Server and Duplex streaming out parameters alternative

[max-ieremenko]

Problem

gRPC protocol is message oriented and in server streaming calls does not allow to pass back to a caller external information.
The only way to achieve this is by using response headers.

As an example file download from server. Together with content i need also file creation date, owner information and file version:

[OperationContract]
Task<IAsyncEnumerable<byte[]>> DownloadFile(string fileName, CallContext context);

// server-side implementation
async Task<IAsyncEnumerable<byte[]>> DownloadFile(string fileName, CallContext context)
{
    DateTime fileCreationDate = ... // resolve file creation date
    string fileOwner = ... // resolve owner information
    string fileVersion = ... // resolve file version

    // pass external information in response headers
    ServerCallContext serverContext = context;
    await serverContext.WriteResponseHeadersAsync(new Metadata
    {
        { "creation-date", fileCreationDate.ToString() },
        { "owner", fileOwner },
        { "version", fileVersion  }
    });

    // pass file content via server streaming
    return ReadFileContent(fileName);
}
private IAsyncEnumerable<byte[]> ReadFileContent(string fileName)
{
    // convert file content to gRPC byte[] stream
    // the code is for demonstration purposes only !!!
    using (var stream = File.OpenRead(fileName))
    {
        var buffer = new byte[4 * 1204];
        while ((await stream.ReadAsync(buffer, 0, buffer.Length)) > 0)
        {
            yield return buffer;
        }
    }
}

// client-side implementation
var context = new CallContext();
var content = await DownloadFile("file name", context);

// extract external information from response headers
var fileCreationDate = context.ResponseHeaders.First(i => i.Key == "creation-date").Value;
var fileOwner = context.ResponseHeaders.First(i => i.Key == "owner").Value;
var fileVersion = context.ResponseHeaders.First(i => i.Key == "version").Value;

// download file content
await foreach(var buffer in content)
{
    // ...
}

Idea

The idea is to move "response headers" related implementation into ServiceModel.Grpc framework:

// no CallContext in the call, only CancellationToken
// pass external information in the method return type
[OperationContract]
Task<(DateTime CreationDate, string Owner, string Version, IAsyncEnumerable<byte[]> Content)> DownloadFile(string fileName, CancellationToken token);

// server-side implementation
async Task<(DateTime, string, string, IAsyncEnumerable<byte[]>)> DownloadFile(string fileName, CancellationToken token)
{
    DateTime fileCreationDate = ... // resolve file creation date
    string fileOwner = ... // resolve owner information
    string fileVersion = ... // resolve file version

    return (fileCreationDate, fileOwner, version, ReadFileContent(fileName));
}

// client-side implementation
var (fileCreationDate, fileOwner, fileVersion, content) = await DownloadFile("file name", CancellationToken);

// download file content
await foreach(var buffer in content)
{
    // ...
}

[max-ieremenko]

released in version 1.1.7