Dubbo/gRPC：实现跨语言通信时，自定义负载均衡算法的接口实现

Dubbo/gRPC：实现跨语言通信时，自定义负载均衡算法的接口实现

大家好，今天我们来深入探讨一下在使用 Dubbo 或 gRPC 进行跨语言通信时，如何实现自定义负载均衡算法的接口。负载均衡在分布式系统中至关重要，它能够将请求有效地分发到多个服务提供者，从而提高系统的可用性、可伸缩性和性能。Dubbo 和 gRPC 作为流行的 RPC 框架，都提供了扩展负载均衡策略的机制。我们将从理论到实践，详细讲解如何利用这些机制实现自定义的负载均衡算法。

1. 负载均衡的重要性与常见策略

在微服务架构中，一个服务通常会部署多个实例，以应对高并发和提高可用性。当客户端发起请求时，需要选择一个合适的实例来处理请求。负载均衡器就是负责这个选择过程的组件。

常见的负载均衡策略包括：

• 轮询 (Round Robin): 依次选择服务实例，保证每个实例都能被均匀地访问。
• 随机 (Random): 随机选择一个服务实例。
• 加权轮询 (Weighted Round Robin): 根据服务实例的权重进行轮询，权重高的实例被选中的概率更高。
• 加权随机 (Weighted Random): 根据服务实例的权重进行随机选择。
• 最少活跃连接 (Least Active Connections): 选择当前活跃连接数最少的服务实例。
• 一致性哈希 (Consistent Hashing): 将请求的某个属性（例如用户 ID）进行哈希，然后根据哈希值选择服务实例，保证相同的请求总是被路由到同一个实例。

这些策略在很多场景下都能满足需求，但在某些特定场景下，可能需要根据业务特点定制负载均衡算法，以达到更好的效果。例如，根据地理位置选择最近的实例，或者根据服务实例的负载情况动态调整权重。

2. Dubbo 的负载均衡扩展机制

Dubbo 使用 SPI (Service Provider Interface) 机制来实现负载均衡策略的扩展。要实现自定义的负载均衡算法，需要：

1. 实现 LoadBalance 接口： 该接口定义了选择服务实例的方法。
2. 在 META-INF/dubbo/org.apache.dubbo.rpc.cluster.LoadBalance 文件中配置实现类。

LoadBalance 接口的定义如下：

package org.apache.dubbo.rpc.cluster;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.RpcException;

import java.util.List;

public interface LoadBalance {

    /**
     * Select one invoker from the list with the load balance algorithm.
     *
     * @param invokers   invokers.
     * @param url        service url.
     * @param invocation invocation.
     * @return selected invoker.
     * @throws RpcException when there's exception in selecting invokers.
     */
    <T> Invoker<T> select(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException;

}

其中：

• invokers: 可用的服务实例列表。
• url: 服务 URL，包含服务的配置信息。
• invocation: RPC 调用信息，包含方法名、参数等。
• 返回值：选中的服务实例。

示例：实现一个简单的基于随机数奇偶性的负载均衡算法

假设我们希望根据随机数来选择服务实例，如果随机数为奇数，则选择第一个实例，否则选择第二个实例（假设至少有两个实例）。

package com.example.dubbo.loadbalance;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.cluster.LoadBalance;

import java.util.List;
import java.util.Random;

public class OddEvenLoadBalance implements LoadBalance {

    @Override
    public <T> Invoker<T> select(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException {
        if (invokers == null || invokers.isEmpty()) {
            return null;
        }
        if (invokers.size() == 1) {
            return invokers.get(0);
        }

        Random random = new Random();
        int randomNumber = random.nextInt();

        if (randomNumber % 2 == 0) {
            // 偶数，选择第二个实例
            return invokers.get(1);
        } else {
            // 奇数，选择第一个实例
            return invokers.get(0);
        }
    }
}

然后，在 src/main/resources/META-INF/dubbo/org.apache.dubbo.rpc.cluster.LoadBalance 文件中配置该实现类：

oddeven=com.example.dubbo.loadbalance.OddEvenLoadBalance

最后，在 Dubbo 的配置文件中指定使用该负载均衡策略：

<dubbo:reference id="demoService" interface="com.example.DemoService" loadbalance="oddeven"/>

或者使用注解：

@Reference(loadbalance = "oddeven")
private DemoService demoService;

