Returning non-FHIR content from a plugin

Some plugins may need to return content that is not a FHIR Resource. You currently cannot do that through the IVonkResponse. But there is another way: write directly to the HttpContext.Response. The plugin with order 1110 makes the IVonkContext accessible. That means if you pick an order higher than 1110 you can read the IVonkContext.Request and .Arguments, and write directly to the HttpContext.Response.

If you write the response yourself, you also need to set the StatusCode and Content-Type on the HttpContext.Response.


If you write to the HttpContext.Response directly, the payload of the IVonkContext.Response is ignored.

The steps to take are:

  1. Configure your plugin with an order higher than 1110

  2. Write the HttpContext.Response.Body directly

  3. Set other properties of the HttpContext.Response (like StatusCode) yourself.

An example of such a plugin would look like this. Note that this is now regular ASP.NET Core Middleware, not a service like in Template for a plugin.

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using System.Text;
using System.Threading.Tasks;
using Vonk.Core.Context;
using Vonk.Core.Context.Features;
using Vonk.Core.Pluggability;

namespace com.mycompany.vonk.myplugin
    public class MyPluginMiddleware
        private readonly RequestDelegate _next;

        public MyPluginMiddleware(RequestDelegate next)
            _next = next;

        public async Task Invoke(HttpContext httpContext)
            var vonkContext = httpContext.Vonk();
            var (request, args, _) = vonkContext.Parts();
            if (VonkInteraction.system_custom.HasFlag(request.Interaction))
                //write something directly to the HttpContext.Response. Now you also have to set the Content-Type header and the Content-Length yourself.
                var message = "This is a response that is not a FHIR resource";
                string contentLength = Encoding.UTF8.GetByteCount(message).ToString();

                httpContext.Response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
                httpContext.Response.Headers.Add("Content-Length", contentLength);
                httpContext.Response.StatusCode = 200;
                await httpContext.Response.WriteAsync("This is a response that is not a FHIR resource");
                await _next(httpContext);

    [VonkConfiguration(order: 1115)] //note the order: higher than 1110
    public static class MyPluginConfiguration
        public static IServiceCollection AddMyPluginServices(IServiceCollection services)
            //No services to register in this example, but if you create services to do the actual work - register them here.
            return services;

        public static IApplicationBuilder UseMyPlugin(IApplicationBuilder app)
            app.UseMiddleware<MyPluginMiddleware>(); //You cannot use the extension methods that allow you to filter the requests.
            return app;

Custom authorization plugin

This feature can also be used to implement custom authorization. You can find a template for that in this gist.

If you just return a statuscode, you could use the regular IVonkContext.Response. If you want to return e.g. a custom json object, you should use the method described above. Do not forget to set the Content-Type (to application/json for a custom json object).

For more information about access control in plugins and facades, see Access Control in Plugins and Facades.