type CallbackFuncs struct {
    StreamOpenFunc     func(context.Context, int64, string) error
    StreamClosedFunc   func(int64)
    StreamRequestFunc  func(int64, *discovery.DiscoveryRequest) error
    StreamResponseFunc func(int64, *discovery.DiscoveryRequest, *discovery.DiscoveryResponse)
    FetchRequestFunc   func(context.Context, *discovery.DiscoveryRequest) error
    FetchResponseFunc  func(*discovery.DiscoveryRequest, *discovery.DiscoveryResponse)
}

type Callbacks interface {
    // OnStreamOpen is called once an xDS stream is open with a stream ID and the type URL (or "" for ADS).
    // Returning an error will end processing and close the stream. OnStreamClosed will still be called.
    OnStreamOpen(context.Context, int64, string) error
    // OnStreamClosed is called immediately prior to closing an xDS stream with a stream ID.
    OnStreamClosed(int64)
    // OnStreamRequest is called once a request is received on a stream.
    // Returning an error will end processing and close the stream. OnStreamClosed will still be called.
    OnStreamRequest(int64, *discovery.DiscoveryRequest) error
    // OnStreamResponse is called immediately prior to sending a response on a stream.
    OnStreamResponse(int64, *discovery.DiscoveryRequest, *discovery.DiscoveryResponse)
    // OnFetchRequest is called for each Fetch request. Returning an error will end processing of the
    // request and respond with an error.
    OnFetchRequest(context.Context, *discovery.DiscoveryRequest) error
    // OnFetchResponse is called immediately prior to sending a response.
    OnFetchResponse(*discovery.DiscoveryRequest, *discovery.DiscoveryResponse)
}

type HTTPGateway struct {
    // Log is an optional log for errors in response write
    Log log.Logger

    // Server is the underlying gRPC server
    Server Server
}

type Server interface {
    endpointservice.EndpointDiscoveryServiceServer
    clusterservice.ClusterDiscoveryServiceServer
    routeservice.RouteDiscoveryServiceServer
    listenerservice.ListenerDiscoveryServiceServer
    discoverygrpc.AggregatedDiscoveryServiceServer
    secretservice.SecretDiscoveryServiceServer
    runtimeservice.RuntimeDiscoveryServiceServer

    // Fetch is the universal fetch method.
    Fetch(context.Context, *discovery.DiscoveryRequest) (*discovery.DiscoveryResponse, error)
    // StreamHandler is the universal stream method.
    StreamHandler(stream Stream, typeURL string) error
}

type Stream interface {
    grpc.ServerStream

    Send(*discovery.DiscoveryResponse) error
    Recv() (*discovery.DiscoveryRequest, error)
}