注意： 这个例子只是为了演示 Dubbo 负载均衡的扩展机制，实际应用中应该选择更合适的算法。并且需要处理 invokers 数量小于2的情况。

3. gRPC 的负载均衡扩展机制

gRPC 的负载均衡机制更加灵活，它允许客户端和服务端都参与负载均衡决策。通常情况下，客户端负责选择服务实例，服务端负责提供服务实例的信息。

gRPC 提供了 NameResolver 和 LoadBalancer 接口来实现负载均衡策略的扩展。

• NameResolver: 负责解析服务名称，获取服务实例的地址列表。
• LoadBalancer: 负责根据地址列表选择一个合适的实例。

3.1 NameResolver

NameResolver 的作用是将服务名解析为一组 EquivalentAddressGroup。 EquivalentAddressGroup 包含了一组等价的地址，gRPC 会尝试连接这些地址，直到连接成功。

package io.grpc;

import java.net.URI;
import java.util.List;
import java.util.Map;
import java.util.concurrent.Executor;

import javax.annotation.concurrent.ThreadSafe;

/**
 * A pluggable component that resolves a target {@link URI} to a list of addresses.
 *
 * <p>Implementations must be thread-safe, since they may be called from different threads.
 */
@ThreadSafe
public abstract class NameResolver {

  /**
   * Returns the authority used to authenticate the channel.  Never return {@code null}.
   */
  public abstract String getServiceAuthority();

  /**
   * Starts the resolution process.
   *
   * <p>When an update to the resolved list of addresses is available, the resolver must call
   * {@link Listener#onAddresses(List, Attributes)}.  If the resolution fails, the resolver must
   * call {@link Listener#onError(Status)}.
   *
   * <p>This method is idempotent.
   *
   * @param listener the listener of resolution results
   * @param executor the executor to be used to run {@code listener}.
   */
  public abstract void start(Listener listener, Executor executor);

  /**
   * Re-resolves the name.
   *
   * <p>This method is only a hint. Implementations are free to ignore it.
   *
   * <p>This method is idempotent.
   */
  public abstract void refresh();

  /**
   * Stops the resolution process.
   *
   * <p>After this method is called, this resolver is considered terminated, and the result of
   * future calls to {@link #start} are undefined.
   */
  public abstract void shutdown();

  /**
   * Receives address updates from the resolver.
   */
  public abstract static class Listener {
    /**
     * Receives the updated list of addresses.
     *
     * <p>The addresses are considered equivalent to each other.  The order of addresses in the
     * list might matter, as the channel attempts to connect to them in that order.
     *
     * @param addresses the updated list of addresses.  Must not be {@code null}, but may be
     *     empty.
     * @param attributes extra attributes associated with the address.  Must not be {@code null}.
     */
    public abstract void onAddresses(List<EquivalentAddressGroup> addresses, Attributes attributes);

    /**
     * Receives an error from the resolver.  All existing connections will be closed.  The channel
     * will remain in the {@link ConnectivityState#TRANSIENT_FAILURE} state.  The channel will
     * attempt to re-resolve the name after a delay.
     *
     * @param error the error reported by the resolver.  Must not be {@code null}.
     */
    public abstract void onError(Status error);

    /**
     * Receives service configuration from the resolver.  The service configuration will be
     * applied to the channel.
     *
     * @param serviceConfig the service configuration.
     */
    public void onServiceConfig(Map<String, ?> serviceConfig) {}
  }
}

3.2 LoadBalancer

LoadBalancer 负责根据 NameResolver 提供的地址列表选择一个或多个子通道 (Subchannel) 来进行连接。

package io.grpc;

import java.util.List;
import java.util.Map;

import javax.annotation.concurrent.ThreadSafe;

/**
 * Pluggable component that receives resolved addresses from {@link NameResolver} and provides
 * {@link Subchannel}s to the gRPC channel.
 *
 * <p>Implementations must be thread-safe, since they may be called from different threads.
 */
@ThreadSafe
public abstract class LoadBalancer {

  /**
   * Called when the address list is updated.
   *
   * <p>Implementations must not block in this method.
   *
   * @param addresses the addresses resolved.  Must not be {@code null}, but may be empty.
   * @param attributes extra attributes associated with the address.  Must not be {@code null}.
   * @param config the configuration of this LoadBalancer.
   */
  public abstract void handleResolvedAddresses(
      List<EquivalentAddressGroup> addresses, Attributes attributes, Config config);

  /**
   * Called when a subchannel's state has changed.
   *
   * <p>Implementations must not block in this method.
   *
   * @param subchannel the subchannel whose state has changed.
   * @param state the new state.
   */
  public abstract void handleSubchannelState(Subchannel subchannel, ConnectivityStateInfo state);

  /**
   * Shuts down the load balancer.
   *
   * <p>After this method is called, this load balancer is considered terminated, and the result
   * of future calls to any method are undefined.
   */
  public abstract void shutdown();

