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* Copyright 2019 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 implements the xds resolver.
// At this point, the resolver is named xds-experimental, and doesn't do very
// much at all, except for returning a hard-coded service config which selects
// the xds_experimental balancer.
package resolver
import (
const (
// The JSON form of the hard-coded service config which picks the
// xds_experimental balancer with round_robin as the child policy.
jsonSC = `{
"round_robin": {}
// xDS balancer name is xds_experimental while resolver scheme is
// xds-experimental since "_" is not a valid character in the URL.
xdsScheme = "xds-experimental"
// NewBuilder creates a new implementation of the resolver.Builder interface
// for the xDS resolver.
func NewBuilder() resolver.Builder {
return &xdsBuilder{}
type xdsBuilder struct{}
// Build helps implement the resolver.Builder interface.
func (b *xdsBuilder) Build(t resolver.Target, cc resolver.ClientConn, o resolver.BuildOption) (resolver.Resolver, error) {
// The xds balancer must have been registered at this point for the service
// config to be parsed properly.
scpr := cc.ParseServiceConfig(jsonSC)
if scpr.Err != nil {
panic(fmt.Sprintf("error parsing service config %q: %v", jsonSC, scpr.Err))
// We return a resolver which bacically does nothing. The hard-coded service
// config returned here picks the xds balancer.
cc.UpdateState(resolver.State{ServiceConfig: scpr})
return &xdsResolver{}, nil
// Name helps implement the resolver.Builder interface.
func (*xdsBuilder) Scheme() string {
return xdsScheme
type xdsResolver struct{}
// ResolveNow is a no-op at this point.
func (*xdsResolver) ResolveNow(o resolver.ResolveNowOption) {}
// Close is a no-op at this point.
func (*xdsResolver) Close() {}
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