这篇文章是记录自己查看客户端grpc负载均衡源码的过程，并没有太详细的讲解，参考价值不大，可以直接跳过，主要给自己看的。

一.主要接口：Balancer Resolver

1.Balancer定义

Resolver定义

具体位置为

1.grpc源码对解析器(resolver)Builder接口有三个结构体进行实现:dns,passthrough,unix。在clientconn.go文件中通过引用包在相应包的初始化方法中进行了注册

2.进入internal/resolver/passthrough/passthrough.go文件中init初始化函数中看到了将passthroughBuilder进行了注册。调用了resolver包的Register函数

func init() {resolver.Register(&passthroughBuilder{})
}

3.Register将注册后的结构保存在全局变量m中，是一个map。key为Scheme，value为Builder接口。我们看到grpc默认的scheme是passthrough即透传什么也不做

package resolverimport ("context""net""net/url""strings""google.golang.org/grpc/attributes""google.golang.org/grpc/credentials""google.golang.org/grpc/internal/pretty""google.golang.org/grpc/serviceconfig"
)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
}

4.再看下passthroughBuilder，看到它实现了Buider接口，Build方法返回实现Resolver接口的结构passthroughResolver。其中passthroughBuilder用来创建解析器，passthroughResolver为创建的解析器

package passthroughimport ("errors""google.golang.org/grpc/resolver"
)const scheme = "passthrough"type passthroughBuilder struct{}func (*passthroughBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {if target.Endpoint() == "" && opts.Dialer == nil {return nil, errors.New("passthrough: received empty target in Build()")}r := &passthroughResolver{target: target,cc:     cc,}r.start()return r, nil
}func (*passthroughBuilder) Scheme() string {return scheme
}type passthroughResolver struct {target resolver.Targetcc     resolver.ClientConn
}func (r *passthroughResolver) start() {r.cc.UpdateState(resolver.State{Addresses: []resolver.Address{{Addr: r.target.Endpoint()}}})
}func (*passthroughResolver) ResolveNow(o resolver.ResolveNowOptions) {}func (*passthroughResolver) Close() {}func init() {resolver.Register(&passthroughBuilder{})
}

5.这段代码中返回的resolverBuilder就是从上面找到注册的builder，resolver.Get就是1.3中对应的代码

7.生成解析器的封装newCCResolverWrapper，并调用build将创建解析器，这个过程中调用了解析器的start方法，这个方法比较重要的是做了两件事情。1.触发选择balancer信号。2.触发状态修改（其中有负载均衡即连接的创建）

上面分别向ccBalancerWrapper的channel中发送了switchToUpdate和ccStateUpdate

balancer选择逻辑

baseBalancer会将所有地址都创建一个连接

invoke的时候，会调用picker选择连接

以上路径最终调用这个方法，可以看到是轮询使用连接