  /**
   * Creates a new {@link Subchannel} which represents one particular server.
   *
   * <p>Implementations must not block in this method.
   *
   * @param addressGroup addresses of the server.
   * @param attrs extra attributes to be passed to the subchannel.
   * @return the newly created {@link Subchannel}.
   */
  public abstract Subchannel newSubchannel(EquivalentAddressGroup addressGroup, Attributes attrs);

  /**
   * Selects a {@link Subchannel} for the next RPC.
   *
   * <p>The {@code params} will contain {@link io.grpc.LoadBalancer.PickSubchannelArgs} which
   * provides metadata of the RPC, such as {@link io.grpc.MethodDescriptor}.
   *
   * <p>Implementations must not block in this method.
   *
   * @param params parameters for the pick operation
   * @return the result of the pick operation.  Must not be {@code null}.
   */
  public abstract PickResult pickSubchannel(PickSubchannelArgs params);

  /**
   * A channel to a server.
   */
  public abstract static class Subchannel {

    /**
     * Requests the {@link LoadBalancer} to update the state of the Subchannel.  The LoadBalancer
     * will eventually receive a {@link #handleSubchannelState(Subchannel, ConnectivityStateInfo)}
     * callback with the updated state.
     *
     * @param requestConnection whether the Subchannel should attempt to connect.  If {@code false},
     *     then the Subchannel is only being updated because of some other event.
     */
    public abstract void requestConnection();

    /**
     * Shuts down the subchannel.  After this method is called, the subchannel is considered
     * terminated, and the result of future calls to any method are undefined.
     */
    public abstract void shutdown();

    /**
     * Returns the address group of this subchannel.
     */
    public abstract EquivalentAddressGroup getAddresses();

    /**
     * Returns extra attributes associated with the subchannel.
     */
    public abstract Attributes getAttributes();
  }

  /**
   * The result of {@link #pickSubchannel}.
   */
  public abstract static class PickResult {

    /**
     * Returns the picked subchannel.
     *
     * @return the picked subchannel, or {@code null} if no subchannel was picked.  If
     *     {@code null}, then either {@link #getStatus()} must be non-OK, or {@link #getStream()}
     *     must be non-{@code null}.
     */
    public abstract Subchannel getSubchannel();

    /**
     * Returns the status if no subchannel was picked.
     *
     * @return the status if no subchannel was picked, or {@link Status#OK} if a subchannel was
     *     picked.
     */
    public abstract Status getStatus();

    /**
     * Returns a {@link ClientStream} to be used instead of a subchannel.
     *
     * <p>It is expected that if this method returns non-{@code null}, then {@link #getStatus()}
     * must be non-OK.
     *
     * @return a {@link ClientStream} to be used instead of a subchannel.
     */
    public abstract ClientStream getStream();

    /**
     * Releases any resources held by this object.  It is called when this PickResult is no longer
     * needed.
     */
    public void release() {}

    /**
     * Creates a PickResult with a subchannel.
     */
    public static PickResult withSubchannel(Subchannel subchannel) {
      return new PickResult() {
        @Override
        public Subchannel getSubchannel() {
          return subchannel;
        }

        @Override
        public Status getStatus() {
          return Status.OK;
        }

        @Override
        public ClientStream getStream() {
          return null;
        }

        @Override
        public String toString() {
          return "PickResult{subchannel=" + subchannel + "}";
        }
      };
    }

    /**
     * Creates a PickResult with a status.
     */
    public static PickResult withError(Status status) {
      return new PickResult() {
        @Override
        public Subchannel getSubchannel() {
          return null;
        }

        @Override
        public Status getStatus() {
          return status;
        }

        @Override
        public ClientStream getStream() {
          return null;
        }

        @Override
        public String toString() {
          return "PickResult{status=" + status + "}";
        }
      };
    }
  }

  /**
   * The configuration of this LoadBalancer.
   */
  public interface Config {}

  /**
   * Arguments provided to {@link #pickSubchannel}.
   */
  public abstract static class PickSubchannelArgs {
    /**
     * Returns the headers to be used for the RPC.
     */
    public abstract Metadata getHeaders();

    /**
     * Returns the {@link MethodDescriptor} of the RPC.
     */
    public abstract MethodDescriptor<?, ?> getMethodDescriptor();

    /**
     * Returns the {@link CallOptions} of the RPC.
     */
    public abstract CallOptions getCallOptions();
  }
}

