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常见的RPC框架主要分为轻重两种。较轻的框架一般只负责通信，如rmi、webservice、restful、Thrift、gRPC等。较重的框架一般包括完整的服务发现、负载均衡策略等等如BAT三家的Dubbo、brpc、Tars之类。
框架选择时个人认为首先要考虑的是框架的历史和项目的活跃程度。一个历史悠久的活跃项目（大概至少可以保证每两到三个月有一次小版本的更新）可以保证各种bug早已暴露并修复，让我们可以更专注于我们自己的项目本身，而不是要担心究竟是我们自己的代码有问题还是框架本身就有问题。
重量级RPC框架有一个主要问题就是结构复杂，另外主语言之外的代码质量也不太容易保证。个人认为活跃的社区以及一个活跃的开源管理团队是这些重型RPC框架项目成功的必要前提条件。比如我们项目组试用过腾讯的Tars，C++同学表示没有任何问题，然后JAVA同学表示java版本有许多bug，修复bug的pull request需要两个多月才能得到merge，而官方jar包也将近两年没有更新过了。
轻量级rpc框架中，restful可以被视作标杆。由于restful基于http协议，天然被各种框架支持，而且非常灵活。restful的缺点有两方面，一是过于灵活，缺少根据协议生成服务端和客户端代码的工具，联调往往要花更多的时间；二是大部分序列化基于json或者xml，相对来讲效率不理想。和restful相比，其它很多轻量级框架都有这样或者那样的缺点，有的缺少跨语言支持（rmi），有的既繁琐又缺乏效率优势（webservice）。个人认为其中相对理想的是gRPC和Thrift。
Protobuf是一种google推出的非常流行的跨语言序列化/反序列化框架。在Protobuf2中就已经出现了用rpc定义服务的概念，但是一直缺少一种流行的rpc框架支持。当Http2推出之后，google将Http2和protobuf3结合，推出了gRPC。gRPC继承了Protobuf和Http2的优点，包括：
同时gRPC还提供了简单地服务发现和负载均衡功能。虽然这并不是gRPC框架的重点，但是开发者可以非常容易的自己扩展gRPC这些功能，实现自己的策略或应用最新的相关方面技术，而不用像重型RPC框架一样受制于框架本身是否支持。
Thrift是Facebook推出的一种RPC框架，从性能上来讲远优于gRPC。但是在实际调研时发现有一个很麻烦的问题：Thrift的客户端是线程不安全的——这意味着在Spring中无法以单例形式注入到Bean中。解决方案有三种：
相比较而言gRPC就友好多了，本身有简单而且可扩展的服务发现和负载均衡功能，底层基于Netty所以线程安全，在不需要极限压榨性能的情况下是非常好的选择。当然如果需要极限压榨性能Thrift也未必够看。
gRPC中有一个特殊的关键字stream，表示可以以流式输入或输出多个protobuf对象。注意只有异步非阻塞的客户端支持以stream形式输入，同步阻塞客户端不支持以stream形式输入。
syntax = "proto3"; //gRPC必须使用proto3
option java_multiple_files = true;
option java_package = "cn.lmh.examples.grpc.proto";
service RouteGuide {
// 输入一个坐标，返回坐标和时间(1:1)
rpc getPoint(Point) returns (LocationNote) {}
// 输入一个矩形，以stream形式返回一系列点(1:n)
rpc listPoints(Rectangle) returns (stream Point) {}
// 以stream形式输入一系列点，返回点的数量和总共花费的时间(m:1)
rpc recordRoute(stream Point) returns (RouteSummary) {}
// 以stream形式输入一系列点，以stream形式返回已输入点的数量和总共花费的时间(m:n)
rpc getPointStream(stream Point) returns (stream RouteSummary) {}
}
message Point {
int32 latitude = 1;
int32 longitude = 2;
}
message Rectangle {
Point lo = 1;
Point hi = 2;
}
message LocationNote {
Point location = 1;
int64 timestamp = 2;
}
message RouteSummary {
int32 point_count = 1;
int64 elapsed_time = 2;
}
由于protoc的gRPC插件需要自己编译，而且存在环境问题。推荐使用gradle或者maven的protobuf插件。入门示例项目使用了gradle，根目录build.gradle配置如下:
plugins {
id 'java'
id 'idea'
id 'wrapper'
}
ext {
groupId = 'cn.lmh.leviathan'
proto = [
version : "3.9.0",
"grpc" :[
version : "1.23.0"
]
]
}
allprojects{
apply plugin: 'java'
apply plugin: 'idea'
sourceCompatibility=JavaVersion.VERSION_1_8
targetCompatibility=JavaVersion.VERSION_1_8
project.group = 'cn.lmh.examples'
compileJava.options.encoding = 'UTF-8'
}
subprojects{
repositories {
mavenCentral()
mavenLocal();
};
configurations {
compile
}
dependencies {
compile "io.grpc:grpc-netty-shaded:${proto.grpc.version}"
compile "io.grpc:grpc-protobuf:${proto.grpc.version}"
compile "io.grpc:grpc-stub:${proto.grpc.version}"
testCompile group: 'junit', name: 'junit', version: '4.12'
}
}
子项目build.gradle如下：
plugins{
id 'com.google.protobuf' version '0.8.10' //引入protobuf插件
}
sourceSets{
main{
proto {
srcDir 'src/main/proto' //指定.proto文件所在的位置
}
}
}
protobuf {
generatedFilesBaseDir = "$projectDir/src" //生成文件的根目录
protoc {
artifact = "com.google.protobuf:protoc:${proto.version}" //protoc的版本
}
plugins {
grpc {
artifact = "io.grpc:protoc-gen-grpc-java:${proto.grpc.version}" //gRPC的版本
}
}
generateProtoTasks {
all()*.plugins {
grpc {
outputSubDir = "java" //grpc生成文件的子目录
}
}
}
}
我们的入门子项目名称叫做starter，配置好build.gradle之后，执行gradlew :starter:generateProto就可以在src/main/java下生成对应的文件：
无论客户端以异步非阻塞还是同步阻塞形式调用，gRPC服务端的Response都是异步形式。对于异步的Request或者Response，都需要实现gRPC的io.grpc.stub.StreamObserver接口。io.grpc.stub.StreamObserver接口有三个方法：
onNext:表示接收/发送一个对象onError:处理异常onCompleted:表示Request或Response结束当Request发送到服务端端时，会异步调用requestObserver的onNext方法，直到结束时调用requestObserver的onCompleted方法；服务端调用responseObserver的onNext把Response返回给客户端，直到调用responseObserver的onCompleted方法通知客户端Response结束。服务端代码如下：
public class RouteGuideServer {
private final int port;
private final Server server;
public RouteGuideServer(int port) throws IOException {
this.port = port;
server = ServerBuilder.forPort(port).addService(new RouteGuideService())
.build();
}
/**
* Start server.
