Subscriptions in GraphQL .NET Core doesn't send response to client

Question

I'm building a GraphQL API in Asp Net Core 3.1. I'm trying to add a subscription that send a new entity to subscriber when it is added. When I execute the subscription from the /ui/playground the handshake with server seems to be successed:

GraphQL http request websocket:

When I execute the mutation that adds a review it successed and create a record on the db, but the above screen reamins as it is, no updates, no data receveived from the socket.

This is the mutation:

Field<ReviewType>(
    "createReview",
    arguments: new QueryArguments(
        new QueryArgument<NonNullGraphType<ReviewInput>> 
        { 
            Name = "reviewInput"
        }
    ),
    resolve: context =>
    {
        var review = context.GetArgument<Review>("reviewInput");
        var rev = reviewService.Add(review);
        return rev;
    }
);

ShopSubscrition.cs

public partial class ShopSubscription : ObjectGraphType
{
    private readonly IReviewService _reviewService;

    public ShopSubscription(IReviewService reviewService)
    {
        _reviewService = reviewService;
        AddField(new EventStreamFieldType
        {
            Name = "reviewAdded",
            Type = typeof(ReviewType),
            Resolver = new FuncFieldResolver<Review>((context) => context.Source as Review),
            Subscriber = new EventStreamResolver<Review>((context) => _reviewService.ReviewAdded())
        });
    }
}

ReviewService.cs

public class ReviewService : IReviewService
{
    private readonly IReviewRepository _reviewRepository;
    private readonly ISubject<Review> _sub = new ReplaySubject<Review>(1);

    public ReviewService(IReviewRepository reviewRepository)
    {
        _reviewRepository = reviewRepository;
    }

    public Review Add(Review review)
    {
        var addedEntity = _reviewRepository.Add(review);
        _sub.OnNext(review);
        return review;
    }

    public IObservable<Review> ReviewAdded()
    {
        return _sub.AsObservable();
    }
}

I post the Startup.cs too, maybe it can helps.

Startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors();

        services.AddDbContext<ShopDbContext>(opt =>
            opt.UseSqlServer(Configuration.GetConnectionString("Default")));

        services.AddScoped<ISupplierRepository, SupplierRepository>();
        services.AddScoped<IProductRepository, ProductRepository>();
        services.AddScoped<IReviewRepository, ReviewRepository>();

        services.AddScoped<IReviewService, ReviewService>();

        services.AddSingleton<IDataLoaderContextAccessor, DataLoaderContextAccessor>();
        services.AddSingleton<DataLoaderDocumentListener>();

        services.AddScoped<ShopSchema>();
        services.AddGraphQL(options =>
        {
            options.EnableMetrics = false;
        })
            .AddWebSockets()
            .AddSystemTextJson()
            .AddGraphTypes(typeof(ShopSchema), ServiceLifetime.Scoped).AddDataLoader();

        services.AddControllers()
            .AddNewtonsoftJson(o => o.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore);
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env, ShopDbContext dbContext)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseCors("Cors");
        app.UseHttpsRedirection();
        app.UseRouting();
        app.UseAuthorization();
        app.UseWebSockets();
        app.UseGraphQLWebSockets<ShopSchema>("/graphql");

        app.UseGraphQL<ShopSchema>();
        app.UseGraphQLPlayground(options: new GraphQLPlaygroundOptions { GraphQLEndPoint ="/graphql", SchemaPollingEnabled = false });

        
        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
        dbContext.SeedData();
    }
}

Thanks in advance for help.

Answer 1

After a deeper investigation I found out the problem. I post the solution, maybe could help someone.

The problem was about Dependency Injection: the class containing the subscription "notification logic" (in my case ReviewService.cs) must be registered as Singleton instead of Scoped. This caused a sort of chain reaction that bring the repositories classes to be registered as Transient. This is the working code:

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options => options.AddPolicy("Cors",
            builder =>
            {
                builder.AllowAnyOrigin()
                .AllowAnyMethod()
                .AllowAnyHeader();
            }));

        services.AddDbContext<ShopDbContext>(opt =>
            opt.UseSqlServer(Configuration.GetConnectionString("Default")));

        services.AddTransient<ISupplierRepository, SupplierRepository>();
        services.AddTransient<IProductRepository, ProductRepository>();
        services.AddTransient<IReviewRepository, ReviewRepository>();

        services.AddSingleton<IReviewService, ReviewService>();

        services.AddSingleton<IDataLoaderContextAccessor, DataLoaderContextAccessor>();
        services.AddSingleton<DataLoaderDocumentListener>();

        services.AddScoped<ShopSchema>();
        services.AddGraphQL(options =>
        {
            options.EnableMetrics = false;  // info sulla richiesta (ms, campi richiesti, ecc)
        })
            .AddWebSockets()
            .AddSystemTextJson()
            .AddGraphTypes(typeof(ShopSchema), ServiceLifetime.Scoped).AddDataLoader();

        services.AddControllers()
            .AddNewtonsoftJson(o => o.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore);
    }