horros/EventGridPublisherClientEmulator

EventGridPublisherClientEmulator

This is a class that "emulates" EventGridPublisherClient in the new Azure.Messaging.EventGrid 4.0.0-beta release.

Problem

Developing locally with EventGrid is possible, using something like ngrok to open a tunnel from the outside and adding a new Event Subscription to EventGrid pointing to the ngrok tunnel. This is less than ideal in many cases, especially when developing larger event driven microservices apps.

Solution

Paul Mcilreavy developed the awesome EventGridSimulator that can act as Azure EventGrid on your local computer. However, try as I may, I could not get the EventGridPublisherClient in the EventGrid 4.0.0-beta4 API to play ball, it kept crashing with an error saying "The SSL connection could not be established" and nothing more of use. I tried everything I could think of at the time, even constructing a new HttpClientHandler for which I disabled certificate validation completely and used that as parameter for a new HttpClient which was used as a parameter for a new HttpClientTransport which was set as the transport on a EventGridPublisherClientOptions object that was passed to EventGridPublisherClient, but nothing worked.

I know EventGridPublisherClient does stuff like force the protocol to be HTTPS regardless of what you pass in as the URI and override the query path etc, so I spent a little while writing together an "emulator" of sorts for EventGridPublisherClient. Since EventGridSimulator does not support CloudEvents I didn't implement support for it in the emulator either, though this should be trivial to add. All the "emulator" does is override

async Task<Response> SendEventsAsync(IEnumerable<EventGridEvent> eventGridEvents, CancellationToken cancellationToken = default)

and construct a JSON representation of the events, set the correct headers and post that to the URI specified, in this case EventGridSimulator. Using this you could roll your own local Event Grid-y thing if you wish, but I warmly recommed EventGridSimulator.

Usage

Build the project and add it as a reference in your own project. Or just copy the .cs file to your project.

My quickly cobbled together thing registers a singleton service of type EventGridPublisherClient in Startup.cs, except if the environment is Development, we instantiate a ClientEmulator class instead of the EventGridPublisherClient:

    if (Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") == "Development")
    {
        services.AddSingleton<EventGridPublisherClient>(clientEmulator =>
        {
            return new ClientEmulator(new Uri("https://localhost:60102/api/events?api-version=2018-01-01"), new AzureKeyCredential("DummyKey"));
        }); 
    }
    else
    {
        string topicEndpoint = Environment.GetEnvironmentVariable("EVENT_GRID_TOPIC_ENDPOINT");
        string key = Environment.GetEnvironmentVariable("EVENT_GRID_KEY");
        services.AddSingleton<EventGridPublisherClient>(clientEmulator =>
        {
            return new EventGridPublisherClient(new Uri(topicEndpoint), new AzureKeyCredential(key));
        });
    }

Then I do something like this in my MediatR Notification Handler:

public class ExampleHandler : INotificationHandler<ExampleEvent>
{
    private readonly EventGridPublisherClient client;

    public ExampleHandler(EventGridPublisherClient client) {
        this.client = client;
    }
    
    public async Task Handle(ExampleEvent notification, CancellationToken cancellationToken = default) {
        await client.SendEventsAsync(GetEventsList(notification), cancellationToken);
    }

    static IList<EventGridEvent> GetEventsList(ExampleEvent notification) 
    {
        // Get a list of events etc
    }
}

For any questions, comments or improvements, open an issue.