dubbo提供四种负载均衡策略：随机、轮询、最少活动、一致性hash
一、RandomLoadBalance——随机
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 weight of every invokers int[] weights = new int[length]; // the first invoker's weight int firstWeight = getWeight(invokers.get(0), invocation); weights[0] = firstWeight; // The sum of weights int totalWeight = firstWeight; for (int i = 1; i < length; i++) { int weight = getWeight(invokers.get(i), invocation); // save for later use weights[i] = weight; // Sum totalWeight += weight; if (sameWeight && weight != firstWeight) { sameWeight = false; } } //有权重，按权重随机 if (totalWeight > 0 && !sameWeight) { // 0——totalweight(不包含)中随机一个数 int offset = ThreadLocalRandom.current().nextInt(totalWeight); // 返回随机数对应数组的invoker for (int i = 0; i < length; i++) { offset -= weights[i]; if (offset < 0) { return invokers.get(i); } } } // 所有节点权重相等或为0，从数组中随机返回一个invoker return invokers.get(ThreadLocalRandom.current().nextInt(length)); }
总结：
随机负载均衡：根据每个节点权重，进行随机（使用ThreadLocalRandom保证线程安全），具体分为了两种情况：
1、每个节点权重相同，随机返回一个invoker。
2、权重不相同，根据总权重生成一个随机数，然后判断随机数所处区间，返回对应的invoker。
栗子：3个节点A、B、C权重分别为1、2 、3，取0-6（不包含）中一个随机数n，n-1<0位于A节点，否则n-1-2<0位于B节点，否则n-1-2-3<0位于C节点。
特点：少量请求，可能会发生倾斜，当请求变多时，趋向均衡。
二、RoundRobinLoadBalance——轮询
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>>(); private AtomicBoolean updateLock = new AtomicBoolean(); @Override protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName(); //建立容器，以方法为单位，记录每个节点的权重（支持动态权重），原子变量current(用于实现轮询) 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.get(identifyString); if (weightedRoundRobin == null) { weightedRoundRobin = new WeightedRoundRobin(); weightedRoundRobin.setWeight(weight); map.putIfAbsent(identifyString, weightedRoundRobin); weightedRoundRobin = map.get(identifyString); } if (weight != weightedRoundRobin.getWeight()) { //weight changed weightedRoundRobin.setWeight(weight); } //current.addAndGet(weight) ， 选中current最大的节点，然后current.addAndGet(-totalWeight) //这里实现轮询的逻辑有点看不懂 long cur = weightedRoundRobin.increaseCurrent(); weightedRoundRobin.setLastUpdate(now); if (cur > maxCurrent) { maxCurrent = cur; selectedInvoker = invoker; selectedWRR = weightedRoundRobin; } totalWeight += weight; } if (!updateLock.get() && invokers.size() != map.size()) { if (updateLock.compareAndSet(false, true)) { try { // 写时复制策略 ConcurrentMap<String, WeightedRoundRobin> newMap = new ConcurrentHashMap<>(map); // 解决倾斜，invoker超过60s未调用，提高优先级 newMap.entrySet().removeIf(item -> now - item.getValue().getLastUpdate() > RECYCLE_PERIOD); methodWeightMap.put(key, newMap); } finally { updateLock.set(false); } } } if (selectedInvoker != null) { selectedWRR.sel(totalWeight); return selectedInvoker; } // should not happen here return invokers.get(0); }
新版的轮询逻辑有点看不懂：举个栗子，有三个节点，权重为1 2 3
000-->123(选中③后12-3)-->240(选中②后2-20)-->303(选中①后-303)-->-226(选中③后-220)-->-143(选中②后-1-23)-->006(选中③后000)之后循环。
调用顺序：③②①③②③，然后循环。
轮询模式存在响应慢的提供者会累积请求的问题。
三、LeastActiveLoadBalance——最少活跃
/* org.apache.dubbo.rpc.cluster.loadbalance.LeastActiveLoadBalance#doSelect */ 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; //筛选出最不活跃的节点， //给每个节点方法创建一个RpcStatus实例，用于记录节点方法活跃性Map<url,<methodName,rpcStatus>> //找到最小活跃的节点，将它的数组下标，放入数组leastIndexes中 //leastIndexes大小为1时，最小活跃节点仅一个，直接返回 //leastIndexes大小大于1时，最小活跃节点多个，然后用Random类似方法，从多个最小活跃节点中，随机返回一个节点 //所有节点活跃性相同时，Random随机返回一个节点 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 && i > 0 && 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)]); }
总结：
最少活跃负载均衡指的是响应慢的提供者收到更少的请求，如果活跃性相同，跟Random负载均衡一致。
活跃数指的是方法调用前后的计数差，是一个简单的计数器，调用前+1，调用后-1，当某节点响应慢时，单位时间-1比较慢，活跃数就比较大。这个时候会请求那些活跃数小的，响应快的应用。
四、ConsistentHashLoadBalance——一致性Hash
