type Locality struct {
	// Region is the region of the xDS client application.
	Region string
	// Zone is the area within a region.
	Zone string
	// SubZone is the further subdivision within a zone.
	SubZone string
}

type MetricsReporter interface {
	// ReportMetric reports a metric. The metric will be one of the predefined
	// set of types depending on the client (XDSClient or LRSClient).
	//
	// Each client will produce different metrics. Please see the client's
	// documentation for a list of possible metrics events.
	ReportMetric(metric any)
}

type Node struct {
	// ID is a string identifier of the application.
	ID string
	// Cluster is the name of the cluster the application belongs to.
	Cluster string
	// Locality is the location of the application including region, zone,
	// sub-zone.
	Locality Locality
	// Metadata provides additional context about the application by associating
	// arbitrary key-value pairs with it.
	Metadata any
	// UserAgentName is the user agent name of application.
	UserAgentName string
	// UserAgentVersion is the user agent version of application.
	UserAgentVersion string
}

type ServerIdentifier struct {
	// ServerURI is the target URI of the server.
	ServerURI string

	// Extensions can be populated with arbitrary data to be passed to the
	// TransportBuilder and/or xDS Client's ResourceType implementations.
	// This field can be used to provide additional configuration or context
	// specific to the user's needs.
	//
	// The xDS and LRS clients do not interpret the contents of this field.
	// It is the responsibility of the user's custom TransportBuilder and/or
	// ResourceType implementations to handle and interpret these extensions.
	//
	// For example, a custom TransportBuilder might use this field to
	// configure a specific security credentials.
	//
	// Extensions may be any type that is comparable, as they are used as map
	// keys internally. If Extensions are not able to be used as a map key,
	// the client may panic.
	//
	// See: https://go.dev/ref/spec#Comparison_operators
	//
	// Any equivalent extensions in all ServerIdentifiers present in a single
	// client's configuration should have the same value. Not following this
	// restriction may result in excess resource usage.
	Extensions any
}

type Stream interface {
	// Send sends the provided message on the stream.
	Send([]byte) error

	// Recv blocks until the next message is received on the stream.
	Recv() ([]byte, error)
}

type Transport interface {
	// NewStream creates a new streaming call to the server for the specific
	// RPC method name. The returned Stream interface can be used to send and
	// receive messages on the stream.
	NewStream(context.Context, string) (Stream, error)

	// Close closes the Transport.
	Close()
}

type TransportBuilder interface {
	// Build creates a new Transport instance to the server based on the
	// provided ServerIdentifier.
	Build(serverIdentifier ServerIdentifier) (Transport, error)
}