3.3 示例：实现一个简单的 gRPC 客户端负载均衡器

下面是一个简单的 gRPC 客户端负载均衡器的示例，它使用轮询策略选择服务实例。

首先，创建一个自定义的 NameResolver：

package com.example.grpc.loadbalance;

import io.grpc.Attributes;
import io.grpc.EquivalentAddressGroup;
import io.grpc.NameResolver;
import io.grpc.Status;

import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.net.URI;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Executor;

public class SimpleNameResolver extends NameResolver {

    private final URI targetUri;
    private Listener listener;

    public SimpleNameResolver(URI targetUri) {
        this.targetUri = targetUri;
    }

    @Override
    public String getServiceAuthority() {
        return targetUri.getAuthority();
    }

    @Override
    public void start(Listener listener, Executor executor) {
        this.listener = listener;
        resolve();
    }

    @Override
    public void refresh() {
        resolve();
    }

    private void resolve() {
        // 模拟从配置中心获取地址列表
        List<SocketAddress> addresses = new ArrayList<>();
        addresses.add(new InetSocketAddress("localhost", 50051));
        addresses.add(new InetSocketAddress("localhost", 50052));

        List<EquivalentAddressGroup> equivalentAddressGroups = new ArrayList<>();
        for (SocketAddress address : addresses) {
            equivalentAddressGroups.add(new EquivalentAddressGroup(address));
        }

        listener.onAddresses(equivalentAddressGroups, Attributes.EMPTY);
    }

    @Override
    public void shutdown() {
        // Nothing to do
    }
}

然后，创建一个自定义的 NameResolverProvider：

package com.example.grpc.loadbalance;

import io.grpc.NameResolver;
import io.grpc.NameResolverProvider;

import java.net.URI;

public class SimpleNameResolverProvider extends NameResolverProvider {

    @Override
    protected boolean isAvailable() {
        return true;
    }

    @Override
    protected int priority() {
        return 5; // 优先级，数值越大优先级越高
    }

    @Override
    public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
        if ("simple".equals(targetUri.getScheme())) {
            return new SimpleNameResolver(targetUri);
        }
        return null;
    }

    @Override
    public String getDefaultScheme() {
        return "simple";
    }
}

接下来，创建一个自定义的 LoadBalancer：

package com.example.grpc.loadbalance;

import io.grpc.ConnectivityStateInfo;
import io.grpc.EquivalentAddressGroup;
import io.grpc.LoadBalancer;
import io.grpc.PickResult;
import io.grpc.Status;
import io.grpc.Subchannel;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

public class RoundRobinLoadBalancer extends LoadBalancer {

    private List<Subchannel> subchannels = new ArrayList<>();
    private final AtomicInteger nextIndex = new AtomicInteger(0);

    @Override
    public void handleResolvedAddresses(List<EquivalentAddressGroup> addresses, Attributes attributes, Config config) {
        List<Subchannel> newSubchannels = new ArrayList<>();
        for (EquivalentAddressGroup addressGroup : addresses) {
            Subchannel subchannel = newSubchannel(addressGroup, Attributes.EMPTY);
            newSubchannels.add(subchannel);
            subchannel.requestConnection();
        }

        // 关闭旧的 Subchannel
        List<Subchannel> oldSubchannels = this.subchannels;
        this.subchannels = newSubchannels;
        for (Subchannel subchannel : oldSubchannels) {
            subchannel.shutdown();
        }
    }

    @Override
    public void handleSubchannelState(Subchannel subchannel, ConnectivityStateInfo state) {
        // 根据 Subchannel 的状态进行处理
    }

    @Override
    public void shutdown() {
        for (Subchannel subchannel : subchannels) {
            subchannel.shutdown();
        }
    }

    @Override
    public Subchannel newSubchannel(EquivalentAddressGroup addressGroup, Attributes attrs) {
        return Helper.createSubchannel(addressGroup, attrs);
    }

    @Override
    public PickResult pickSubchannel(PickSubchannelArgs params) {
        if (subchannels.isEmpty()) {
            return PickResult.withError(Status.UNAVAILABLE.withDescription("No subchannels available"));
        }

        int index = nextIndex.getAndIncrement() % subchannels.size();
        return PickResult.withSubchannel(subchannels.get(index));
    }
}

最后，创建一个自定义的 LoadBalancerProvider：

package com.example.grpc.loadbalance;

import io.grpc.LoadBalancer;
import io.grpc.LoadBalancerProvider;
import io.grpc.LoadBalancerRegistry;

public class RoundRobinLoadBalancerProvider extends LoadBalancerProvider {

    @Override
    public LoadBalancer newLoadBalancer(Helper helper) {
        return new RoundRobinLoadBalancer();
    }