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { String methodName = RpcUtils.getMethodName(invocation); //key = group+interface+version+methodName 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(); //为每个key创建一个选择器（实际就是一个方法对应一个选择器） //<group.interface.version.method,consistentHashSelector> ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key); if (selector == null || selector.identityHashCode != invokersHashCode) { //① selector为空时创建 //② 原语节点更改，导致invokers.hashCode变动，重新分配 selectors.put(key, new ConsistentHashSelector<T>(invokers, methodName, invokersHashCode)); selector = (ConsistentHashSelector<T>) selectors.get(key); } //选择器选择合适的节点 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(); //默认160个槽位，<dubbo:parameter key="hash.codes" value="160" /> this.replicaNumber = url.getMethodParameter(methodName, HASH_NODES, 160); //默认只对方法第一个参数去hash，<dubbo:parameter key="hash.arguments" value="0" /> 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]); } //① 根据address+replica+h确定invoker的<key,invoker>，与其他invoker的属性无关，所以其他invoker挂掉，<key,invoker>不变 //② 为了解决数据倾斜问题dubbo默认160个虚拟节点是每个invoker都有160个虚拟节点，即一致性hash上会有160*invokers.size个服务节点 for (Invoker<T> invoker : invokers) { String address = invoker.getUrl().getAddress(); for (int i = 0; i < replicaNumber / 4; i++) { byte[] digest = md5(address + i); for (int h = 0; h < 4; h++) { long m = hash(digest, h); virtualInvokers.put(m, invoker); } } } } public Invoker<T> select(Invocation invocation) { //默认去方法的第一个参数 String key = toKey(invocation.getArguments()); //求得参数的md5值 byte[] digest = md5(key); //根据第一个参数md5找到对应的invoker 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) { //一致性hash的实现：hash对应的下一个节点 //例如TreeMap现在有节点 3、6、9 //hash=1时取TreeMap.get(3) //hash=3时取TreeMap.get(3) //hash=4时取TreeMap.get(6) //dubbo会有160*invokers.size个服务节点(value对应实际的invoker) Map.Entry<Long, Invoker<T>> entry = virtualInvokers.ceilingEntry(hash); if (entry == null) { entry = virtualInvokers.firstEntry(); } return entry.getValue(); } //CRC24生成hash值？？ 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; } //md5加密：将参数转化一个byte数组 private byte[] md5(String value) { MessageDigest md5; try { md5 = MessageDigest.getInstance("MD5"); } catch (NoSuchAlgorithmException e) { throw new IllegalStateException(e.getMessage(), e); } md5.reset(); byte[] bytes = value.getBytes(StandardCharsets.UTF_8); md5.update(bytes); return md5.digest(); } }
总结：
一致性Hash负载均衡：是将带有相同参数（默认方法的第一个参数）的请求总是发送给同一个提供者。当某台提供者挂掉时，原本发往该提供者的请求会基于虚拟节点（默认160个）平摊到其他提供者上（具体就是挂点虚拟节点的下一个节点），不会引起剧烈变动
五、预热处理——getWeight()
权重处理主要有一个机器预热处理：越热时间内，根据 运行时间/预热时间 的值控制权重。
/* org.apache.dubbo.rpc.cluster.loadbalance.AbstractLoadBalance */ int getWeight(Invoker<?> invoker, Invocation invocation) { int weight; URL url = invoker.getUrl(); // Multiple registry scenario, load balance among multiple registries. if (REGISTRY_SERVICE_REFERENCE_PATH.equals(url.getServiceInterface())) { weight = url.getParameter(REGISTRY_KEY + "." + WEIGHT_KEY, DEFAULT_WEIGHT); } else { weight = url.getMethodParameter(invocation.getMethodName(), WEIGHT_KEY, DEFAULT_WEIGHT); if (weight > 0) { long timestamp = invoker.getUrl().getParameter(TIMESTAMP_KEY, 0L); if (timestamp > 0L) { long uptime = System.currentTimeMillis() - timestamp; if (uptime < 0) { return 1; } //获取预热时间，默认10分钟 int warmup = invoker.getUrl().getParameter(WARMUP_KEY, DEFAULT_WARMUP); if (uptime > 0 && uptime < warmup) { //预热时间内，降低权重，避免刚启动，请求负载导致启动失败 weight = calculateWarmupWeight((int)uptime, warmup, weight); } } } } return Math.max(weight, 0); } static int calculateWarmupWeight(int uptime, int warmup, int weight) { int ww = (int) ( uptime / ((float) warmup / weight)); //uptime/warmup * weight,与uptime成正比 return ww < 1 ? 1 : (Math.min(ww, weight)); }
六、负载均衡的配置方法
默认random，
<!-- 接口层面，下面配置一个就可以生效 --> <dubbo:service interface="..." loadbalance="roundrobin" /> <dubbo:reference interface="..." loadbalance="roundrobin" /> <!-- 方法层面，下面配置一个就可以生效 --> <dubbo:service interface="..."> <dubbo:method name="..." loadbalance="roundrobin" /> </dubbo:service> <dubbo:reference interface="..."> <dubbo:method name="..." loadbalance="roundrobin" /> </dubbo:reference>