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前言
前几周因为看问题简单熟悉了Dubbo的服务暴露过程，虽然公司不在使用，但是觉得还是有必要多了解了解。
这两天有时间，就想着了解一下Dubbo的服务消费过程。之前对RPC框架也有相关了解，深知作为一款优秀的RPC框架，服务的消费过程必然存在着很多的设计。如远程调用代理层，路由层，容错重试逻辑等等。下面就通过阅读Dubbo的源码来一点点体会吧。
服务调用原理
案例
从最简单的demo入手
private static void runWithRefer() {
ReferenceConfig<DemoService> reference = new ReferenceConfig<>();
reference.setApplication(new ApplicationConfig("dubbo-demo-api-consumer"));
reference.setRegistry(new RegistryConfig("zookeeper://127.0.0.1:2181"));
reference.setInterface(DemoService.class);
DemoService service = reference.get();
String message = service.sayHello("dubbo");
System.out.println(message);
}
可以看到调用服务会指定注册中心，以及需要消费的接口。比较关键的代码就是reference.get(); 这里其实会返回一个代理对象，在内部实现远程调用。
代理对象的创建
reference.get()方法最终会返回ref。
/**
* The interface proxy reference
*/
private transient volatile T ref;
public synchronized T get() {
if (destroyed) {
throw new IllegalStateException("The invoker of ReferenceConfig(" + url + ") has already destroyed!");
}
if (ref == null) {
init();
}
return ref;
}
在init方法中会去创建ref。
ref = createProxy(map);
private T createProxy(Map<String, String> map) {
//省略代码...
if (urls.size() == 1) {
invoker = REF_PROTOCOL.refer(interfaceClass, urls.get(0));
} else {
}
// create service proxy
return (T) PROXY_FACTORY.getProxy(invoker, ProtocolUtils.isGeneric(generic));
}
通过上面代码我们需要了解invoker是如何生成的，先来看看REF_PROTOCOL的实现类是哪一个吧
/**
* The {@link Protocol} implementation with adaptive functionality,it will be different in different scenarios.
* A particular {@link Protocol} implementation is determined by the protocol attribute in the {@link URL}.
* For example:
*
* <li>when the url is registry://224.5.6.7:1234/org.apache.dubbo.registry.RegistryService?application=dubbo-sample,
* then the protocol is <b>RegistryProtocol</b></li>
*
* <li>when the url is dubbo://224.5.6.7:1234/org.apache.dubbo.config.api.DemoService?application=dubbo-sample, then
* the protocol is <b>DubboProtocol</b></li>
* <p>
* Actually，when the {@link ExtensionLoader} init the {@link Protocol} instants,it will automatically wraps two
* layers, and eventually will get a <b>ProtocolFilterWrapper</b> or <b>ProtocolListenerWrapper</b>
*/
private static final Protocol REF_PROTOCOL = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
通过上面的注释可以了解到会调用RegistryProtocol中的refer方法。最终返回MigrationInvoker。然后会创建代理
/**
* JavassistRpcProxyFactory
*/
public class JavassistProxyFactory extends AbstractProxyFactory {
@Override
@SuppressWarnings("unchecked")
public <T> T getProxy(Invoker<T> invoker, Class<?>[] interfaces) {
return (T) Proxy.getProxy(interfaces).newInstance(new InvokerInvocationHandler(invoker));
}
@Override
public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
// TODO Wrapper cannot handle this scenario correctly: the classname contains '$'
final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);
return new AbstractProxyInvoker<T>(proxy, type, url) {
@Override
protected Object doInvoke(T proxy, String methodName,
Class<?>[] parameterTypes,
Object[] arguments) throws Throwable {
return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);
}
};
}
}
有了这个代理对象就可以像调用本地方法一样去进行远程调用了。其本质是调用了InvokerInvocationHandler中的invoke方法。
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (method.getDeclaringClass() == Object.class) {
return method.invoke(invoker, args);
}
String methodName = method.getName();
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 0) {
if ("toString".equals(methodName)) {
return invoker.toString();
} else if ("$destroy".equals(methodName)) {
invoker.destroy();
return null;
} else if ("hashCode".equals(methodName)) {
return invoker.hashCode();
}
} else if (parameterTypes.length == 1 && "equals".equals(methodName)) {
return invoker.equals(args[0]);
}
RpcInvocation rpcInvocation = new RpcInvocation(method, invoker.getInterface().getName(), protocolServiceKey, args);
String serviceKey = invoker.getUrl().getServiceKey();
rpcInvocation.setTargetServiceUniqueName(serviceKey);
// invoker.getUrl() returns consumer url.
