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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
import (
var (
// m is a map from scheme to resolver builder.
m = make(map[string]Builder)
// defaultScheme is the default scheme to use.
defaultScheme = "passthrough"
// 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.
// NOTE: this function must only be called during initialization time (i.e. in
// an init() function), and is not thread-safe. If multiple Resolvers are
// registered with the same name, the one registered last will take effect.
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, nil will be returned.
func Get(scheme string) Builder {
if b, ok := m[scheme]; ok {
return b
return nil
// SetDefaultScheme sets the default scheme that will be used. The default
// default scheme is "passthrough".
// NOTE: this function must only be called during initialization time (i.e. in
// an init() function), and is not thread-safe. The scheme set last overrides
// previously set values.
func SetDefaultScheme(scheme string) {
defaultScheme = scheme
// GetDefaultScheme gets the default scheme that will be used.
func GetDefaultScheme() string {
return defaultScheme
// 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.
// e.g. if Type is GRPCLB, ServerName should be the name of the remote load
// balancer, not the name of the backend.
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 {
// DisableServiceConfig indicates whether resolver should fetch service config data.
DisableServiceConfig bool
// State contains the current Resolver state relevant to the ClientConn.
type State struct {
Addresses []Address // Resolved addresses for the target
// ServiceConfig is the parsed service config; obtained from
// serviceconfig.Parse.
ServiceConfig serviceconfig.Config
// TODO: add Err error
// ClientConn contains the callbacks for resolver to notify any updates
// to the gRPC ClientConn.
// This interface is to be implemented by gRPC. Users should not need a
// brand new implementation of this interface. For the situations like
// testing, the new implementation should embed this interface. This allows
// gRPC to add new methods to this interface.
type ClientConn interface {
// UpdateState updates the state of the ClientConn appropriately.
// 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.
// Deprecated: Use UpdateState instead.
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.
// Deprecated: Use UpdateState instead.
NewServiceConfig(serviceConfig string)
// Target represents a target for gRPC, as specified in:
// It is parsed from the target string that gets passed into Dial or DialContext by the user. And
// grpc passes it to the resolver and the balancer.
// If the target follows the naming spec, and the parsed scheme is registered with grpc, we will
// parse the target string according to the spec. e.g. "dns://some_authority/" will be parsed
// into &Target{Scheme: "dns", Authority: "some_authority", Endpoint: ""}
// If the target does not contain a scheme, we will apply the default scheme, and set the Target to
// be the full target string. e.g. "" will be parsed into
// &Target{Scheme: resolver.GetDefaultScheme(), Endpoint: ""}.
// If the parsed scheme is not registered (i.e. no corresponding resolver available to resolve the
// endpoint), we set the Scheme to be the default scheme, and set the Endpoint to be the full target
// string. e.g. target string "unknown_scheme://authority/endpoint" will be parsed into
// &Target{Scheme: resolver.GetDefaultScheme(), Endpoint: "unknown_scheme://authority/endpoint"}.
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.
// It could be called multiple times concurrently.
// 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)