}

digest.Sum() simply returns what it receives following the check sum of the same old data. This is not what we expect!

Try to solve it

Maybe the correct way:

func (h *HashRing) hashDigest(key string) []byte {
	h.hasher.Write([]byte(key))
	return h.hasher.Sum(nil)
}

I'm not familiar with hash things, and I'm not sure whether it's ok for other hashers. But at least, it works for the default hasher.

I am trying to iterate over all nodes of the hash ring, and there are missing nodes.

Here is a sample:

package main

import (
	"github.com/serialx/hashring"
	"fmt"
	"sort"
)

func generate(n int) map[string]int {
	result := make(map[string]int)
	for i := 0; i < n; i++ {
		result[fmt.Sprintf("node%02d", i)] = i
	}
	return result
}

func main() {
	weights := generate(1000)
	ring := hashring.NewWithWeights(weights)
	fmt.Printf("size %d\n", ring.Size())

	var nodes []string
	var ok bool
	for i := 0; i < ring.Size(); i++ {
		nodes, ok = ring.GetNodes("1", ring.Size()-i)
		if ok {
			break
		}
	}
	sort.Strings(nodes)
	fmt.Printf("actual %d\n", len(nodes))
}

Result:

size 1000
actual 987

I am trying this library, but I consistently get the same node. I am trying to hash on stock ticker symbols, here is an example:

package main

import (
	"log"

	"github.com/serialx/hashring"
)

func main() {
	memcacheServers := []string{
		"192.168.0.246:11212",
		"192.168.0.247:11212",
		"192.168.0.249:11212",
	}

	tickers := []string{
		"MSFT",
		"AAPL",
		"AMZN",
		"AAPL",
		"X",
		"NFLX",
		"AMD",
		"APO",
		"V",
		"SPY",
		"TSLA",
	}

	ring := hashring.New(memcacheServers)
	for _, ticker := range tickers {
		server, _ := ring.GetNode(ticker)
		log.Println("Node:", server)
	}
}

Results:

$ go run ring-experiment.go 
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212
2019/10/16 15:14:59 Node: 192.168.0.249:11212