*/
public void start() throws IOException {
server.start();
System.out.println("Server started, listening on " + port);
Runtime.getRuntime().addShutdownHook(new Thread() {
@Override
public void run() {
RouteGuideServer.this.stop();
}
});
}
/**
* Stop server
*/
public void stop() {
if (server != null) {
server.shutdown();
}
}
/**
* Await termination on the main thread since the grpc library uses daemon threads.
*/
private void blockUntilShutdown() throws InterruptedException {
if (server != null) {
server.awaitTermination();
}
}
public static void main(String[] args) throws Exception {
RouteGuideServer server = new RouteGuideServer(8980);
server.start();
server.blockUntilShutdown();
}
private static class RouteGuideService extends RouteGuideGrpc.RouteGuideImplBase {
@Override
public void getPoint(Point request, StreamObserver<LocationNote> responseObserver) {
LocationNote value = LocationNote
.newBuilder()
.setLocation(request)
.setTimestamp(System.nanoTime())
.build();
responseObserver.onNext(value);
responseObserver.onCompleted();
}
@Override
public void listPoints(Rectangle request, StreamObserver<Point> responseObserver) {
int left = Math.min(request.getLo().getLongitude(), request.getHi().getLongitude());
int right = Math.max(request.getLo().getLongitude(), request.getHi().getLongitude());
int top = Math.max(request.getLo().getLatitude(), request.getHi().getLatitude());
int bottom = Math.max(request.getLo().getLatitude(), request.getHi().getLatitude());
for (int x = left; x <= right; x++) {
for (int y = top; y >= bottom; y--) {
Point point = Point.newBuilder().setLongitude(x).setLatitude(y).build();
responseObserver.onNext(point);
}
}
responseObserver.onCompleted();
}
@Override
public StreamObserver<Point> recordRoute(StreamObserver<RouteSummary> responseObserver) {
return new StreamObserver<Point>() { //返回的是requestObserver
AtomicInteger pointCount = new AtomicInteger(0);
final long startTime = System.nanoTime();
@Override
public void onNext(Point value) {
int count = pointCount.incrementAndGet();
}
@Override
public void onError(Throwable t) {
}
@Override
public void onCompleted() {
RouteSummary result = RouteSummary.newBuilder()
.setElapsedTime(System.nanoTime() - startTime).setPointCount(pointCount.get()).build();
responseObserver.onNext(result);
responseObserver.onCompleted();
}
};
}
@Override
public StreamObserver<Point> getPointStream(StreamObserver<RouteSummary> responseObserver) {
return new StreamObserver<Point>() { //返回的是requestObserver
AtomicInteger pointCount = new AtomicInteger(0);
final long startTime = System.nanoTime();
@Override
public void onNext(Point value) {
int count = pointCount.incrementAndGet();
RouteSummary result = RouteSummary.newBuilder()
.setElapsedTime(System.nanoTime() - startTime).setPointCount(count).build();
responseObserver.onNext(result);
}
@Override
public void onError(Throwable t) {
}
@Override
public void onCompleted() {
responseObserver.onCompleted();
}
};
}
}
}
gRPC的客户端有同步阻塞客户端（blockingStub)和异步非阻塞客户端(Stub）两种。同步客户端使用比较方便，但是性能较低，而且不支持stream形式的Request;异步客户端性能较高，支持stream形式的Request，但是如果想要以同步方式调用需要额外封装。本文将主要以异步为例。
由于gRPC的异步客户端性能较高且功能更完整，所以一般都会采用异步客户端。异步客户端接收到的Response也是以io.grpc.stub.StreamObserver形式。由于客户端的调用可能是在异步进程中但更可能是在同步进程中，所以就存在一个如何把gRPC异步Response转为同步Response的问题。
一个比较常见的思路是写一个io.grpc.stub.StreamObserver实现，里面有一个内置变量保存异步Response的结果，再添加一个阻塞式的get()方法，直到Response结束才把所有结果返回。要知道Response是否结束，需要添加一个Boolean或者AtomicBoolean变量，初始化为false，调用responseObserver.onCompleted()方法时设置为true，这样就可以通过这个变量判断Response是否结束。
阻塞get()方法最常见的思路是get()写一个while循环，直到变量值改为true才退出循环并返回结果。这种方式的优点是简单直接，任何语言都可以简单实现，缺点是由于使用循环可能CPU占用较高。而对于java这种多线程比较完善的语言，另一个比较好思路是Response结束前将线程挂起，当调用responseObserver.onCompleted()方法再唤醒线程。代码如下：
public class CallableStreamObserver<T> implements StreamObserver<T> {
List<T> values = new ArrayList<T>();
boolean isCompleted = false;
Throwable t = null;
@Override
public void onNext(T value) {
this.values.add(value);
}
@Override
public void onError(Throwable t) {
this.isCompleted = true;
notifyAll();
}
@Override
public synchronized void onCompleted() {
this.isCompleted = true;
notifyAll();
}
public List<T> get() throws Throwable {
if (!this.isCompleted) {
synchronized (this) {
this.wait(60 * 1000);
}
}
if (null != t) {
throw this.t;
} else {
return this.values;
}
}
}
public class RouteGuideClient {
private final ManagedChannel channel;
private final RouteGuideGrpc.RouteGuideBlockingStub blockingStub;
private final RouteGuideGrpc.RouteGuideStub asyncStub;
public RouteGuideClient(String host, int port) {
String target = "dns:///" + host + ":" + port;
