http.go

/*
Copyright 2013 Google Inc.

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 groupcache

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"sync"

	"github.com/golang/protobuf/proto"
	"groupcacheNote/consistenthash"
	pb "groupcacheNote/groupcachepb"
)

const defaultBasePath = "/_groupcache/"

const defaultReplicas = 50

// HTTPPool implements PeerPicker for a pool of HTTP peers.
type HTTPPool struct {
	// Context optionally specifies a context for the server to use when it
	// receives a request.
	// If nil, the server uses a nil Context.
	Context func(*http.Request) Context

	// Transport optionally specifies an http.RoundTripper for the client
	// to use when it makes a request.
	// If nil, the client uses http.DefaultTransport.
	Transport func(Context) http.RoundTripper

	// this peer's base URL, e.g. "https://example.net:8000"
	self string // 本地节点url

	// opts specifies the options.
	opts HTTPPoolOptions

	mu sync.Mutex // guards peers and httpGetters
	// 包含 一致性hash map、hash函数的结构体，保存集群节点（都是用url代表，同下）和其对应一致性hash值
	peers *consistenthash.Map
	// 保存节点url和其对应的http数据请求器
	httpGetters map[string]*httpGetter // keyed by e.g. "http://10.0.0.2:8008"
}

// HTTPPoolOptions are the configurations of a HTTPPool.
type HTTPPoolOptions struct {
	// BasePath specifies the HTTP path that will serve groupcache requests.
	// If blank, it defaults to "/_groupcache/".
	BasePath string // peers间url请求路径  /_groupcache/

	// Replicas specifies the number of key replicas on the consistent hash.
	// If blank, it defaults to 50.
	Replicas int // 单一节点在一致性hash map中的虚拟节点数 默认为 50 个

	// HashFn specifies the hash function of the consistent hash.
	// If blank, it defaults to crc32.ChecksumIEEE.
	HashFn consistenthash.Hash // 一致性hash计算函数 默认是 crc32.ChecksumIEEE.
}

// NewHTTPPool initializes an HTTP pool of peers, and registers itself as a PeerPicker.
// For convenience, it also registers itself as an http.Handler with http.DefaultServeMux.
// The self argument should be a valid base URL that points to the current server,
// for example "http://example.net:8000".
func NewHTTPPool(self string) *HTTPPool {
	p := NewHTTPPoolOpts(self, nil)
	http.Handle(p.opts.BasePath, p)
	return p
}

var httpPoolMade bool

// NewHTTPPoolOpts initializes an HTTP pool of peers with the given options.
// Unlike NewHTTPPool, this function does not register the created pool as an HTTP handler.
// The returned *HTTPPool implements http.Handler and must be registered using http.Handle.
func NewHTTPPoolOpts(self string, o *HTTPPoolOptions) *HTTPPool {
	if httpPoolMade {
		panic("groupcache: NewHTTPPool must be called only once")
	}
	httpPoolMade = true

	p := &HTTPPool{
		self:        self,
		httpGetters: make(map[string]*httpGetter),
	}
	if o != nil {
		p.opts = *o
	}
	if p.opts.BasePath == "" {
		p.opts.BasePath = defaultBasePath
	}
	if p.opts.Replicas == 0 {
		p.opts.Replicas = defaultReplicas
	}
	p.peers = consistenthash.New(p.opts.Replicas, p.opts.HashFn)

	RegisterPeerPicker(func() PeerPicker { return p })
	return p
}

// Set updates the pool's list of peers.
// Each peer value should be a valid base URL,
// for example "http://example.net:8000".
func (p *HTTPPool) Set(peers ...string) {
	p.mu.Lock()
	defer p.mu.Unlock()
	p.peers = consistenthash.New(p.opts.Replicas, p.opts.HashFn)
	p.peers.Add(peers...)
	p.httpGetters = make(map[string]*httpGetter, len(peers))
	for _, peer := range peers {
		p.httpGetters[peer] = &httpGetter{transport: p.Transport, baseURL: peer + p.opts.BasePath, baseHost: peer}
	}
}

// httppool选取节点算法
func (p *HTTPPool) PickPeer(key string) (ProtoGetter, bool) {
	p.mu.Lock()
	defer p.mu.Unlock()
	if p.peers.IsEmpty() {
		return nil, false
	}
	// 在这里一致性hash是用于查找服务器url的，根据key来定位具体的服务器
	if peer := p.peers.Get(key); peer != p.self { // 判断获取到的节点是不是本地节点
		return p.httpGetters[peer], true // 返回节点对应的httpGetter
	}
	return nil, false
}

func (p *HTTPPool) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Parse request.
	if !strings.HasPrefix(r.URL.Path, p.opts.BasePath) {
		panic("HTTPPool serving unexpected path: " + r.URL.Path)
	}
	parts := strings.SplitN(r.URL.Path[len(p.opts.BasePath):], "/", 2)
	if len(parts) != 2 {
		http.Error(w, "bad request", http.StatusBadRequest)
		return
	}
	groupName := parts[0]
	key := parts[1]

	// Fetch the value for this group/key.
	group := GetGroup(groupName) // 获取group名字，因为可以有多个group
	if group == nil {
		http.Error(w, "no such group: "+groupName, http.StatusNotFound)
		return
	}
	var ctx Context
	if p.Context != nil {
		ctx = p.Context(r)
	}

	group.Stats.ServerRequests.Add(1) // 设置统计数据
	var value []byte
	err := group.Get(ctx, key, AllocatingByteSliceSink(&value)) // 此处Get方法和例子中的test.go的image_cache.Get方法是一样的，都是响应外部http请求
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	// Write the value to the response body as a proto message.
	body, err := proto.Marshal(&pb.GetResponse{Value: value})
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	w.Header().Set("Content-Type", "application/x-protobuf")
	w.Write(body)
}

//向其他节点发起http请求，获取缓存数据
type httpGetter struct {
	transport func(Context) http.RoundTripper
	baseURL   string
	baseHost  string
}

var bufferPool = sync.Pool{
	New: func() interface{} { return new(bytes.Buffer) },
}

func (h *httpGetter) GetBaseHost() string {
	return h.baseHost
}

//向其他节点发起http请求，获取缓存数据
func (h *httpGetter) Get(context Context, in *pb.GetRequest, out *pb.GetResponse) error {
	u := fmt.Sprintf(
		"%v%v/%v",
		h.baseURL,
		url.QueryEscape(in.GetGroup()),
		url.QueryEscape(in.GetKey()),
	)
	fmt.Println("peer url: ", u)
	req, err := http.NewRequest("GET", u, nil)
	if err != nil {
		return err
	}
	tr := http.DefaultTransport
	if h.transport != nil {
		tr = h.transport(context)
	}
	res, err := tr.RoundTrip(req)
	if err != nil {
		return err
	}
	defer res.Body.Close()
	if res.StatusCode != http.StatusOK {
		return fmt.Errorf("server returned: %v", res.Status)
	}
	b := bufferPool.Get().(*bytes.Buffer)
	b.Reset()
	defer bufferPool.Put(b)
	_, err = io.Copy(b, res.Body)
	if err != nil {
		return fmt.Errorf("reading response body: %v", err)
	}
	err = proto.Unmarshal(b.Bytes(), out)
	if err != nil {
		return fmt.Errorf("decoding response body: %v", err)
	}
	return nil
}