ServiceModel.Grpc exception handling general information

ServiceModel.Grpc does not handle any errors by default, exception are handled by gRPC API (RpcException).

How to activate and configure custom error handling is described here. This page explains how ServiceModel.Grpc error handling works under the hood. The implementation and technical details are subject to change in the future.

Briefly

Error handling customization should be configured on the client and server. When it is done, all calls on the client and server are intercepted by specific Grpc.Core.Interceptors.Interceptor implementation.

Server interceptor on any exception asks for IServerErrorHandler.ProvideFaultOrIgnore, if ServerFaultDetail is provided, the interceptor throws a RpcException with extra metadata. The metadata is transferred to the client.

Client interceptor catches only RpcException, when it is the case IClientErrorHandler.ThrowOrIgnore is responsible for what to do with the exception. All metadata from the server interceptor is available in ClientFaultDetail.

So, exception details are transferred as metadata via gRPC API (RpcException). This means the metadata is a set of http headers.

Server-side error model

IServerErrorHandler.ProvideFaultOrIgnore can provide custom error model to transfer it to the client call.

public struct ServerFaultDetail
{
    // optional StatusCode => RpcException.Status.StatusCode
    public StatusCode? StatusCode { get; set; }

    // optional Message => RpcException.Status.Detail
    public string Message { get; set; }

    // optional user defined error detail
    public object Detail { get; set; }

    // optional Trailers => RpcException.Trailers
    public Metadata Trailers { get; set; }
}

The ServerFaultDetail.Detail can be an instance of any user-defined class. When it is provided, the ServiceModel.Grpc:

• serializes it by the IMarshallerFactory associated with the current service to a byte[] and passes to a client call via binary header
• passes the type (Detail.GetType()) of instance to the client call via string header

Example

// user defined detail
[DataContract]
public class CustomDetail
{
    [DataMember]
    public string SomeProperty { get; set; }

    [DataMember]
    public string AnotherProperty { get; set; }
}

// user defined IServerErrorHandler.ProvideFaultOrIgnore
public ServerFaultDetail? ProvideFaultOrIgnore(ServerCallInterceptorContext context, Exception error)
{
    return new ServerFaultDetail
    {
        Detail = new CustomDetail
        {
            SomeProperty = ...
            AnotherProperty = ...
        }
    };
}

Client-side error model

IClientErrorHandler.ThrowOrIgnore must decide what to throw to the user code.

public readonly struct ClientFaultDetail
{
    // original RpcException
    public RpcException OriginalError { get; }

    // detail provided by IServerErrorHandler.ProvideFaultOrIgnore on server
    public object Detail { get; }
}

When ServerFaultDetail.Detail is provided by server, the ServiceModel.Grpc:

• restores the detail type from string header
• de-serializes detail by the IMarshallerFactory associated with the current service from binary header

Example

// user defined IClientErrorHandler.ThrowOrIgnore
public void ThrowOrIgnore(ClientCallInterceptorContext context, ClientFaultDetail detail)
{
    var customDetail = (CustomDetail)detail.Detail;
    // ...
}

Corner case “operation canceled”

The following code asks gRPC API to cancel the call after 1 second.

[ServiceContract]
public interface ISomeService
{
    [OperationContract]
    Task DoSomething(CancellationToken token);
}

// client call
var tokenSource = new CancellationTokenSource();
var client = IClientFactory.CreateClient<ISomeService>(...);

tokenSource.CancelAfter(TimeSpan.FromSeconds(1));
await client.DoSomething(tokenSource.Token);

What happens to the client after 1 second if the call is not finished?

On the client the real gRPC call will be interrupted by gRPC API, so no metadata is available from the server.

What happens to the server after 1 second if the call is not finished?

User-defined server method will be finished according to the implementation, no metadata is provided to the client by gRPC API.

ServiceModel.Grpc.Interceptors ServerErrorHandlerBase and ClientErrorHandlerBase handle this case properly. During the custom error handling configuration on client and server make sure that the root handlers are inherited from ServerErrorHandlerBase and ClientErrorHandlerBase.

// client call
var tokenSource = new CancellationTokenSource();
var client = IClientFactory.CreateClient<ISomeService>(...);

tokenSource.CancelAfter(TimeSpan.FromSeconds(1));

try
{
    await client.DoSomething(tokenSource.Token);
}
catch (OperationCanceledException)
{
    ....
}