ManagedChannelBuilder<?> channelBuilder = ManagedChannelBuilder
.forTarget(target)
.usePlaintext();
channel = channelBuilder.build();
blockingStub = RouteGuideGrpc.newBlockingStub(channel);
asyncStub = RouteGuideGrpc.newStub(channel);
}
public void shutdown() throws InterruptedException {
channel.shutdown().awaitTermination(5, TimeUnit.SECONDS);
}
public LocationNote getPoint(int lo, int lt, boolean blocking) throws Throwable {
Point point = Point.newBuilder().setLongitude(lo).setLatitude(lt).build();
if(blocking) {
return blockingStub.getPoint(point);
}else{
CallableStreamObserver<LocationNote> responseObserver = new CallableStreamObserver<LocationNote>();
asyncStub.getPoint(point, responseObserver);
return responseObserver.get().get(0);
}
}
public Iterator<Point> listPoints(int left, int top, int right, int bottom, boolean blocking) throws Throwable {
Point hi = Point.newBuilder().setLongitude(left).setLatitude(top).build();
Point lo = Point.newBuilder().setLongitude(right).setLatitude(bottom).build();
Rectangle rec = Rectangle.newBuilder().setHi(hi).setLo(lo).build();
if(blocking){
return blockingStub.listPoints(rec);
}else{
CallableStreamObserver<Point> responseObserver = new CallableStreamObserver<Point>();
asyncStub.listPoints(rec, responseObserver);
return responseObserver.get().iterator();
}
}
public RouteSummary recordRoute(Collection<Point> points) throws Throwable {
CallableStreamObserver<RouteSummary> responseObserver = new CallableStreamObserver<RouteSummary>();
StreamObserver<Point> requestObserver = asyncStub.recordRoute(responseObserver);
points.stream().parallel().forEach(p -> requestObserver.onNext(p));
requestObserver.onCompleted();
return responseObserver.get().get(0);
}
public List<RouteSummary> getPointStream(Collection<Point> points) throws Throwable {
CallableStreamObserver<RouteSummary> responseObserver = new CallableStreamObserver<RouteSummary>();
StreamObserver<Point> requestObserver = asyncStub.getPointStream(responseObserver);
points.stream().parallel().forEach(p -> requestObserver.onNext(p));
requestObserver.onCompleted();
return responseObserver.get();
}
}
gRPC官方将自己分为三层组件：Stub、Channel和Transport。
整个grpc-java项目的代码比较多。从风格上来讲，封装比较多，相对于interface更喜欢使用abstract class，相对于反射更喜欢使用硬编码，而且大量使用了单线程异步调用造成调用栈断裂，与常见的java项目的编码风格有很大差别，阅读起来可能容易不习惯。
在源码层面本文将关注下面这些方面：
通过入门示例可以看到，Channel的初始化过程分三步：
io.grpc.ManagedChannelBuilder;io.grpc.ManagedChannelBuilder对象;io.grpc.ManagedChannel。forTarget方法gRPC这里设计比较繁琐，过程比较绕。forTarget方法的实际功能就是把参数target赋值给io.grpc.ManagedChannelBuilder的内部变量target。
public static ManagedChannelBuilder<?> forTarget(String target) {
return ManagedChannelProvider.provider().builderForTarget(target);
}
io.grpc.ManagedChannelProvider.provider()会返回一个io.grpc.ManagedChannelProvider实现。有哪些io.grpc.ManagedChannelProvider实现是在io.grpc.ManagedChannelProvider中以硬编码形式确定的，这里其实存在利用反射改进的空间。
private static final class HardcodedClasses implements Iterable<Class<?>> {
@Override
public Iterator<Class<?>> iterator() {
List<Class<?>> list = new ArrayList<>();
try {
list.add(Class.forName("io.grpc.okhttp.OkHttpChannelProvider"));
} catch (ClassNotFoundException ex) {
// ignore
}
try {
list.add(Class.forName("io.grpc.netty.NettyChannelProvider"));
} catch (ClassNotFoundException ex) {
// ignore
}
return list.iterator();
}
}
实际上就根据依赖的jar包不同就只有两个实现，一个netty的，一个okhttp的。如果像入门示例项目一样只配置了netty实现,那就只有netty的。io.grpc.netty.NettyChannelProvider的buildForTarget方法调用的是io.grpc.netty.NettyChannelBuilder的forTarget方法。
public NettyChannelBuilder builderForTarget(String target) {
return NettyChannelBuilder.forTarget(target);
}
而io.grpc.netty.NettyChannelBuilder继承自io.grpc.internal.AbstractManagedChannelImplBuilder，forTarget方法实际上调用了父类的构造函数。
NettyChannelBuilder(String target) {
super(target);
}
public static NettyChannelBuilder forTarget(String target) {
return new NettyChannelBuilder(target);
}
io.grpc.internal.AbstractManagedChannelImplBuilder的构造函数最终会是把参数赋值给target变量。
protected AbstractManagedChannelImplBuilder(String target) {
this.target = Preconditions.checkNotNull(target, "target");
this.directServerAddress = null;
}
build方法从前文可以看到，实际初始化的io.grpc.ManagedChannelBuilder实际上是io.grpc.netty.NettyChannelBuilder，其build方法实现在其父类io.grpc.internal.AbstractManagedChannelImplBuilder中。
public ManagedChannel build() {
return new ManagedChannelOrphanWrapper(new ManagedChannelImpl(
this,
buildTransportFactory(),
// TODO(carl-mastrangelo): Allow clients to pass this in
new ExponentialBackoffPolicy.Provider(),
SharedResourcePool.forResource(GrpcUtil.SHARED_CHANNEL_EXECUTOR),
GrpcUtil.STOPWATCH_SUPPLIER,
getEffectiveInterceptors(),
TimeProvider.SYSTEM_TIME_PROVIDER));
}
