在微服务体系中,任何一个服务都可以使用多个实例进行部署,那么对于一个请求发送到哪个实例来进行处理,需要一个策略来选择处理节点。
Selector是基于注册中心registry来进行节点选择和状态标记的。在选择过程中可以使用不同的算法进行选择。
Selector接口定义为
// Selector builds on the registry as a mechanism to pick nodes
// and mark their status. This allows host pools and other things
// to be built using various algorithms.
type Selector interface {
Init(opts ...Option) error
Options() Options
// Select 返回一个函数,返回值函数应该返回下一个节点
Select(service string, opts ...SelectOption) (Next, error)
// Mark sets the success/error against a node
Mark(service string, node *registry.Node, err error)
// Reset returns state back to zero for a service
Reset(service string)
// Close renders the selector unusable
Close() error
// Name of the selector
String() string
}
如果需要不同的选择策略, 可以自定义选择策略
// Next is a function that returns the next node
// based on the selector's strategy
type Next func() (*registry.Node, error)
// Filter is used to filter a service during the selection process
type Filter func([]*registry.Service) []*registry.Service
// Strategy is a selection strategy e.g random, round robin
type Strategy func([]*registry.Service) Next
selector默认的实现
func NewSelector(opts ...Option) Selector {
sopts := Options{
Strategy: Random,
}
for _, opt := range opts {
opt(&sopts)
}
if sopts.Registry == nil {
sopts.Registry = registry.DefaultRegistry
}
s := ®istrySelector{
so: sopts,
}
s.rc = s.newCache()
return s
}
在默认的实现中,节点选择策略为随机选择
func init() {
rand.Seed(time.Now().UnixNano())
}
// Random is a random strategy algorithm for node selection
func Random(services []*registry.Service) Next {
nodes := make([]*registry.Node, 0, len(services))
for _, service := range services {
nodes = append(nodes, service.Nodes...)
}
return func() (*registry.Node, error) {
if len(nodes) == 0 {
return nil, ErrNoneAvailable
}
i := rand.Int() % len(nodes)
return nodes[i], nil
}
}
节点选择流程如下:
- 根据指定的服务名,获取服务列表
- 根据服务选项中的过滤器,进行过滤
- 如果还存在多个节点,那么根据选择策略,选择一个节点返回
func (c *registrySelector) Select(service string, opts ...SelectOption) (Next, error) {
sopts := SelectOptions{
Strategy: c.so.Strategy,
}
for _, opt := range opts {
opt(&sopts)
}
// get the service
// try the cache first
// if that fails go directly to the registry
services, err := c.rc.GetService(service)
if err != nil {
if err == registry.ErrNotFound {
return nil, ErrNotFound
}
return nil, err
}
// apply the filters
for _, filter := range sopts.Filters {
services = filter(services)
}
// if there's nothing left, return
if len(services) == 0 {
return nil, ErrNoneAvailable
}
return sopts.Strategy(services), nil
}
从examples中介绍的客户端负载案例中,我们可以看看如何去使用
func main() {
cmd.Init()
client.DefaultClient = client.NewClient(
client.Selector(FirstNodeSelector()),
)
fmt.Println("\n--- Call example ---")
for i := 0; i < 10; i++ {
call(i)
}
}
FirstNodeSelector的返回值是一个firstNodeSelector
负载器,他是Selector接口的实现者,逻辑非常简单,实现的效果就是永远选择服务列表中的第一个
// Built in random hashed node selector
type firstNodeSelector struct {
opts selector.Options
}
func (n *firstNodeSelector) Init(opts ...selector.Option) error {
for _, o := range opts {
o(&n.opts)
}
return nil
}
func (n *firstNodeSelector) Options() selector.Options {
return n.opts
}
func (n *firstNodeSelector) Select(service string, opts ...selector.SelectOption) (selector.Next, error) {
services, err := n.opts.Registry.GetService(service)
if err != nil {
return nil, err
}
if len(services) == 0 {
return nil, selector.ErrNotFound
}
var sopts selector.SelectOptions
for _, opt := range opts {
opt(&sopts)
}
for _, filter := range sopts.Filters {
services = filter(services)
}
if len(services) == 0 {
return nil, selector.ErrNotFound
}
if len(services[0].Nodes) == 0 {
return nil, selector.ErrNotFound
}
return func() (*registry.Node, error) {
return services[0].Nodes[0], nil
}, nil
}
func (n *firstNodeSelector) Mark(service string, node *registry.Node, err error) {
return
}
func (n *firstNodeSelector) Reset(service string) {
return
}
func (n *firstNodeSelector) Close() error {
return nil
}
func (n *firstNodeSelector) String() string {
return "first"
}
继续看client的代码, 在实例化client的时候,初始化我们的选择器,这也体现了,只要我们是实现了Selector接口,自己定义选择器实现,也是没问题的。
client.DefaultClient = client.NewClient(
client.Selector(FirstNodeSelector()),
)
在服务调用时,从下面的client.Call方法实现,可以看看整个流程是如何运作的
func call(i int) {
// Create new request to service go.micro.srv.example, method Example.Call
req := client.NewRequest("go.micro.srv.example", "Example.Call", &example.Request{
Name: "John",
})
rsp := &example.Response{}
// Call service
if err := client.Call(context.Background(), req, rsp); err != nil {
fmt.Println("call err: ", err, rsp)
return
}
fmt.Println("Call:", i, "rsp:", rsp.Msg)
}
如果你使用的grpc,
func (g *grpcClient) Call(ctx context.Context, req client.Request, rsp interface{}, opts ...client.CallOption) error {
if req == nil {
return errors.InternalServerError("go.micro.client", "req is nil")
} else if rsp == nil {
return errors.InternalServerError("go.micro.client", "rsp is nil")
}
// make a copy of call opts
callOpts := g.opts.CallOptions
for _, opt := range opts {
opt(&callOpts)
}
next, err := g.next(req, callOpts)
if err != nil {
return err
}
// check if we already have a deadline
d, ok := ctx.Deadline()
if !ok {
// no deadline so we create a new one
var cancel context.CancelFunc
ctx, cancel = context.WithTimeout(ctx, callOpts.RequestTimeout)
defer cancel()
} else {
// got a deadline so no need to setup context
// but we need to set the timeout we pass along
opt := client.WithRequestTimeout(time.Until(d))
opt(&callOpts)
}
// should we noop right here?
