聊聊dubbo-go的ConsistentHashLoadBalance

序

本文主要研究一下dubbo-go的ConsistentHashLoadBalance

ConsistentHashLoadBalance

dubbo-go-v1.4.2/cluster/loadbalance/consistent_hash.go

const (
    // ConsistentHash ...
    ConsistentHash = "consistenthash"
    // HashNodes ...
    HashNodes = "hash.nodes"
    // HashArguments ...
    HashArguments = "hash.arguments"
)

var (
    selectors = make(map[string]*ConsistentHashSelector)
    re        = regexp.MustCompile(constant.COMMA_SPLIT_PATTERN)
)

func init() {
    extension.SetLoadbalance(ConsistentHash, NewConsistentHashLoadBalance)
}

// ConsistentHashLoadBalance ...
type ConsistentHashLoadBalance struct {
}

// NewConsistentHashLoadBalance ...
func NewConsistentHashLoadBalance() cluster.LoadBalance {
    return &ConsistentHashLoadBalance{}
}

• ConsistentHashLoadBalance的init方法设置了名为consistenthash的ConsistentHashLoadBalance到extension中

Select

dubbo-go-v1.4.2/cluster/loadbalance/consistent_hash.go

// Select ...
func (lb *ConsistentHashLoadBalance) Select(invokers []protocol.Invoker, invocation protocol.Invocation) protocol.Invoker {
    methodName := invocation.MethodName()
    key := invokers[0].GetUrl().ServiceKey() + "." + methodName

    // hash the invokers
    bs := make([]byte, 0)
    for _, invoker := range invokers {
        b, err := json.Marshal(invoker)
        if err != nil {
            return nil
        }
        bs = append(bs, b...)
    }
    hashCode := crc32.ChecksumIEEE(bs)
    selector, ok := selectors[key]
    if !ok || selector.hashCode != hashCode {
        selectors[key] = newConsistentHashSelector(invokers, methodName, hashCode)
        selector = selectors[key]
    }
    return selector.Select(invocation)
}

• Select方法遍历invokers挨个执行json.Marshal(invoker)，将bytes[]添加到bs中，之后通过crc32.ChecksumIEEE(bs)计算hashCode，然后对比selectors[key]的hashCode与计算出来的hashCode是否一致，不一致则通过newConsistentHashSelector重新设置一个，最后执行selector.Select(invocation)

ConsistentHashSelector

dubbo-go-v1.4.2/cluster/loadbalance/consistent_hash.go

// ConsistentHashSelector ...
type ConsistentHashSelector struct {
    hashCode        uint32
    replicaNum      int
    virtualInvokers map[uint32]protocol.Invoker
    keys            Uint32Slice
    argumentIndex   []int
}

• ConsistentHashSelector定义了hashCode、replicaNum、virtualInvokers、keys、argumentIndex属性

newConsistentHashSelector

dubbo-go-v1.4.2/cluster/loadbalance/consistent_hash.go

func newConsistentHashSelector(invokers []protocol.Invoker, methodName string,
    hashCode uint32) *ConsistentHashSelector {

    selector := &ConsistentHashSelector{}
    selector.virtualInvokers = make(map[uint32]protocol.Invoker)
    selector.hashCode = hashCode
    url := invokers[0].GetUrl()
    selector.replicaNum = int(url.GetMethodParamInt(methodName, HashNodes, 160))
    indices := re.Split(url.GetMethodParam(methodName, HashArguments, "0"), -1)
    for _, index := range indices {
        i, err := strconv.Atoi(index)
        if err != nil {
            return nil
        }
        selector.argumentIndex = append(selector.argumentIndex, i)
    }
    for _, invoker := range invokers {
        u := invoker.GetUrl()
        address := u.Ip + ":" + u.Port
        for i := 0; i < selector.replicaNum/4; i++ {
            digest := md5.Sum([]byte(address + strconv.Itoa(i)))
            for j := 0; j < 4; j++ {
                key := selector.hash(digest, j)
                selector.keys = append(selector.keys, key)
                selector.virtualInvokers[key] = invoker
            }
        }
    }
    sort.Sort(selector.keys)
    return selector
}

• newConsistentHashSelector方法实例化ConsistentHashSelector，并初始化virtualInvokers、hashCode、argumentIndex、keys、virtualInvokers属性

Select

dubbo-go-v1.4.2/cluster/loadbalance/consistent_hash.go

// Select ...
func (c *ConsistentHashSelector) Select(invocation protocol.Invocation) protocol.Invoker {
    key := c.toKey(invocation.Arguments())
    digest := md5.Sum([]byte(key))
    return c.selectForKey(c.hash(digest, 0))
}

func (c *ConsistentHashSelector) toKey(args []interface{}) string {
    var sb strings.Builder
    for i := range c.argumentIndex {
        if i >= 0 && i < len(args) {
            fmt.Fprint(&sb, args[i].(string))
        }
    }
    return sb.String()
}

func (c *ConsistentHashSelector) selectForKey(hash uint32) protocol.Invoker {
    idx := sort.Search(len(c.keys), func(i int) bool {
        return c.keys[i] >= hash
    })
    if idx == len(c.keys) {
        idx = 0
    }
    return c.virtualInvokers[c.keys[idx]]
}

func (c *ConsistentHashSelector) hash(digest [16]byte, i int) uint32 {
    return uint32((digest[3+i*4]&0xFF)<<24) | uint32((digest[2+i*4]&0xFF)<<16) |
        uint32((digest[1+i*4]&0xFF)<<8) | uint32(digest[i*4]&0xFF)&0xFFFFFFF
}

• Select方法通过c.toKey(invocation.Arguments())获取key，再通过md5.Sum([]byte(key))计算digest，最后通过c.selectForKey(c.hash(digest, 0))选取Invoker

小结

ConsistentHashLoadBalance的Select方法遍历invokers挨个执行json.Marshal(invoker)，将bytes[]添加到bs中，之后通过crc32.ChecksumIEEE(bs)计算hashCode，然后对比selectors[key]的hashCode与计算出来的hashCode是否一致，不一致则通过newConsistentHashSelector重新设置一个，最后执行selector.Select(invocation)

doc

• consistent_hash