    @Override
    public boolean isAvailable() {
        return true;
    }

    @Override
    public int getPriority() {
        return 5;
    }

    @Override
    public String getPolicyName() {
        return "round_robin";
    }

    public static void register() {
        LoadBalancerRegistry.getDefaultRegistry().register(new RoundRobinLoadBalancerProvider());
    }
}

在客户端启动时，需要注册 SimpleNameResolverProvider 和 RoundRobinLoadBalancerProvider：

public class GrpcClient {
    public static void main(String[] args) throws InterruptedException {
        SimpleNameResolverProvider.register();
        RoundRobinLoadBalancerProvider.register();

        ManagedChannel channel = ManagedChannelBuilder.forTarget("simple://localhost").defaultLoadBalancingPolicy("round_robin").usePlaintext().build();

        // 使用 channel 调用 gRPC 服务
    }
}

注意： 这个例子只是为了演示 gRPC 客户端负载均衡的扩展机制，实际应用中应该选择更合适的算法，并考虑错误处理和重试机制。

4. 自定义负载均衡策略的设计原则

在设计自定义负载均衡策略时，需要考虑以下几个原则：

• 业务相关性： 负载均衡策略应该与业务场景紧密结合，根据业务特点选择合适的算法。
• 性能： 负载均衡算法的性能应该尽可能高，避免成为系统的瓶颈。
• 可扩展性： 负载均衡策略应该易于扩展，方便添加新的算法。
• 容错性： 负载均衡策略应该能够处理服务实例的故障，保证系统的可用性。
• 可监控性： 负载均衡策略应该提供监控指标，方便观察其运行状态。

5. 常见场景下的自定义负载均衡策略

• 基于地理位置的负载均衡： 根据客户端的 IP 地址选择最近的服务实例，可以提高用户体验。
• 基于服务实例负载的负载均衡： 根据服务实例的 CPU 使用率、内存使用率等指标动态调整权重，可以避免某些实例过载。
• 基于请求内容的负载均衡： 根据请求的内容（例如用户 ID）选择服务实例，可以实现会话保持等功能。

示例：基于服务实例负载的负载均衡

假设我们希望根据服务实例的 CPU 使用率来动态调整权重。首先，需要服务实例提供 CPU 使用率的指标。然后，在负载均衡算法中，根据 CPU 使用率计算权重，并使用加权轮询或加权随机算法选择服务实例。

在 Dubbo 中，可以通过自定义 LoadBalance 接口实现，并从注册中心获取服务实例的 CPU 使用率信息。在 gRPC 中，可以通过扩展 LoadBalancer 接口实现，并使用 gRPC 的 Health Check 机制获取服务实例的健康状态和负载信息。

6. 使用元数据进行更灵活的路由控制

Dubbo 和 gRPC 都支持使用元数据（Metadata）来进行更灵活的路由控制。元数据可以附加在请求或服务实例上，负载均衡器可以根据元数据选择服务实例。

Dubbo:

可以在 Dubbo 的配置文件中配置路由规则，根据请求的元数据选择服务实例。

<dubbo:router url="condition://0.0.0.0/com.example.DemoService?category=routers&amp;dynamic=false">
    <dubbo:param key="rule" value="method = sayHello => host = 10.20.153.10"/>
</dubbo:router>

gRPC:

可以在客户端的 CallOptions 中添加元数据，然后在负载均衡器中根据元数据选择服务实例。

Metadata metadata = new Metadata();
metadata.put(Metadata.Key.of("user-id", Metadata.ASCII_STRING_MARSHALLER), "123");

CallOptions callOptions = CallOptions.DEFAULT.withOption(ClientCall.HEADER_METADATA_KEY, metadata);

// 使用 callOptions 调用 gRPC 服务

7. 总结

负载均衡是分布式系统中的关键组件，Dubbo 和 gRPC 都提供了灵活的扩展机制，允许开发者根据业务特点定制负载均衡算法。在设计自定义负载均衡策略时，需要考虑业务相关性、性能、可扩展性、容错性和可监控性等原则。通过合理地选择和配置负载均衡策略，可以提高系统的可用性、可伸缩性和性能。

8. 关键点回顾：定制负载均衡策略的要点

• Dubbo: 通过实现 LoadBalance 接口并配置 SPI 文件来扩展负载均衡策略。
• gRPC: 通过扩展 NameResolver 和 LoadBalancer 接口来实现客户端负载均衡。
• 元数据: Dubbo 和 gRPC 都支持使用元数据进行更灵活的路由控制。

希望今天的分享对大家有所帮助，谢谢！