这里的io.grpc.internal.ManagedChannelOrphanWrapper和io.grpc.internal.ManagedChannelImpl其实都是io.grpc.ManagedChannel的实现。io.grpc.internal.ManagedChannelOrphanWrapper从功能上分析没有任何作用，io.grpc.internal.ManagedChannelOrphanWrapper会为io.grpc.ManagedChannel创建弱引用，并被放置到ReferenceQueue中。如果Channel是单例的，那么意义不大；如果客户端被重复创建却没有被关闭，那么ReferenceQueue中会留下相应的引用记录，可能有助于排查问题。
io.grpc.internal.ManagedChannelImpl构造方法的几个参数中，除了第一个参数是builder本身，第二个参数是用来创建Transport的Factory，第三个参数是后台连接重试策略，第四个参数是gRPC的全局线程池，第五个和第七个都是和时间相关的对象，主要用于日志中，第六个是客户端调用时的interceptor。在io.grpc.netty.NettyChannelBuilder中，buildTransportFactory方法会创建一个io.grpc.netty.NettyChannelBuilder.NettyTransportFactory。
前文的入门示例中直接写了target，只能连接单个Server。如果有多个可以提供服务的Server，那么就需要有一种方式通过单个target发现这些Server。在io.grpc.ManagedChannelBuilder中有一个nameResolverFactory方法，可以用来指定如何解析target地址，发现多个服务端。
nameResolverFactory方法这个方法的实现也在io.grpc.internal.AbstractManagedChannelImplBuilder中，如果用户有自己的io.grpc.NameResolver.Factory实现的话可以通过nameResolverFactory方法指定,gRPC就会使用用户自己的io.grpc.NameResolver.Factroy实现代替gRPC自己的默认实现，否则会使用io.grpc.NameResolverRegistry中的默认实现。
io.grpc.NameResolverRegistry会通过硬编码加载io.grpc.NameResolverProvider实现，并创建一个与之有关的io.grpc.NameResolver.Factory的实现。目前硬编码加载的io.grpc.NameResolverProvider实现只有io.grpc.internal.DnsNameResolverProvider一种。
private final NameResolver.Factory factory = new NameResolverFactory();
@GuardedBy("this")
private final LinkedHashSet<NameResolverProvider> allProviders = new LinkedHashSet<>();
private synchronized void addProvider(NameResolverProvider provider) {
checkArgument(provider.isAvailable(), "isAvailable() returned false");
allProviders.add(provider);
}
public static synchronized NameResolverRegistry getDefaultRegistry() {
if (instance == null) {
List<NameResolverProvider> providerList = ServiceProviders.loadAll(
NameResolverProvider.class,
getHardCodedClasses(),
NameResolverProvider.class.getClassLoader(),
new NameResolverPriorityAccessor());
if (providerList.isEmpty()) {
logger.warning("No NameResolverProviders found via ServiceLoader, including for DNS. This "
+ "is probably due to a broken build. If using ProGuard, check your configuration");
}
instance = new NameResolverRegistry();
for (NameResolverProvider provider : providerList) {
logger.fine("Service loader found " + provider);
if (provider.isAvailable()) {
instance.addProvider(provider);
}
}
instance.refreshProviders();
}
return instance;
}
public NameResolver.Factory asFactory() {
return factory;
}
@VisibleForTesting
static List<Class<?>> getHardCodedClasses() {
ArrayList<Class<?>> list = new ArrayList<>();
try {
list.add(Class.forName("io.grpc.internal.DnsNameResolverProvider"));
} catch (ClassNotFoundException e) {
logger.log(Level.FINE, "Unable to find DNS NameResolver", e);
}
return Collections.unmodifiableList(list);
}
private final class NameResolverFactory extends NameResolver.Factory {
@Override
@Nullable
public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
List<NameResolverProvider> providers = providers();
for (NameResolverProvider provider : providers) {
NameResolver resolver = provider.newNameResolver(targetUri, args);
if (resolver != null) {
return resolver;
}
}
return null;
}
@Override
public String getDefaultScheme() {
List<NameResolverProvider> providers = providers();
if (providers.isEmpty()) {
return "unknown";
}
return providers.get(0).getDefaultScheme();
}
}
getDefaultSchema会匹配target中的schema（如dns），如果匹配的上，就使用相应的NameResolver.Factory，返回NameResolver决定真正的服务访问地址。
io.grpc.NameResolver我们来看io.grpc.NameResolver
public abstract class NameResolver {
public abstract String getServiceAuthority();
public void start(final Listener listener) {
if (listener instanceof Listener2) {
start((Listener2) listener);
} else {
start(new Listener2() {
@Override
public void onError(Status error) {
listener.onError(error);
}
@Override
public void onResult(ResolutionResult resolutionResult) {
listener.onAddresses(resolutionResult.getAddresses(), resolutionResult.getAttributes());
}
});
}
}
public void start(Listener2 listener) {
start((Listener) listener);
}
public abstract void shutdown();
public void refresh() {}
@ThreadSafe
public interface Listener {
void onAddresses(List<EquivalentAddressGroup> servers, @ResolutionResultAttr Attributes attributes);
void onError(Status error);
}
public abstract static class Listener2 implements Listener {
@Override
public final void onAddresses(
List<EquivalentAddressGroup> servers, @ResolutionResultAttr Attributes attributes) {
onResult(
ResolutionResult.newBuilder().setAddresses(servers).setAttributes(attributes).build());
}
public abstract void onResult(ResolutionResult resolutionResult);
