Source: serviceconfig.go in package google.golang.org/grpc/internal/serviceconfig

* * Copyright 2020 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 * * http:www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. *

Package serviceconfig contains utility functions to parse service config.

package serviceconfig

import (
	"encoding/json"
	"fmt"

	"google.golang.org/grpc/balancer"
	"google.golang.org/grpc/grpclog"
	externalserviceconfig "google.golang.org/grpc/serviceconfig"
)

var logger = grpclog.Component("core")

BalancerConfig wraps the name and config associated with one load balancing policy. It corresponds to a single entry of the loadBalancingConfig field from ServiceConfig. It implements the json.Unmarshaler interface. https://github.com/grpc/grpc-proto/blob/54713b1e8bc6ed2d4f25fb4dff527842150b91b2/grpc/service_config/service_config.proto#L247

type BalancerConfig struct {
	Name   string
	Config externalserviceconfig.LoadBalancingConfig
}

type intermediateBalancerConfig []map[string]json.RawMessage

UnmarshalJSON implements the json.Unmarshaler interface. ServiceConfig contains a list of loadBalancingConfigs, each with a name and config. This method iterates through that list in order, and stops at the first policy that is supported. - If the config for the first supported policy is invalid, the whole service config is invalid. - If the list doesn't contain any supported policy, the whole service config is invalid.

func (bc *BalancerConfig) UnmarshalJSON(b []byte) error {
	var ir intermediateBalancerConfig
	err := json.Unmarshal(b, &ir)
	if err != nil {
		return err
	}

	for i, lbcfg := range ir {
		if len(lbcfg) != 1 {
			return fmt.Errorf("invalid loadBalancingConfig: entry %v does not contain exactly 1 policy/config pair: %q", i, lbcfg)
		}

		var (
			name    string
			jsonCfg json.RawMessage

Get the key:value pair from the map. We have already made sure that the map contains a single entry.

		for name, jsonCfg = range lbcfg {
		}

		builder := balancer.Get(name)

If the balancer is not registered, move on to the next config. This is not an error.

			continue
		}
		bc.Name = name

		parser, ok := builder.(balancer.ConfigParser)
		if !ok {
			if string(jsonCfg) != "{}" {
				logger.Warningf("non-empty balancer configuration %q, but balancer does not implement ParseConfig", string(jsonCfg))

Stop at this, though the builder doesn't support parsing config.

			return nil
		}

		cfg, err := parser.ParseConfig(jsonCfg)
		if err != nil {
			return fmt.Errorf("error parsing loadBalancingConfig for policy %q: %v", name, err)
		}
		bc.Config = cfg
		return nil

This is reached when the for loop iterates over all entries, but didn't return. This means we had a loadBalancingConfig slice but did not encounter a registered policy. The config is considered invalid in this case.

	return fmt.Errorf("invalid loadBalancingConfig: no supported policies found")