How to implement asynchronous RPC calls? What are the patterns and advantages of asynchronous calls? - 面试题

Asynchronous RPC calls are an important technology for improving system performance and concurrency, allowing clients to handle other tasks while waiting for responses:

Asynchronous Call Patterns:

1. Future/Promise Pattern

Principle: Returns Future object immediately after call, get result through Future
Advantages: Simple to use, doesn't block calling thread
Disadvantages: Need to actively get result, code may not be elegant

Implementation Example:

java
// Dubbo asynchronous call
<dubbo:reference interface="com.example.UserService" 
                 async="true"/>

// Usage
userService.getUser(1L);
Future<User> future = RpcContext.getContext().getFuture();
User user = future.get(1000, TimeUnit.MILLISECONDS);

2. Callback Pattern

Principle: Pass callback function when calling, execute callback when result returns
Advantages: Event-driven, suitable for asynchronous processing
Disadvantages: Callback hell, poor code readability

Implementation Example:

java
public interface AsyncCallback<T> {
    void onSuccess(T result);
    void onFailure(Throwable t);
}

// Usage
userService.getUserAsync(1L, new AsyncCallback<User>() {
    @Override
    public void onSuccess(User user) {
        // Handle success result
    }
    
    @Override
    public void onFailure(Throwable t) {
        // Handle failure
    }
});

3. Reactive Programming

Principle: Use reactive streams to handle asynchronous data
Advantages: Elegant code, supports backpressure, suitable for stream processing
Disadvantages: Steep learning curve

Implementation Example:

java
// Reactor
Mono<User> userMono = userService.getUserReactive(1L);
userMono.subscribe(
    user -> System.out.println(user),
    error -> System.err.println(error)
);

// RxJava
Observable<User> userObs = userService.getUserRx(1L);
userObs.subscribe(
    user -> System.out.println(user),
    error -> System.err.println(error)
);

4. gRPC Asynchronous Call

Principle: Use StreamObserver to handle asynchronous responses
Advantages: Supports streaming communication, deeply integrated with gRPC

Implementation Example:

java
// Unary asynchronous call
stub.sayHello(request, new StreamObserver<HelloResponse>() {
    @Override
    public void onNext(HelloResponse response) {
        // Handle response
    }
    
    @Override
    public void onError(Throwable t) {
        // Handle error
    }
    
    @Override
    public void onCompleted() {
        // Call completed
    }
});

// Bidirectional streaming
StreamObserver<Request> requestObserver = stub.bidirectionalStream(
    new StreamObserver<Response>() {
        @Override
        public void onNext(Response response) {
            // Handle response
        }
        
        @Override
        public void onError(Throwable t) {
            // Handle error
        }
        
        @Override
        public void onCompleted() {
            // Call completed
        }
    });

// Send requests
requestObserver.onNext(request1);
requestObserver.onNext(request2);
requestObserver.onCompleted();

5. CompletableFuture

Principle: Asynchronous programming tool introduced in Java 8
Advantages: Powerful features, supports chained calls

Implementation Example:

java
CompletableFuture<User> future = CompletableFuture.supplyAsync(
    () -> userService.getUser(1L)
);

// Chained calls
future.thenAccept(user -> System.out.println(user))
      .exceptionally(t -> {
          System.err.println(t);
          return null;
      });

// Combine multiple Futures
CompletableFuture<User> userFuture = userService.getUserAsync(1L);
CompletableFuture<Order> orderFuture = orderService.getOrderAsync(1L);

CompletableFuture<Result> resultFuture = userFuture.thenCombineAsync(
    orderFuture,
    (user, order) -> new Result(user, order)
);

Advantages of Asynchronous Calls:

1. Improve Concurrency

Doesn't block calling thread
Can handle multiple requests simultaneously
Fully utilize system resources

2. Reduce Latency

Client can initiate multiple calls in parallel
Reduce waiting time
Improve response speed

3. Improve Throughput

Process more requests per unit time
Suitable for high concurrency scenarios

4. Better User Experience

Avoid UI freezing
Implement real-time updates

Challenges of Asynchronous Calls:

1. Code Complexity

Asynchronous code is hard to understand and debug
Complex error handling
Need to handle thread safety issues

2. Context Propagation

Context may be lost during asynchronous calls
Need to explicitly pass context information
Solution: Use ThreadLocal, TransmittableThreadLocal

3. Timeout Control

Need to set reasonable timeout
Avoid infinite waiting

Implementation Example:

java
CompletableFuture<User> future = userService.getUserAsync(1L);
try {
    User user = future.get(1000, TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
    future.cancel(true);
}

4. Resource Management

Need to reasonably manage thread pools
Avoid resource exhaustion

Implementation Example:

java
ExecutorService executor = Executors.newFixedThreadPool(10);
CompletableFuture<User> future = CompletableFuture.supplyAsync(
    () -> userService.getUser(1L),
    executor
);

Best Practices:

1. Reasonably Choose Asynchronous Pattern

Simple scenarios: Future/Promise
Event-driven: Callback pattern
Stream processing: Reactive programming
High performance requirements: CompletableFuture

2. Comprehensive Error Handling

Catch all exceptions
Provide meaningful error messages
Implement retry mechanism

3. Timeout Control

Set reasonable timeout
Cancel request after timeout
Avoid resource leaks

4. Resource Management

Use thread pools to manage threads
Release resources in time
Avoid memory leaks

5. Monitoring and Logging

Log asynchronous calls
Monitor asynchronous call performance
Discover problems in time

Applicable Scenarios:

High concurrency scenarios
Need to call multiple services in parallel
Stream data processing
Scenarios with high real-time requirements
Long-running tasks