Interceptors allow us to hook into protocol-specific events. We can, for example, intercept an incoming HTTP request or a client connecting or disconnecting a WebSocket session.

IHttpRequestInterceptor

Each GraphQL request sent via HTTP can be intercepted using an IHttpRequestInterceptor before it is being executed. Per default Hot Chocolate registers a DefaultHttpRequestInterceptor for this purpose.

We can create a new class inheriting from DefaultHttpRequestInterceptor to provide our own logic for request interception.

public class HttpRequestInterceptor : DefaultHttpRequestInterceptor
{
    public override ValueTask OnCreateAsync(HttpContext context,
        IRequestExecutor requestExecutor, OperationRequestBuilder requestBuilder,
        CancellationToken cancellationToken)
    {
        return base.OnCreateAsync(context, requestExecutor, requestBuilder,
            cancellationToken);
    }
}

Once we have defined our custom HttpRequestInterceptor, we also have to register it.

builder.Services
    .AddGraphQLServer()
    .AddHttpRequestInterceptor<HttpRequestInterceptor>();

If needed, we can also inject services into our custom HttpRequestInterceptor using its constructor.

OnCreateAsync

This method is invoked for every GraphQL request sent via HTTP. It is a great place to set global state variables, extend the identity of the authenticated user or do anything that we want to do on a per-request basis.

public override ValueTask OnCreateAsync(HttpContext context,
    IRequestExecutor requestExecutor, OperationRequestBuilder requestBuilder,
    CancellationToken cancellationToken)
{
    return base.OnCreateAsync(context, requestExecutor, requestBuilder,
        cancellationToken);
}

Most of the configuration will be done through the OperationRequestBuilder, injected as argument to this method.

Learn more about the OperationRequestBuilder

If we want to fail the request, before it is being executed, we can throw a GraphQLException. The middleware will then translate this exception to a proper GraphQL error response for the client.

ISocketSessionInterceptor

Each GraphQL request sent over WebSockets can be intercepted using an ISocketSessionInterceptor before it is being executed. Since WebSockets are long lived connections, we can also intercept specific lifecycle events, such as connecting or disconnecting. Per default Hot Chocolate registers a DefaultSocketSessionInterceptor for this purpose.

We can create a new class inheriting from DefaultSocketSessionInterceptor to provide our own logic for request / lifecycle interception.

public class SocketSessionInterceptor : DefaultSocketSessionInterceptor
{
    public override ValueTask<ConnectionStatus> OnConnectAsync(
        ISocketConnection connection, InitializeConnectionMessage message,
        CancellationToken cancellationToken)
    {
        return base.OnConnectAsync(connection, message, cancellationToken);
    }

    public override ValueTask OnRequestAsync(ISocketConnection connection,
        OperationRequestBuilder requestBuilder,
        CancellationToken cancellationToken)
    {
        return base.OnRequestAsync(connection, requestBuilder,
            cancellationToken);
    }

    public override ValueTask OnCloseAsync(ISocketConnection connection,
        CancellationToken cancellationToken)
    {
        return base.OnCloseAsync(connection, cancellationToken);
    }
}

Once we have defined our custom SocketSessionInterceptor, we also have to register it.

builder.Services
    .AddGraphQLServer()
    .AddSocketSessionInterceptor<SocketSessionInterceptor>();

If needed, we can also inject services into our custom HttpRequestInterceptor using its constructor.

We do not have to override every method shown above, we can also only override the ones we are interested in.

OnConnectAsync

This method is invoked once, when a client attempts to initialize a WebSocket connection. We have the option to either accept or reject specific connection requests.

public async override ValueTask<ConnectionStatus> OnConnectAsync(
    ISocketConnection connection, InitializeConnectionMessage message,
    CancellationToken cancellationToken)
{
    if (condition)
    {
        return ConnectionStatus.Reject("Connection rejected for X reason!");
    }

    return ConnectionStatus.Accept();
}

We also get access to the InitializeConnectionMessage. If a client sends a payload with this message, for example an auth token, we can access the Payload like the following.

public async override ValueTask<ConnectionStatus> OnConnectAsync(
    ISocketConnection connection, InitializeConnectionMessage message,
    CancellationToken cancellationToken)
{
    if (message.Payload?.TryGetValue("MyKey", out object? value) == true)
    {
        // ...
    }

    return ConnectionStatus.Accept();
}

OnRequestAsync

This method is invoked for every GraphQL request a client sends using the already established WebSocket connection. It is a great place to set global state variables, extend the identity of the authenticated user or anything that we want to do on a per-request basis.

public override ValueTask OnRequestAsync(ISocketConnection connection,
    OperationRequestBuilder requestBuilder, CancellationToken cancellationToken)
{
    return base.OnRequestAsync(connection, requestBuilder, cancellationToken);
}

Most of the configuration will be done through the OperationRequestBuilder, injected as argument to this method.

Learn more about the OperationRequestBuilder

If we want to fail the request, before it is being executed, we can throw a GraphQLException. The middleware will then translate this exception to a proper GraphQL error response for the client.

OnCloseAsync

This method is invoked, once a client closes the WebSocket connection or the connection is terminated in any other way.

OperationRequestBuilder

The OperationRequestBuilder allows us to influence the execution of a GraphQL request.

It has many capabilities, but most of them are only used internally. In the following we are going to cover the methods that are most relevant to us as consumers.

Properties

We can set Properties, also called Global State, on the OperationRequestBuilder, which can then be referenced in middleware, field resolvers, etc.

Learn more about Global State

SetProperty

SetProperty allows us to add a key-value pair, where the key is a string and the value can be anything, i.e. an object.

requestBuilder.SetProperty("name", "value");
requestBuilder.SetProperty("name", 123);
requestBuilder.SetProperty("name", new User { Name = "Joe" });

There is also TrySetProperty, which only adds the property, if it hasn't yet been added.

requestBuilder.TryAddProperty("name", 123);

SetProperties

SetProperties allows us to set all properties at once.

var properties = new Dictionary<string, object>
{
    { "name", "value" }
};

requestBuilder.SetProperties(properties);

SetServices

SetServices allows us to add an IServiceProvider which should be used for dependency injection during the request.

var provider = new ServiceCollection()
    .AddSingleton<ExampleService>()
    .BuildServiceProvider();

requestBuilder.SetServices(provider);

There is also TrySetServices, which only sets the IServiceProvider, if it hasn't yet been set.

AllowIntrospection

If we have disabled introspection globally, AllowIntrospection allows us to enable it for specific requests.

requestBuilder.AllowIntrospection();