How to transform a POST request as GET with request body as query strings?

[sreejith-ms]

Hi,

I have a requirement to read the request body and transform it to query strings.

The request is

POST: /search_foo_data
Body: {
 "a": 1,
 "b": 2,
}

I want to transform it as
GET: /foo?a=1&b=2

I tried with the following code

.AddTransforms(builderContext =>
{
    if (builderContext.Route.RouteId == "route2")
    {
        builderContext.AddRequestTransform(async transformContext =>
        {
            var data = await transformContext.HttpContext.Request.ReadFromJsonAsync<Foo>();
            transformContext.ProxyRequest.Headers.Add("a", data.A);
        });
    }
})

but get's the exception

Request: An error was encountered before receiving a response.
System.Net.Http.HttpRequestException: An error occurred while sending the request.
 ---> System.IO.IOException: The request was aborted.
 ---> System.Net.Http.Http2StreamException: The HTTP/2 server reset the stream. HTTP/2 error code 'PROTOCOL_ERROR' (0x1).
   --- End of inner exception stack trace ---
   at System.Net.Http.Http2Connection.ThrowRequestAborted(Exception innerException)
   at System.Net.Http.Http2Connection.Http2Stream.CheckResponseBodyState()
   at System.Net.Http.Http2Connection.Http2Stream.TryEnsureHeaders()
   at System.Net.Http.Http2Connection.Http2Stream.ReadResponseHeadersAsync(CancellationToken cancellationToken)
   at System.Net.Http.Http2Connection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.Http2Connection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   at Yarp.ReverseProxy.Forwarder.HttpForwarder.SendAsync(HttpContext context, String destinationPrefix, HttpMessageInvoker httpClient, ForwarderRequestConfig requestConfig, HttpTransformer transformer)
[12:23:28 INF] Executed endpoint 'route2'

Any guidelines or examples for this?

[MihaZupan]

ReadFromJsonAsync will consume the request body stream, so when YARP tries to proxy the request, there will be a mismatch between the Content-Length header and the content actually available.

I would suggest transforming the request body into the query before invoking the forwarder.

For this to work you will also have to change the request's method to GET and clear the Content-Length header as well.

app.MapReverseProxy(proxyPipeline =>
{
    proxyPipeline.Use(static async (context, next) =>
    {
        if (context.Request.Method == HttpMethods.Post)
        {
            var proxyFeature = context.GetReverseProxyFeature();
            if (proxyFeature?.Route.Config.RouteId == "route2")
            {
                context.Request.Method = HttpMethods.Get;
                context.Request.Headers.ContentLength = null;

                var data = await context.Request.ReadFromJsonAsync<Foo>();
                if (data is not null)
                {
                    var query = context.Request.QueryString;
                    query = query.Add("a", data.a);
                    query = query.Add("b", data.b);
                    context.Request.QueryString = query;
                }
            }
        }
        await next();
    });

    proxyPipeline.UseSessionAffinity();
    proxyPipeline.UseLoadBalancing();
});

[sreejith-ms]

Thanks, @MihaZupan that worked !!