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* Copyright 2017 gRPC authors.
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
// Package resolver defines APIs for name resolution in gRPC.
// All APIs in this package are experimental.
package resolver
var (
// m is a map from scheme to resolver builder.
m = make(map[string]Builder)
// defaultScheme is the default scheme to use.
defaultScheme string
// TODO(bar) install dns resolver in init(){}.
// Register registers the resolver builder to the resolver map.
// b.Scheme will be used as the scheme registered with this builder.
func Register(b Builder) {
m[b.Scheme()] = b
// Get returns the resolver builder registered with the given scheme.
// If no builder is register with the scheme, the default scheme will
// be used.
// If the default scheme is not modified, "dns" will be the default
// scheme, and the preinstalled dns resolver will be used.
// If the default scheme is modified, and a resolver is registered with
// the scheme, that resolver will be returned.
// If the default scheme is modified, and no resolver is registered with
// the scheme, nil will be returned.
func Get(scheme string) Builder {
if b, ok := m[scheme]; ok {
return b
if b, ok := m[defaultScheme]; ok {
return b
return nil
// SetDefaultScheme sets the default scheme that will be used.
// The default default scheme is "dns".
func SetDefaultScheme(scheme string) {
defaultScheme = scheme
// AddressType indicates the address type returned by name resolution.
type AddressType uint8
const (
// Backend indicates the address is for a backend server.
Backend AddressType = iota
// GRPCLB indicates the address is for a grpclb load balancer.
// Address represents a server the client connects to.
// This is the EXPERIMENTAL API and may be changed or extended in the future.
type Address struct {
// Addr is the server address on which a connection will be established.
Addr string
// Type is the type of this address.
Type AddressType
// ServerName is the name of this address.
// It's the name of the grpc load balancer, which will be used for authentication.
ServerName string
// Metadata is the information associated with Addr, which may be used
// to make load balancing decision.
Metadata interface{}
// BuildOption includes additional information for the builder to create
// the resolver.
type BuildOption struct {
// ClientConn contains the callbacks for resolver to notify any updates
// to the gRPC ClientConn.
type ClientConn interface {
// NewAddress is called by resolver to notify ClientConn a new list
// of resolved addresses.
// The address list should be the complete list of resolved addresses.
NewAddress(addresses []Address)
// NewServiceConfig is called by resolver to notify ClientConn a new
// service config. The service config should be provided as a json string.
NewServiceConfig(serviceConfig string)
// Target represents a target for gRPC, as specified in:
type Target struct {
Scheme string
Authority string
Endpoint string
// Builder creates a resolver that will be used to watch name resolution updates.
type Builder interface {
// Build creates a new resolver for the given target.
// gRPC dial calls Build synchronously, and fails if the returned error is
// not nil.
Build(target Target, cc ClientConn, opts BuildOption) (Resolver, error)
// Scheme returns the scheme supported by this resolver.
// Scheme is defined at
Scheme() string
// ResolveNowOption includes additional information for ResolveNow.
type ResolveNowOption struct{}
// Resolver watches for the updates on the specified target.
// Updates include address updates and service config updates.
type Resolver interface {
// ResolveNow will be called by gRPC to try to resolve the target name again.
// It's just a hint, resolver can ignore this if it's not necessary.
// Close closes the resolver.
// UnregisterForTesting removes the resolver builder with the given scheme from the
// resolver map.
// This function is for testing only.
func UnregisterForTesting(scheme string) {
delete(m, scheme)