@Override
public abstract void onError(Status error);
}
public static final class ResolutionResult {
private final List<EquivalentAddressGroup> addresses;
@ResolutionResultAttr
private final Attributes attributes;
@Nullable
private final ConfigOrError serviceConfig;
ResolutionResult(
List<EquivalentAddressGroup> addresses,
@ResolutionResultAttr Attributes attributes,
ConfigOrError serviceConfig) {
this.addresses = Collections.unmodifiableList(new ArrayList<>(addresses));
this.attributes = checkNotNull(attributes, "attributes");
this.serviceConfig = serviceConfig;
}
public static Builder newBuilder() {
return new Builder();
}
public Builder toBuilder() {
return newBuilder()
.setAddresses(addresses)
.setAttributes(attributes)
.setServiceConfig(serviceConfig);
}
public List<EquivalentAddressGroup> getAddresses() {
return addresses;
}
@ResolutionResultAttr
public Attributes getAttributes() {
return attributes;
}
@Nullable
public ConfigOrError getServiceConfig() {
return serviceConfig;
}
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/1770")
public static final class Builder {
private List<EquivalentAddressGroup> addresses = Collections.emptyList();
private Attributes attributes = Attributes.EMPTY;
@Nullable
private ConfigOrError serviceConfig;
Builder() {}
public Builder setAddresses(List<EquivalentAddressGroup> addresses) {
this.addresses = addresses;
return this;
}
public Builder setAttributes(Attributes attributes) {
this.attributes = attributes;
return this;
}
public Builder setServiceConfig(@Nullable ConfigOrError serviceConfig) {
this.serviceConfig = serviceConfig;
return this;
}
public ResolutionResult build() {
return new ResolutionResult(addresses, attributes, serviceConfig);
}
}
}
}
在客户端首次连接服务端的时候会调用Listener2的start方法，需要更新的时候会调用refresh方法。当Listener2接收到服务端地址时，会调用onResult方法。
io.grpc.internal.DnsNameResolver由于gRPC支持长连接，所以如果直连的话只会访问一个域名下的一台服务器，即首次连接时通过DNS返回IP地址。io.grpc.internal.DnsNameResolverProvider是对io.grpc.internal.DnsNameResolver的简单封装，只支持以dns:///开头的地址。io.grpc.internal.DnsNameResolver会根据target获取该host下所有关联的IP，即通过DNS解析出所有的服务端IP地址。
public final class DnsNameResolverProvider extends NameResolverProvider {
private static final String SCHEME = "dns";
@Override
public DnsNameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
if (SCHEME.equals(targetUri.getScheme())) {
String targetPath = Preconditions.checkNotNull(targetUri.getPath(), "targetPath");
Preconditions.checkArgument(targetPath.startsWith("/"),
"the path component (%s) of the target (%s) must start with '/'", targetPath, targetUri);
String name = targetPath.substring(1);
return new DnsNameResolver(
targetUri.getAuthority(),
name,
args,
GrpcUtil.SHARED_CHANNEL_EXECUTOR,
Stopwatch.createUnstarted(),
InternalServiceProviders.isAndroid(getClass().getClassLoader()));
} else {
return null;
}
}
@Override
public String getDefaultScheme() {
return SCHEME;
}
@Override
protected boolean isAvailable() {
return true;
}
@Override
protected int priority() {
return 5;
}
}
可以看到io.grpc.internal.DnsNameResolver中的start和refresh方法都调用的是resolve方法，而resolve方法是执行了一个继承自Runnable的Resolve接口。
在有代理的情况下，Resolve的resolveInternal会根据代理返回的ProxiedSocketAddress创建EquivalentAddressGroup作为服务端列表返回，并设置空config；否则会调用resolveAll方法获取服务端列表，并调用parseServiceConfig方法设置config。resolveAll方法返回的ResolutionResults有三个变量addresses、txtRecords和balancerAddresses。
@VisibleForTesting
static ResolutionResults resolveAll(
AddressResolver addressResolver,
@Nullable ResourceResolver resourceResolver,
boolean requestSrvRecords,
boolean requestTxtRecords,
String name) {
List<? extends InetAddress> addresses = Collections.emptyList();
Exception addressesException = null;
List<EquivalentAddressGroup> balancerAddresses = Collections.emptyList();
Exception balancerAddressesException = null;
List<String> txtRecords = Collections.emptyList();
Exception txtRecordsException = null;
try {
addresses = addressResolver.resolveAddress(name);
} catch (Exception e) {
addressesException = e;
}
if (resourceResolver != null) {
if (requestSrvRecords) {
try {
balancerAddresses =
resourceResolver.resolveSrv(addressResolver, GRPCLB_NAME_PREFIX + name);
} catch (Exception e) {
balancerAddressesException = e;
}
}
if (requestTxtRecords) {
boolean balancerLookupFailedOrNotAttempted =
!requestSrvRecords || balancerAddressesException != null;
boolean dontResolveTxt =
(addressesException != null) && balancerLookupFailedOrNotAttempted;
if (!dontResolveTxt) {
try {
txtRecords = resourceResolver.resolveTxt(SERVICE_CONFIG_NAME_PREFIX + name);
} catch (Exception e) {
txtRecordsException = e;
}
}
}
}
try {
if (addressesException != null
