java - gRPC 如何在客户端等待来自 StreamingServer 的标头
问题描述
对于 proto 文件定义的简单 gRPC 服务:
service HelloService {
rpc sayHello() returns (stream string)
}
在我开始处理来自服务器的响应之前,我如何在客户端等待首先等待标头到达?我尝试使用 ClientInterceptor 并覆盖 onHeaders() 方法,但是在对 sayHello() 的调用已经完成后会调用它。如何验证 sayHello 客户端中的特定标头并根据标头的有效性继续调用?
public class SomeHeaderInterceptor implements ClientInterceptor {
private static final String FULL_METHOD_NAME = "sayHello";
public static CallOptions.Key<String> someHeader = CallOptions.Key.of("some_header_active", "false");
@Override
public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(MethodDescriptor<ReqT, RespT> methodDescriptor,
CallOptions callOptions, Channel channel) {
return new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(channel.newCall(methodDescriptor, callOptions)) {
@Override
public void start(Listener<RespT> responseListener, Metadata headers) {
super.start(new ForwardingClientCallListener.SimpleForwardingClientCallListener<RespT>(responseListener) {
@Override
public void onHeaders(Metadata headers) {
Metadata.Key<String> SAYHELLO_ACTIVE_HEADER = Metadata.Key.of("some_header_active",
Metadata.ASCII_STRING_MARSHALLER);
if (methodDescriptor.getFullMethodName().equals(FULL_METHOD_NAME)) {
if (!headers.containsKey(SAYHELLO_ACTIVE_HEADER)) {
LOGGER.logError("some_header activation missing from header: " + headers);
} else {
callOptions.withOption(someHeader, "true");
Context.current().withValue(Context.key("test"), "testvalue");
}
}
super.onHeaders(headers);
}
}, headers);
}
};
}
sayHello 的代码如下:
public Iterator<String> sayHello() {
Iterator<String> stream = blockingStub.sayHello();
// wait for the sayhello active header
boolean isActive = Boolean.parseBoolean(blockingStub.getCallOptions().getOption(SomeHeaderInterceptor. someHeader));
System.out.println("the some_header header value is: " + isActive);
System.out.println("the context key : " + Context.key("test").get(Context.current()));
return stream;
}
在上面的 sayHello() 代码中,它不等待标头到达并在 onHeaders() 中设置上下文。我怎样才能做到这一点?在我验证服务器发送的流中存在 some_header 后,我只想将流返回给客户端调用者。
解决方案
标头由服务器在第一条消息之前发送,因此最简单的方法是调用stream.hasNext()
,这将阻止等待消息。在许多 RPC 中,第一条消息在响应头之后很快就出现了,所以这会很好地工作。
顺便说一句,我注意到您正在尝试使用CallOptions
and Context
:
callOptions.withOption(someHeader, "true");
Context.current().withValue(Context.key("test"), "testvalue");
这两行都没有真正做任何事情,因为这两个对象都是不可变的。这些with*
调用创建了一个新实例,因此您必须使用该行的返回值来执行任何操作。此外,CallOptions 和 Context 主要以相反的方向传递信息,例如从客户端应用程序到拦截器。要“反转”方向,应用程序需要设置一个可变值,如 AtomicReference 或回调函数,然后拦截器可以与该值交互。
如果服务器响应标头和第一条消息之间可能存在明显的延迟,那么事情就会变得更加复杂。调用 sayHello() 的代码:
CompletableFuture<Boolean> future = new CompletableFuture<>();
Iterator<String> stream = blockingStub
.withOption(SomeHeaderInterceptor.SOME_HEADER, future)
.sayHello();
// wait for the sayhello active header
boolean isActive = future.get();
然后在拦截器中:
private static final String FULL_METHOD_NAME =
//"helloworld.Greeter/SayHello";
GreeterGrpc.getSayHelloMethod().getFullMethodName();
public static final CallOptions.Key<CompletableFuture<Boolean>> SOME_HEADER =
CallOptions.Key.create("some_header_active", CompletableFuture.completedFuture(false));
private static final Metadata.Key<String> SAYHELLO_ACTIVE_HEADER =
Metadata.Key.of("some_header_active", Metadata.ASCII_STRING_MARSHALLER);
@Override
public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(MethodDescriptor<ReqT, RespT> methodDescriptor,
CallOptions callOptions, Channel channel) {
CompletableFuture<Boolean> future = callOptions.getOption(SOME_HEADER);
if (!methodDescriptor.getFullMethodName().equals(FULL_METHOD_NAME)) {
future.complete(false);
return channel.newCall(methodDescriptor, callOptions);
}
return new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(channel.newCall(methodDescriptor, callOptions)) {
@Override
public void start(Listener<RespT> responseListener, Metadata headers) {
super.start(new ForwardingClientCallListener.SimpleForwardingClientCallListener<RespT>(responseListener) {
@Override
public void onHeaders(Metadata headers) {
if (!headers.containsKey(SAYHELLO_ACTIVE_HEADER)) {
// Could also be future.complete(false)
future.completeExceptionally(new Exception("some_header activation missing from header: " + headers));
} else {
future.complete(true);
}
super.onHeaders(headers);
}
@Override
public void onClose(Status status, Metadata trailers) {
// onHeaders() might not have been called, especially if there was an error
if (!future.isDone()) {
future.completeExceptionally(status.asRuntimeException(trailers));
}
super.onClose(status, trailers);
}
}, headers);
}
};
}
如果您只需要进行验证,而不是实际延迟,那么您可以避免延迟并在验证失败时让拦截器调用失败:
private static final String FULL_METHOD_NAME =
//"helloworld.Greeter/SayHello";
GreeterGrpc.getSayHelloMethod().getFullMethodName();
private static final Metadata.Key<String> SAYHELLO_ACTIVE_HEADER =
Metadata.Key.of("some_header_active", Metadata.ASCII_STRING_MARSHALLER);
@Override
public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(MethodDescriptor<ReqT, RespT> methodDescriptor,
CallOptions callOptions, Channel channel) {
if (!methodDescriptor.getFullMethodName().equals(FULL_METHOD_NAME)) {
return channel.newCall(methodDescriptor, callOptions);
}
// We use context to cancel since it is thread-safe, whereas ClientCall.cancel is not
CancellableContext context = Context.current().withCancellation();
class ValidatingListener extends ForwardingClientCallListener<RespT> {
private Listener<RespT> responseListener;
public ValidatingListener(Listener<RespT> responseListener) {
this.responseListener = responseListener;
}
@Override
protected Listener<RespT> delegate() {
return responseListener;
}
@Override
public void onHeaders(Metadata headers) {
if (!headers.containsKey(SAYHELLO_ACTIVE_HEADER)) {
Listener<RespT> saved = responseListener;
responseListener = new Listener<RespT>() {}; // noop listener; throw away future events
Status status = Status.UNKNOWN.withDescription(
"some_header activation missing from header: " + headers);
context.cancel(status.asRuntimeException());
saved.onClose(status, new Metadata());
return;
}
// Validation successful
super.onHeaders(headers);
}
@Override
public void onClose(Status status, Metadata trailers) {
context.close();
}
}
Context toRestore = context.attach();
try {
return new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(channel.newCall(methodDescriptor, callOptions)) {
@Override
public void start(Listener<RespT> responseListener, Metadata headers) {
super.start(new ValidatingListener(responseListener), headers);
}
};
} finally {
context.detach(toRestore);
}
}
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