How to forward HTTP response to client

Question

I have a client (Xamarin) and two Web API servers, A and B. The client makes a request to A which uses the request parameters to make another request to B. How do I return the response that A receives from B to the client such that B's use of C is transparent to the client.

For example, if I make a request from A to B using HttpClient how do I forward the HttpResponseMessage to the client in a controller's action?

Answer 1

Look at aspnet/AspLabs repository for a good example of a transparent HTTP proxy. This proxy is a middleware handling client's requests to A and making its own requests to B.

To provide a complete answer, this is the part of its source code which is actually forwarding a HttpResponseMessage:

using (var responseMessage = await _httpClient.SendAsync(
    requestMessage, HttpCompletionOption.ResponseHeadersRead, context.RequestAborted))
{
    context.Response.StatusCode = (int)responseMessage.StatusCode;
    foreach (var header in responseMessage.Headers)
    {
        context.Response.Headers[header.Key] = header.Value.ToArray();
    }

    foreach (var header in responseMessage.Content.Headers)
    {
        context.Response.Headers[header.Key] = header.Value.ToArray();
    }

    // SendAsync removes chunking from the response. This removes the header so it doesn't expect a chunked response.
    context.Response.Headers.Remove("transfer-encoding");
    await responseMessage.Content.CopyToAsync(context.Response.Body);
}

There is full code copying a received HttpResponseMessage to the HttpContext.Response in the repository.

Answer 2

I took IIya's answer and adapted it for asp.net MVC 5

 public async Task GetFile(CancellationToken cancellationToken)
    {
        var context = HttpContext;
        var _httpClient = StaticHttpClient.Instance;
        var requestMessage = new HttpRequestMessage()
        {
            RequestUri = new Uri("https://localhost:44305/api/files/123"),
        };
        using (var responseMessage = await _httpClient.SendAsync(
requestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken))
        {
            Response.StatusCode = (int)responseMessage.StatusCode;
            foreach (var header in responseMessage.Headers)
            {
                Response.Headers[header.Key] = header.Value.First();
            }
            foreach (var header in responseMessage.Content.Headers)
            {
                Response.Headers[header.Key] = header.Value.First();
            }
            Response.Headers.Remove("transfer-encoding");
            //Response.BinaryWrite(await responseMessage.Content.ReadAsByteArrayAsync());
            await responseMessage.Content.CopyToAsync(Response.OutputStream);
            Response.Flush();
            Response.Close();
            context.ApplicationInstance.CompleteRequest();
        }
    }