在集群负载均衡时,Dubbo提供4种均衡策略:Random LoadBalance(default)、RoundRobin LoadBalance、LeastActive LoadBalance与ConsistentHash LoadBalance。

Random LoadBalance

  • 随机,按权重设置随机概率。
  • 在一个截面上碰撞的概率高,但调用量越大分布越均匀,而且按概率使用权重后也比较均匀,有利于动态调整提供者权重。
源码
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public class RandomLoadBalance extends AbstractLoadBalance {

public static final String NAME = "random";

/**
* Select one invoker between a list using a random criteria
* @param invokers List of possible invokers
* @param url URL
* @param invocation Invocation
* @param <T>
* @return The selected invoker
*/
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
// Number of invokers
int length = invokers.size();
// Every invoker has the same weight?
boolean sameWeight = true;
// the maxWeight of every invokers, the minWeight = 0 or the maxWeight of the last invoker
int[] weights = new int[length];
// The sum of weights
int totalWeight = 0;
for (int i = 0; i < length; i++) {
int weight = getWeight(invokers.get(i), invocation);
// Sum
totalWeight += weight;
// save for later use
weights[i] = totalWeight;
if (sameWeight && totalWeight != weight * (i + 1)) {
sameWeight = false;
}
}
if (totalWeight > 0 && !sameWeight) {
// If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on totalWeight.
int offset = ThreadLocalRandom.current().nextInt(totalWeight);
// Return a invoker based on the random value.
for (int i = 0; i < length; i++) {
if (offset < weights[i]) {
return invokers.get(i);
}
}
}
// If all invokers have the same weight value or totalWeight=0, return evenly.
return invokers.get(ThreadLocalRandom.current().nextInt(length));
}

}

RoundRobin LoadBalance

  • 轮询,按公约后的权重设置轮询比率。
  • 存在慢的提供者累积请求的问题,比如:第二台机器很慢,但没挂,当请求调到第二台时就卡在那,久而久之,所有请求都卡在调到第二台上。
源码
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public class RoundRobinLoadBalance extends AbstractLoadBalance {
public static final String NAME = "roundrobin";

private static final int RECYCLE_PERIOD = 60000;

protected static class WeightedRoundRobin {
private int weight;
private AtomicLong current = new AtomicLong(0);
private long lastUpdate;

public int getWeight() {
return weight;
}

public void setWeight(int weight) {
this.weight = weight;
current.set(0);
}

public long increaseCurrent() {
return current.addAndGet(weight);
}

public void sel(int total) {
current.addAndGet(-1 * total);
}

public long getLastUpdate() {
return lastUpdate;
}

public void setLastUpdate(long lastUpdate) {
this.lastUpdate = lastUpdate;
}
}

private ConcurrentMap<String, ConcurrentMap<String, WeightedRoundRobin>> methodWeightMap = new ConcurrentHashMap<String, ConcurrentMap<String, WeightedRoundRobin>>();

/**
* get invoker addr list cached for specified invocation
* <p>
* <b>for unit test only</b>
*
* @param invokers
* @param invocation
* @return
*/
protected <T> Collection<String> getInvokerAddrList(List<Invoker<T>> invokers, Invocation invocation) {
String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName();
Map<String, WeightedRoundRobin> map = methodWeightMap.get(key);
if (map != null) {
return map.keySet();
}
return null;
}

@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName();
ConcurrentMap<String, WeightedRoundRobin> map = methodWeightMap.computeIfAbsent(key, k -> new ConcurrentHashMap<>());
int totalWeight = 0;
long maxCurrent = Long.MIN_VALUE;
long now = System.currentTimeMillis();
Invoker<T> selectedInvoker = null;
WeightedRoundRobin selectedWRR = null;
for (Invoker<T> invoker : invokers) {
String identifyString = invoker.getUrl().toIdentityString();
int weight = getWeight(invoker, invocation);
WeightedRoundRobin weightedRoundRobin = map.computeIfAbsent(identifyString, k -> {
WeightedRoundRobin wrr = new WeightedRoundRobin();
wrr.setWeight(weight);
return wrr;
});

if (weight != weightedRoundRobin.getWeight()) {
//weight changed
weightedRoundRobin.setWeight(weight);
}
long cur = weightedRoundRobin.increaseCurrent();
weightedRoundRobin.setLastUpdate(now);
if (cur > maxCurrent) {
maxCurrent = cur;
selectedInvoker = invoker;
selectedWRR = weightedRoundRobin;
}
totalWeight += weight;
}
if (invokers.size() != map.size()) {
map.entrySet().removeIf(item -> now - item.getValue().getLastUpdate() > RECYCLE_PERIOD);
}
if (selectedInvoker != null) {
selectedWRR.sel(totalWeight);
return selectedInvoker;
}
// should not happen here
return invokers.get(0);
}

}

LeastActive LoadBalance

  • 最少活跃调用数,相同活跃数的随机,活跃数指调用前后计数差。
  • 使慢的提供者收到更少请求,因为越慢的提供者的调用前后计数差会越大。
源码
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public class LeastActiveLoadBalance extends AbstractLoadBalance {

public static final String NAME = "leastactive";

@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
// Number of invokers
int length = invokers.size();
// The least active value of all invokers
int leastActive = -1;
// The number of invokers having the same least active value (leastActive)
int leastCount = 0;
// The index of invokers having the same least active value (leastActive)
int[] leastIndexes = new int[length];
// the weight of every invokers
int[] weights = new int[length];
// The sum of the warmup weights of all the least active invokers
int totalWeight = 0;
// The weight of the first least active invoker
int firstWeight = 0;
// Every least active invoker has the same weight value?
boolean sameWeight = true;


