gRPC下游的优雅停机

Question

syntax = "proto3";

package pb;

message SimpleRequest {
    int64 number = 1;
}

message SimpleResponse {
    int64 doubled = 1;
}

// All the calls in this serivce preform the action of doubling a number.
// The streams will continuously send the next double, eg. 1, 2, 4, 8, 16.
service Test {
    // This RPC streams from the server only.
    rpc Downstream(SimpleRequest) returns (stream SimpleResponse);
}

我能够成功地打开一个流，并不断地从服务器获得下一个翻倍的数字。

我的go运行代码如下所示：

ctxDownstream, cancel := context.WithCancel(ctx)
downstream, err := testClient.Downstream(ctxDownstream, &pb.SimpleRequest{Number: 1})
for {
    responseDownstream, err := downstream.Recv()
    if err != io.EOF {
        println(fmt.Sprintf("downstream response: %d, error: %v", responseDownstream.Doubled, err))

        if responseDownstream.Doubled >= 32 {
            break
        }
    }
}
cancel() // !!This is not a graceful shutdown
println(fmt.Sprintf("%v", downstream.Trailer()))

我遇到的问题是使用上下文取消意味着我的downstream.trailer（）响应为空。是否有一种方法可以从客户端优雅地关闭此连接并接收downstream.trailer（）。

func (b *binding) Downstream(req *pb.SimpleRequest, stream pb.Test_DownstreamServer) error {
    request := req

    r := make(chan *pb.SimpleResponse)
    e := make(chan error)
    ticker := time.NewTicker(200 * time.Millisecond)
    defer func() { ticker.Stop(); close(r); close(e) }()

    go func() {
        defer func() { recover() }()
        for {
            select {
            case <-ticker.C:
                response, err := b.Endpoint(stream.Context(), request)
                if err != nil {
                    e <- err
                }
                r <- response
            }
        }
    }()

    for {
        select {
        case err := <-e:
            return err
        case response := <-r:
            if err := stream.Send(response); err != nil {
                return err
            }
            request.Number = response.Doubled
        case <-stream.Context().Done():
            return nil
        }
    }
}

您仍然需要用一些信息填充预告片。我使用grpc.StreamServerInterceptor来完成这项工作。

Answer 1

根据grpc go文档

Trailer从服务器返回Trailer元数据（如果有的话）。只有在Stream.CloseAndRecv返回或Stream.Recv返回非零错误（包括IO.EOF）之后才可以调用它。

因此，如果您想在客户机中阅读预告片，请尝试如下所示

ctxDownstream, cancel := context.WithCancel(ctx)
defer cancel()
for {
  ...
  // on error or EOF
  break;
}
println(fmt.Sprintf("%v", downstream.Trailer()))