select {
case <-ctx.Done():
return errors.New("go.micro.client", fmt.Sprintf("%v", ctx.Err()), 408)
default:
}
// make copy of call method
gcall := g.call
// wrap the call in reverse
for i := len(callOpts.CallWrappers); i > 0; i-- {
gcall = callOpts.CallWrappers[i-1](gcall)
}
// return errors.New("go.micro.client", "request timeout", 408)
call := func(i int) error {
// call backoff first. Someone may want an initial start delay
t, err := callOpts.Backoff(ctx, req, i)
if err != nil {
return errors.InternalServerError("go.micro.client", err.Error())
}
// only sleep if greater than 0
if t.Seconds() > 0 {
time.Sleep(t)
}
// select next node
node, err := next()
service := req.Service()
if err != nil {
if err == selector.ErrNotFound {
return errors.InternalServerError("go.micro.client", "service %s: %s", service, err.Error())
}
return errors.InternalServerError("go.micro.client", "error selecting %s node: %s", service, err.Error())
}
// make the call
err = gcall(ctx, node, req, rsp, callOpts)
g.opts.Selector.Mark(service, node, err)
if verr, ok := err.(*errors.Error); ok {
return verr
}
return err
}
ch := make(chan error, callOpts.Retries+1)
var gerr error
for i := 0; i <= callOpts.Retries; i++ {
go func(i int) {
ch <- call(i)
}(i)
select {
case <-ctx.Done():
return errors.New("go.micro.client", fmt.Sprintf("%v", ctx.Err()), 408)
case err := <-ch:
// if the call succeeded lets bail early
if err == nil {
return nil
}
retry, rerr := callOpts.Retry(ctx, req, i, err)
if rerr != nil {
return rerr
}
if !retry {
return err
}
gerr = err
}
}
return gerr
}
他会调用next方法,来获取节点
func (g *grpcClient) next(request client.Request, opts client.CallOptions) (selector.Next, error) {
service, address, _ := pnet.Proxy(request.Service(), opts.Address)
// return remote address
if len(address) > 0 {
return func() (*registry.Node, error) {
return ®istry.Node{
Address: address[0],
}, nil
}, nil
}
// get next nodes from the selector
next, err := g.opts.Selector.Select(service, opts.SelectOptions...)
if err != nil {
if err == selector.ErrNotFound {
return nil, errors.InternalServerError("go.micro.client", "service %s: %s", service, err.Error())
}
return nil, errors.InternalServerError("go.micro.client", "error selecting %s node: %s", service, err.Error())
}
return next, nil
}
在真正的调用之前,会进行节点选择
// return errors.New("go.micro.client", "request timeout", 408)
call := func(i int) error {
// call backoff first. Someone may want an initial start delay
t, err := callOpts.Backoff(ctx, req, i)
if err != nil {
return errors.InternalServerError("go.micro.client", err.Error())
}
// only sleep if greater than 0
if t.Seconds() > 0 {
time.Sleep(t)
}
// select next node
node, err := next()
service := req.Service()
if err != nil {
if err == selector.ErrNotFound {
return errors.InternalServerError("go.micro.client", "service %s: %s", service, err.Error())
}
return errors.InternalServerError("go.micro.client", "error selecting %s node: %s", service, err.Error())
}
// make the call
err = gcall(ctx, node, req, rsp, callOpts)
g.opts.Selector.Mark(service, node, err)
if verr, ok := err.(*errors.Error); ok {
return verr
}
return err
}
以上就是节点选择器的大致内容,如果想要了解更详细的内容,可以去这里go-micro看更详细的代码。