&& (balancerAddressesException != null || balancerAddresses.isEmpty())) {
Throwables.throwIfUnchecked(addressesException);
throw new RuntimeException(addressesException);
}
} finally {
if (addressesException != null) {
logger.log(Level.FINE, "Address resolution failure", addressesException);
}
if (balancerAddressesException != null) {
logger.log(Level.FINE, "Balancer resolution failure", balancerAddressesException);
}
if (txtRecordsException != null) {
logger.log(Level.FINE, "ServiceConfig resolution failure", txtRecordsException);
}
}
return new ResolutionResults(addresses, txtRecords, balancerAddresses);
}
addressResolver的resolveAddress方法实际是调用JDK的java.net.InetAddress的getAllByName方法，即根据host通过DNS返回一系列服务端列表。resourceResolver根据LDAP协议获取指定命名空间下的服务端列表地址。txtRecords和balancerAddresses是和LDAP相关的参数，方法入参requestSrvRecords和requestTxtRecords的默认值都是false。由于LDAP不是特别常用，这里就不深入展开了。
NameResolverListener的onResult
当NameResolverListener获取解析结果后会调用onResult方法，进而会调用io.grpc.LoadBalancer的handleResolvedAddresses方法。
io.grpc.ManagedChannel初始化的时候可以通过defaultLoadBalancingPolicy方法指定负载均衡策略，实际是根据defaultLoadBalancingPolicy创建了一个io.grpc.internal.AutoConfiguredLoadBalancerFactory对象。io.grpc.internal.AutoConfiguredLoadBalancerFactory则通过io.grpc.LoadBalancerRegistry获取对应名称的负载均衡策略。io.grpc.LoadBalancerProvider的getPolicyName方法指定负载均衡策略名称，newLoadBalancer返回负载均衡io.grpc.LoadBalancer的具体实现。如果想要添加自定义负载均衡策略，需要调用io.grpc.LoadBalancerRegistry的registry方法，并自己实现io.grpc.LoadBalancerProvider和io.grpc.LoadBalancer，并指定负载均衡策略名称即可。
io.grpc.LoadBalancer.SubchannelPickerio.grpc.LoadBalancer的核心逻辑实际在SubchannelPicker中。pickSubchannel方法会返回的PickResult中包含真正可用的subchannel，用来进行后续的数据传输。
public abstract static class SubchannelPicker {
/**
* Make a balancing decision for a new RPC.
*
* @param args the pick arguments
* @since 1.3.0
*/
public abstract PickResult pickSubchannel(PickSubchannelArgs args);
}
gRPC默认提供了两种负载均衡实现策略：prick_first和round_robin。前者总会使用第一个可用的服务端，后者则是简单轮询。
handleResolvedAddresses当服务端列表更新时，会调用io.grpc.LoadBalancer的handleResolvedAddresses方法更新可用的subchannel。
@Override
public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
List<EquivalentAddressGroup> servers = resolvedAddresses.getAddresses();
if (subchannel == null) {
final Subchannel subchannel = helper.createSubchannel(
CreateSubchannelArgs.newBuilder()
.setAddresses(servers)
.build());
subchannel.start(new SubchannelStateListener() {
@Override
public void onSubchannelState(ConnectivityStateInfo stateInfo) {
processSubchannelState(subchannel, stateInfo);
}
});
this.subchannel = subchannel;
helper.updateBalancingState(CONNECTING, new Picker(PickResult.withSubchannel(subchannel)));
subchannel.requestConnection();
} else {
subchannel.updateAddresses(servers);
}
}
如果是首次调用(subchannel == null) 会创建subchannel，其实现是io.grpc.internal.ManagedChannelImpl.SubchannelImpl，创建的过程中会创建io.grpc.internal.InternalSubchannel。然后调用io.grpc.internal.ManagedChannelImpl的updateBalancingState方法，把subchannelPicker更新为实现Picker，然后开启subchannel的连接。
在开启subchannel的连接过程中，会调用io.grpc.internal.InternalSubchannel的obtainActiveTransport方法。
这里的transportFactory就是上面提到io.grpc.ManagedChannelBuilder调用build初始化时调用buildTransportFactory方法返回的，依赖于Transport层的具体实现。在netty实现中，返回的是io.grpc.netty.NettyClientTransport。
gRPC客户端发起Request时，stub会调用ClientCalls的startCall方法，最终会调用io.grpc.internal.ManagedChannelImpl.ChannelTransportProvider的get方法获取io.grc.internal.ClientTransport。
public ClientTransport get(PickSubchannelArgs args) {
SubchannelPicker pickerCopy = subchannelPicker;
if (shutdown.get()) {
return delayedTransport;
}
if (pickerCopy == null) {
final class ExitIdleModeForTransport implements Runnable {
@Override
public void run() {
exitIdleMode();
}
}
syncContext.execute(new ExitIdleModeForTransport());
return delayedTransport;
}
PickResult pickResult = pickerCopy.pickSubchannel(args);
ClientTransport transport = GrpcUtil.getTransportFromPickResult(
pickResult, args.getCallOptions().isWaitForReady());
if (transport != null) {
return transport;
}
return delayedTransport;
}
如果subchannelPicker存在，会使用subchannelPicker进行选择；如果是首次访问服务端时subchannel肯定不存在，会使用syncContext异步执行exitIdleMode方法初始化。syncContext是一个单线程执行队列，可以保证先提交的任务先执行。delayedTransport的执行也依赖于syncContext，这就保证了delayedTransport中的方法执行一定会在exitIdleMode方法之后。
exidIdleMode方法exitIdleMode方法会初始化NameResolver和LoadBalancer，并会启动NameResolverListener。当解析完成后会调用NameResolverListener的onResult方法，进而调用LoadBalancer的handleResolvedAddresses方法创建subchannelPicker、创建并连接subchannel。
@VisibleForTesting
void exitIdleMode() {
syncContext.throwIfNotInThisSynchronizationContext();
if (shutdown.get() || panicMode) {
return;
}
if (inUseStateAggregator.isInUse()) {
cancelIdleTimer(false);
} else {
