gRPC 使用自定义的 NameResolver
在使用注册中心时，gRPC 并未提供注册中心的服务发现，需要自己实现 NameResolverProvider 和 NameResolver
NameResolver 里面重写了 start 和 refresh 方法，这两个方法都调用一个 resolve 方法做服务发现；
resovle 方法内部通过服务名从注册中心拉取服务实例列表，然后调用 Listener 的 onResult方法，将实例列表传递给 LoadBalancer，完成服务解析
在服务运行期间，因为实例可能会发生变化，所以可以通过定时执行触发服务解析；如果注册中心支持，也可以通过回调触发
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| public class CustomNameResolver extends NameResolver {
private final ScheduledExecutorService executorService = new ScheduledThreadPoolExecutor(10);
private final String authority;
private Listener2 listener;
public CustomNameResolver(String authority) {
this.authority = authority;
}
@Override
public String getServiceAuthority() {
return this.authority;
}
@Override
public void shutdown() {
this.executorService.shutdown();
}
@Override
public void start(Listener2 listener) {
this.listener = listener;
this.resolve();
}
@Override
public void refresh() {
this.resolve();
}
private void resolve() {
executorService.scheduleAtFixedRate(() -> {
List<InetSocketAddress> addressList = getAddressList(this.authority);
if (addressList == null || addressList.size() == 0) {
return;
}
List<SocketAddress> socketAddressList = addressList.stream()
.map(this::toSocketAddress)
.collect(Collectors.toList());
EquivalentAddressGroup equivalentAddressGroup = new EquivalentAddressGroup(socketAddressList);
Attributes.Key<String> key = Attributes.Key.create("CustomKey");
String value = "CustomValue";
Attributes attributes = Attributes.newBuilder().set(key, value).build();
ConfigOrError configOrError = ConfigOrError.fromError(Status.NOT_FOUND);
ResolutionResult resolutionResult = ResolutionResult.newBuilder()
.setAddresses(Arrays.asList(equivalentAddressGroup))
.setAttributes(attributes)
.setServiceConfig(configOrError)
.build();
this.listener.onResult(resolutionResult);
}, 0, 5, TimeUnit.SECONDS);
}
private SocketAddress toSocketAddress(InetSocketAddress address) {
return new InetSocketAddress(address.getHostName(), address.getPort());
}
private List<InetSocketAddress> getAddressList(String authority) {
InetSocketAddress inetSocketAddress = new InetSocketAddress("localhost", 1234);
InetSocketAddress inetSocketAddress2 = new InetSocketAddress("127.0.0.1", 1234);
return Arrays.asList(inetSocketAddress, inetSocketAddress2);
}
}
|
NameResolverProvider 主要用于注册 NameResolver，可以设置默认的协议，是否可用，优先级等
优先级有效值是 0-10，gRPC 默认的 DnsNameResolver 优先级是5，所以自定义的优先级要大于5
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| public class CustomNameResolverProvider extends NameResolverProvider {
@Override
public NameResolver newNameResolver(URI targetUri, NameResolver.Args args) {
return new CustomNameResolver(targetUri.toString());
}
@Override
protected boolean isAvailable() {
return true;
}
@Override
protected int priority() {
return 10;
}
@Override
public String getDefaultScheme() {
return "http";
}
}
|
在新版本(1.21+)之后，NameResolver 推荐使用 NameResolverRegistry 进行注册；
注册要在创建 Channel 之前执行
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| NameResolverRegistry.getDefaultRegistry().register(new CustomNameResolverProvider());
this.channel = ManagedChannelBuilder
.forTarget("server")
.usePlaintext()
.build();
|
在 Channel 调用 build 方式时，会在 io.grpc.internal.ManagedChannelImpl#ManagedChannelImpl的构造方法中获取 NameResolver.Factory，这个属性的值是由调用 io.grpc.internal.AbstractManagedChannelImplBuilder#getNameResolverFactory 方法获取的，这个方法里面的属性值来自于 io.grpc.NameResolverRegistry#asFactory；NameResolverRegistry 自己通过内部类 NameResolverFactory创建了NameResovler.Factory 的实例，调用 Factory 的 newNameResolver时，从 provider 属性中获取根据优先级排序后的 NameResolver，创建实例并返回第一个创建的有效实例