// Filter out all the least active invokers
for (int i = 0; i < length; i++) {
Invoker<T> invoker = invokers.get(i);
// Get the active number of the invoker
int active = RpcStatus.getStatus(invoker.getUrl(), invocation.getMethodName()).getActive();
// Get the weight of the invoker's configuration. The default value is 100.
int afterWarmup = getWeight(invoker, invocation);
// save for later use
weights[i] = afterWarmup;
// If it is the first invoker or the active number of the invoker is less than the current least active number
if (leastActive == -1 || active < leastActive) {
// Reset the active number of the current invoker to the least active number
leastActive = active;
// Reset the number of least active invokers
leastCount = 1;
// Put the first least active invoker first in leastIndexes
leastIndexes[0] = i;
// Reset totalWeight
totalWeight = afterWarmup;
// Record the weight the first least active invoker
firstWeight = afterWarmup;
// Each invoke has the same weight (only one invoker here)
sameWeight = true;
// If current invoker's active value equals with leaseActive, then accumulating.
} else if (active == leastActive) {
// Record the index of the least active invoker in leastIndexes order
leastIndexes[leastCount++] = i;
// Accumulate the total weight of the least active invoker
totalWeight += afterWarmup;
// If every invoker has the same weight?
if (sameWeight && afterWarmup != firstWeight) {
sameWeight = false;
}
}
}
// Choose an invoker from all the least active invokers
if (leastCount == 1) {
// If we got exactly one invoker having the least active value, return this invoker directly.
return invokers.get(leastIndexes[0]);
}
if (!sameWeight && totalWeight > 0) {
// If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on
// totalWeight.
int offsetWeight = ThreadLocalRandom.current().nextInt(totalWeight);
// Return a invoker based on the random value.
for (int i = 0; i < leastCount; i++) {
int leastIndex = leastIndexes[i];
offsetWeight -= weights[leastIndex];
if (offsetWeight < 0) {
return invokers.get(leastIndex);
}
}
}
// If all invokers have the same weight value or totalWeight=0, return evenly.
return invokers.get(leastIndexes[ThreadLocalRandom.current().nextInt(leastCount)]);
}
}

ConsistentHash LoadBalance

  • 一致性 Hash,相同参数的请求总是发到同一提供者。
  • 当某一台提供者挂时,原本发往该提供者的请求,基于虚拟节点,平摊到其它提供者,不会引起剧烈变动。
源码
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public class ConsistentHashLoadBalance extends AbstractLoadBalance {
public static final String NAME = "consistenthash";

/**
* Hash nodes name
*/
public static final String HASH_NODES = "hash.nodes";

/**
* Hash arguments name
*/
public static final String HASH_ARGUMENTS = "hash.arguments";

private final ConcurrentMap<String, ConsistentHashSelector<?>> selectors = new ConcurrentHashMap<String, ConsistentHashSelector<?>>();

@SuppressWarnings("unchecked")
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
String methodName = RpcUtils.getMethodName(invocation);
String key = invokers.get(0).getUrl().getServiceKey() + "." + methodName;
// using the hashcode of list to compute the hash only pay attention to the elements in the list
int invokersHashCode = invokers.hashCode();
ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key);
if (selector == null || selector.identityHashCode != invokersHashCode) {
selectors.put(key, new ConsistentHashSelector<T>(invokers, methodName, invokersHashCode));
selector = (ConsistentHashSelector<T>) selectors.get(key);
}
return selector.select(invocation);
}

private static final class ConsistentHashSelector<T> {

private final TreeMap<Long, Invoker<T>> virtualInvokers;

private final int replicaNumber;

private final int identityHashCode;

private final int[] argumentIndex;

ConsistentHashSelector(List<Invoker<T>> invokers, String methodName, int identityHashCode) {
this.virtualInvokers = new TreeMap<Long, Invoker<T>>();
this.identityHashCode = identityHashCode;
URL url = invokers.get(0).getUrl();
this.replicaNumber = url.getMethodParameter(methodName, HASH_NODES, 160);
String[] index = COMMA_SPLIT_PATTERN.split(url.getMethodParameter(methodName, HASH_ARGUMENTS, "0"));
argumentIndex = new int[index.length];
for (int i = 0; i < index.length; i++) {
argumentIndex[i] = Integer.parseInt(index[i]);
}
for (Invoker<T> invoker : invokers) {
String address = invoker.getUrl().getAddress();
for (int i = 0; i < replicaNumber / 4; i++) {
byte[] digest = Bytes.getMD5(address + i);
for (int h = 0; h < 4; h++) {
long m = hash(digest, h);
virtualInvokers.put(m, invoker);
}
}
}
}

public Invoker<T> select(Invocation invocation) {
String key = toKey(invocation.getArguments());
byte[] digest = Bytes.getMD5(key);
return selectForKey(hash(digest, 0));
}

private String toKey(Object[] args) {
StringBuilder buf = new StringBuilder();
for (int i : argumentIndex) {
if (i >= 0 && i < args.length) {
buf.append(args[i]);
}
}
return buf.toString();
}

private Invoker<T> selectForKey(long hash) {
Map.Entry<Long, Invoker<T>> entry = virtualInvokers.ceilingEntry(hash);
if (entry == null) {
entry = virtualInvokers.firstEntry();
}
return entry.getValue();
}

private long hash(byte[] digest, int number) {
return (((long) (digest[3 + number * 4] & 0xFF) << 24)
| ((long) (digest[2 + number * 4] & 0xFF) << 16)
| ((long) (digest[1 + number * 4] & 0xFF) << 8)
| (digest[number * 4] & 0xFF))
& 0xFFFFFFFFL;
}
}

}