RpcContext.setRpcContext(invoker.getUrl());
if (consumerModel != null) {
rpcInvocation.put(Constants.CONSUMER_MODEL, consumerModel);
rpcInvocation.put(Constants.METHOD_MODEL, consumerModel.getMethodModel(method));
}
return invoker.invoke(rpcInvocation).recreate();
}
到此，会发现最终还是调用了Invoker中的invoke方法。在Invoker中做了容错，路由，远程调用。
路由+容错处理
到此我们需要重点关注代理类中的Invoker。MigrationInvoker的invoke方法：
public Result invoke(Invocation invocation) throws RpcException {
if (!checkInvokerAvailable(serviceDiscoveryInvoker)) {
if (logger.isDebugEnabled()) {
logger.debug("Using interface addresses to handle invocation, interface " + type.getName() + ", total address size " + (invoker.getDirectory().getAllInvokers() == null ? "is null" : invoker.getDirectory().getAllInvokers().size()));
}
return invoker.invoke(invocation);
}
if (!checkInvokerAvailable(invoker)) {
if (logger.isDebugEnabled()) {
logger.debug("Using instance addresses to handle invocation, interface " + type.getName() + ", total address size " + (serviceDiscoveryInvoker.getDirectory().getAllInvokers() == null ? " is null " : serviceDiscoveryInvoker.getDirectory().getAllInvokers().size()));
}
return serviceDiscoveryInvoker.invoke(invocation);
}
return currentAvailableInvoker.invoke(invocation);
}
这里的invoker是在如下方法中进行的初始化
public synchronized void refreshInterfaceInvoker() {
clearListener(invoker);
if (needRefresh(invoker)) {
// FIXME invoker.destroy();
if (logger.isDebugEnabled()) {
logger.debug("Re-subscribing interface addresses for interface " + type.getName());
}
invoker = registryProtocol.getInvoker(cluster, registry, type, url);
if (migrationMultiRegistry) {
setListener(serviceDiscoveryInvoker, () -> {
this.setAddressChanged();
});
}
}
}
最终获取到默认的invoker是FailoverClusterInvoker。ClusterInvoker有多种实现，默认是FailoverClusterInvoker。(这种策略简单来说就是当调用一个服务提供者失败后，会继续选择其他节点的服务提供者进行调用，When invoke fails, log the initial error and retry other invokers)。
public class FailoverCluster extends AbstractCluster {
public static final String NAME = "failover";
@Override
public <T> AbstractClusterInvoker<T> doJoin(Directory<T> directory) throws RpcException {
return new FailoverClusterInvoker<>(directory);
}
}
上面的invoker.invoke(invocation);方法最终会先调用到父类AbstractClusterInvoker的invoke方法，然后调用到实现类FailoverClusterInvoker中的invoke方法中。
AbstractClusterInvoker.invoke
@Override
public Result invoke(final Invocation invocation) throws RpcException {
checkWhetherDestroyed();
// binding attachments into invocation.
Map<String, Object> contextAttachments = RpcContext.getContext().getObjectAttachments();
if (contextAttachments != null && contextAttachments.size() != 0) {
((RpcInvocation) invocation).addObjectAttachments(contextAttachments);
}
List<Invoker<T>> invokers = list(invocation);
LoadBalance loadbalance = initLoadBalance(invokers, invocation);
RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
return doInvoke(invocation, invokers, loadbalance);
}
可以看到在父类中先获取到了所有的invokers，这里的invokers代表了已经建立远程连接的服务提供者列表。并且初始化了LocaBanlace，用于根据不同策略选择服务地址进行调用，如轮训，随机。这里有多种实现，基于SPI进行配置。
protected LoadBalance initLoadBalance(List<Invoker<T>> invokers, Invocation invocation) {
if (CollectionUtils.isNotEmpty(invokers)) {
return ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(invokers.get(0).getUrl()
.getMethodParameter(RpcUtils.getMethodName(invocation), LOADBALANCE_KEY, DEFAULT_LOADBALANCE));
} else {
return ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(DEFAULT_LOADBALANCE);
}
}
接着来看看FailoverClusterInvoker.invoker的实现(When invoke fails, log the initial error and retry other invokers)
@Override
@SuppressWarnings({"unchecked", "rawtypes"})
public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
List<Invoker<T>> copyInvokers = invokers;
checkInvokers(copyInvokers, invocation);
String methodName = RpcUtils.getMethodName(invocation);
int len = calculateInvokeTimes(methodName);
// retry loop.
