Implementing GraphQL subscriptions

GraphQL subscriptions, by default, work over WebSockets but can also work over Server-Sent Events (SSE), SignalR, or even gRPC.

Imagine that a client app wants to be notified when a product has its unit price reduced. It would be great if the client could subscribe to an event that gets triggered whenever a unit price is reduced, instead of having to query for changes to the unit prices.

Adding a subscription and topic to the GraphQL service

Let’s add this feature to our GraphQL service using subscriptions:

1. Add a new class file named ProductDiscount.cs.
2. Modify the contents to define a model to notify a client about a product’s unit price reduction, as shown in the following code:

   namespace Northwind.GraphQL.Service;
   public class ProductDiscount
   {
     public int? ProductId { get; set; }
     public decimal? OriginalUnitPrice { get; set; }
     public decimal? NewUnitPrice { get; set; }
   }
   

3. Add...