rescheduleIdleTimer();
}
if (lbHelper != null) {
return;
}
channelLogger.log(ChannelLogLevel.INFO, "Exiting idle mode");
LbHelperImpl lbHelper = new LbHelperImpl();
lbHelper.lb = loadBalancerFactory.newLoadBalancer(lbHelper);
this.lbHelper = lbHelper;
NameResolverListener listener = new NameResolverListener(lbHelper, nameResolver);
nameResolver.start(listener);
nameResolverStarted = true;
}
发送Request时会调用ConnectionClientTransport的newStream方法返回一个io.grpc.internal.ClientStream对象,而首次调用会通过delayedTransport延迟调用newStream方法。
netty实现会返回一个io.grpc.netty.shaded.io.grpc.netty.NettyClientStream对象。io.grpc.internal.ClientStream下有两个子类,TransportState负责处理传输状态，Sink负责写入数据。
在进行一系列http2相关设置后，会调用io.grpc.internal.ClientStream的start方法，为TransportState设置监听并通过Sink写入Header。
@Override
public final void start(ClientStreamListener listener) {
transportState().setListener(listener);
if (!useGet) {
abstractClientStreamSink().writeHeaders(headers, null);
headers = null;
}
}
初始化结束后，调用requestObserver的onNext方法会调用io.grpc.internal.ClientCallImpl的sendMessage方法，将protobuf对象转换成InputStream，并作为参数调用io.grpc.internal.ClientStream的writeMessage方法，进而调用io.grpc.internal.MessageFramer的writePayload方法，最终调用writeToOutputStream方法将内容写入Http的OutputStream。如果是参数是stream形式会继续调用flush。
调用requestObserver的onCompleted方法会调用io.grpc.internal.ClientCallImpl的halfClose方法，进而会调用io.grpc.internal.MessageFramer的endOfMessages，flush并结束发送消息。
客户端接受到Response会调用ClientStreamListener的messagesAvailable方法，并通过同步线程池最终调用StreamObserver的onNext方法接收数据。
当返回结束时会调用TransportState的transportReportStatus方法关闭请求，进而调用ClientStreamListener的closed方法关闭监听，进而调用StreamObserver的onClose方法。
gRPC发送的请求发送方法是POST，路径是/{methodName}，content-type为content-type = application/grpc+proto。
HEADERS (flags = END_HEADERS)
:method = POST
:scheme = http
:path = /RouteGuide/getPoint
grpc-timeout = 1S
content-type = application/grpc+proto
grpc-encoding = gzip
DATA (flags = END_STREAM)
<Length-Prefixed Message>
HEADERS (flags = END_HEADERS)
:status = 200
grpc-encoding = gzip
content-type = application/grpc+proto
DATA
<Length-Prefixed Message>
HEADERS (flags = END_STREAM, END_HEADERS)
grpc-status = 0 # OK
trace-proto-bin = jher831yy13JHy3hc
NameResolver.Factory实现public class CuratorNameResolver extends NameResolver {
CuratorFramework curatorFramework;
String basePath;
String serviceAuthority;
private Listener2 listener;
public CuratorNameResolver(CuratorFramework curatorFramework, String basePath, String serviceAuthority) {
this.curatorFramework = curatorFramework;
this.basePath = basePath;
this.serviceAuthority = serviceAuthority;
}
@Override
public void start(Listener2 listener) {
this.curatorFramework.start();
this.listener = listener;
refresh();
}
@Override
public void refresh() {
List<EquivalentAddressGroup> servers = new ArrayList<>();
try {
List<EquivalentAddressGroup> addresses = curatorFramework.getChildren()
.forPath(basePath)
.stream().map(address ->{
try {
URI uri = new URI("http://" + address);
return new EquivalentAddressGroup(
new InetSocketAddress(uri.getHost(), uri.getPort()));
}catch (Exception e){
listener.onError(Status.INTERNAL);
return null;
}
}).collect(Collectors.toList());
listener.onResult(ResolutionResult.newBuilder().setAddresses(addresses).build());
} catch (Exception e) {
listener.onError(Status.INTERNAL);
}
}
@Override
public String getServiceAuthority() {
return this.serviceAuthority;
}
@Override
public void shutdown() {
this.curatorFramework.close();
}
public static class Factory extends NameResolver.Factory{
@Override
public NameResolver newNameResolver(URI targetUri, Args args) {
String address = targetUri.getHost() + ":" + targetUri.getPort();
String authority = null == targetUri.getAuthority() ? address : targetUri.getAuthority();
CuratorFramework curator = CuratorFrameworkFactory.builder()
.connectString(address)
.retryPolicy(new ExponentialBackoffRetry(1000, 5))
.connectionTimeoutMs(1000)
.sessionTimeoutMs(60000)
.build();
return new CuratorNameResolver(curator, targetUri.getPath(), authority);
}
@Override
public String getDefaultScheme() {
return "zookeeper";
}
}
}
public class RandomLoadBalancer extends LoadBalancer{
LoadBalancer.Helper helper;
private final Map<EquivalentAddressGroup, Subchannel> subchannels =
new HashMap<>();
static final Attributes.Key<Ref<ConnectivityStateInfo>> STATE_INFO =
Attributes.Key.create("state-info");
public RandomLoadBalancer(LoadBalancer.Helper helper) {
this.helper = helper;
}
@Override
public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
List<EquivalentAddressGroup> servers = resolvedAddresses.getAddresses();
for(EquivalentAddressGroup server : servers){