RpcException le = null; // last exception.
List<Invoker<T>> invoked = new ArrayList<Invoker<T>>(copyInvokers.size()); // invoked invokers.
Set<String> providers = new HashSet<String>(len);
for (int i = 0; i < len; i++) {
//Reselect before retry to avoid a change of candidate `invokers`.
//NOTE: if `invokers` changed, then `invoked` also lose accuracy.
if (i > 0) {
checkWhetherDestroyed();
copyInvokers = list(invocation);
// check again
checkInvokers(copyInvokers, invocation);
}
Invoker<T> invoker = select(loadbalance, invocation, copyInvokers, invoked);
invoked.add(invoker);
RpcContext.getContext().setInvokers((List) invoked);
try {
Result result = invoker.invoke(invocation);
if (le != null && logger.isWarnEnabled()) {
logger.warn("Although retry the method " + methodName
+ " in the service " + getInterface().getName()
+ " was successful by the provider " + invoker.getUrl().getAddress()
+ ", but there have been failed providers " + providers
+ " (" + providers.size() + "/" + copyInvokers.size()
+ ") from the registry " + directory.getUrl().getAddress()
+ " on the consumer " + NetUtils.getLocalHost()
+ " using the dubbo version " + Version.getVersion() + ". Last error is: "
+ le.getMessage(), le);
}
return result;
} catch (RpcException e) {
if (e.isBiz()) { // biz exception.
throw e;
}
le = e;
} catch (Throwable e) {
le = new RpcException(e.getMessage(), e);
} finally {
providers.add(invoker.getUrl().getAddress());
}
}
throw new RpcException(le.getCode(), "Failed to invoke the method "
+ methodName + " in the service " + getInterface().getName()
+ ". Tried " + len + " times of the providers " + providers
+ " (" + providers.size() + "/" + copyInvokers.size()
+ ") from the registry " + directory.getUrl().getAddress()
+ " on the consumer " + NetUtils.getLocalHost() + " using the dubbo version "
+ Version.getVersion() + ". Last error is: "
+ le.getMessage(), le.getCause() != null ? le.getCause() : le);
}
可以看到其实现是在一个循环中来进行的，首先基于loadbalance进行select，选择一个invoker。这里的invoker其实对应的是DubboInvoker(已经和对应的服务提供者建立好了netty 连接)。然后调用其invoke方法。如果失败了，则会继续循环，重新根据loadbalance进行select获取另一个DubboInvoker(另一个连接)。
DubboInvoker远程调用
最后我们来看看其做了什么吧
@Override
protected Result doInvoke(final Invocation invocation) throws Throwable {
RpcInvocation inv = (RpcInvocation) invocation;
final String methodName = RpcUtils.getMethodName(invocation);
inv.setAttachment(PATH_KEY, getUrl().getPath());
inv.setAttachment(VERSION_KEY, version);
ExchangeClient currentClient;
if (clients.length == 1) {
currentClient = clients[0];
} else {
currentClient = clients[index.getAndIncrement() % clients.length];
}
try {
boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
int timeout = calculateTimeout(invocation, methodName);
invocation.put(TIMEOUT_KEY, timeout);
if (isOneway) {
boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
currentClient.send(inv, isSent);
return AsyncRpcResult.newDefaultAsyncResult(invocation);
} else {
ExecutorService executor = getCallbackExecutor(getUrl(), inv);
CompletableFuture<AppResponse> appResponseFuture =
currentClient.request(inv, timeout, executor).thenApply(obj -> (AppResponse) obj);
// save for 2.6.x compatibility, for example, TraceFilter in Zipkin uses com.alibaba.xxx.FutureAdapter
FutureContext.getContext().setCompatibleFuture(appResponseFuture);
AsyncRpcResult result = new AsyncRpcResult(appResponseFuture, inv);
result.setExecutor(executor);
return result;
}
} catch (TimeoutException e) {
throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
} catch (RemotingException e) {
throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
}
}
这里就是向服务端发送请求，进行远程调用。服务端收到请求后，就会找到对应的服务实现类进行调用了。