List<EquivalentAddressGroup> serverSingletonListt = Collections.singletonList(server);
Subchannel exists = subchannels.getOrDefault(server, null);
if(null != exists){
exists.updateAddresses(serverSingletonListt);
continue;
}
Attributes.Builder subchannelAttrs = Attributes.newBuilder()
.set(STATE_INFO,
new Ref<>(ConnectivityStateInfo.forNonError(IDLE)));
final Subchannel subchannel = helper.createSubchannel(CreateSubchannelArgs.newBuilder()
.setAddresses(serverSingletonListt)
.setAttributes(subchannelAttrs.build())
.build());
subchannels.put(server, subchannel);
subchannel.start(new SubchannelStateListener() {
@Override
public void onSubchannelState(ConnectivityStateInfo state) {
for(Map.Entry<EquivalentAddressGroup, Subchannel> entry : subchannels.entrySet()){
if(subchannel == entry.getValue()){
if (state.getState() == SHUTDOWN) {
subchannels.remove(entry.getKey());
}
if (state.getState() == IDLE) {
subchannel.requestConnection();
}
subchannel.getAttributes().get(STATE_INFO).value = state;
updateBalancingState();
return;
}
}
}
});
subchannel.requestConnection();
}
updateBalancingState();
}
@Override
public void handleNameResolutionError(Status error) {
shutdown();
helper.updateBalancingState(TRANSIENT_FAILURE, new SubchannelPicker() {
@Override
public PickResult pickSubchannel(PickSubchannelArgs args) {
return PickResult.withError(error);
}
});
}
private void updateBalancingState(){
boolean ready = true;
for(Subchannel subchannel : this.subchannels.values()){
if(subchannel.getAttributes().get(STATE_INFO).value.getState() != READY){
helper.updateBalancingState(CONNECTING, new RandomSubchannelPick(subchannels.values()));
return;
}
}
helper.updateBalancingState(ConnectivityState.READY, new RandomSubchannelPick(subchannels.values()));
}
@Override
public void shutdown() {
for(Iterator<Map.Entry<EquivalentAddressGroup, Subchannel>> itr = subchannels.entrySet().iterator(); itr.hasNext();){
Map.Entry<EquivalentAddressGroup, Subchannel> e = itr.next();
e.getValue().shutdown();
itr.remove();
}
}
class RandomSubchannelPick extends SubchannelPicker{
Subchannel[] subchannels;
Random random = new Random(System.currentTimeMillis());
public RandomSubchannelPick(Collection<Subchannel> subchannels) {
this.subchannels = subchannels.stream().toArray(Subchannel[]::new);
}
@Override
public PickResult pickSubchannel(PickSubchannelArgs args) {
int idx = random.nextInt(subchannels.length);
return PickResult.withSubchannel(subchannels[idx]);
}
}
public static class Provider extends LoadBalancerProvider{
@Override
public boolean isAvailable() {
return true;
}
@Override
public int getPriority() {
return 100;
}
@Override
public String getPolicyName() {
return "random";
}
@Override
public LoadBalancer newLoadBalancer(LoadBalancer.Helper helper) {
return new RandomLoadBalancer(helper);
}
}
static final class Ref<T> {
T value;
Ref(T value) {
this.value = value;
}
}
}
服务端需要把自己的服务地址注册到zookeeper。
private final int port;
private final Server server;
private String registryPath;
private String address;
CuratorFramework curator = CuratorFrameworkFactory.builder()
.connectString("127.0.0.1:2181")
.retryPolicy(new ExponentialBackoffRetry(1000, 5))
.connectionTimeoutMs(1000)
.sessionTimeoutMs(60000)
.build();;
public GreetingServer(int port, String registryPath) throws IOException {
this.port = port;
server = ServerBuilder.forPort(port).addService(new GreetingService())
.build();
this.registryPath = registryPath;
this.address = "localhost:" + port; //本机网卡不能正确显示地址，直接写死localhost
}
/**
* Start server.
*/
public void start() throws Exception {
this.curator.start();
server.start();;
this.curator.create()
.creatingParentContainersIfNeeded()
.withMode(CreateMode.EPHEMERAL)
.forPath(registryPath + "/" + address, ("http://" + address).getBytes());
System.out.println("Server started, listening on " + address);
Runtime.getRuntime().addShutdownHook(new Thread() {
@Override
public void run() {
GreetingServer.this.stop();
}
});
}
客户端需要注册自定义的NameResolverFactory和LoadBalancer。
public GreetingClient(String host, int port, String path) {
String target = "zookeeper://" + host + ":" + port + path;
CuratorNameResolver.Factory factory = new CuratorNameResolver.Factory();
LoadBalancerRegistry.getDefaultRegistry().register(new RandomLoadBalancer.Provider());
ManagedChannelBuilder<?> channelBuilder = ManagedChannelBuilder
.forTarget(target)
.nameResolverFactory(factory)
.defaultLoadBalancingPolicy("random")
.usePlaintext();
channel = channelBuilder.build();
blockingStub = GreetingGrpc.newBlockingStub(channel);
}
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