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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
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 groupcache
import (
pb ""
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. ""
self string
// opts specifies the options.
opts HTTPPoolOptions
mu sync.Mutex // guards peers and httpGetters
peers *consistenthash.Map
httpGetters map[string]*httpGetter // keyed by e.g. ""
// 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
// Replicas specifies the number of key replicas on the consistent hash.
// If blank, it defaults to 50.
Replicas int
// HashFn specifies the hash function of the consistent hash.
// If blank, it defaults to crc32.ChecksumIEEE.
HashFn consistenthash.Hash
// 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 "".
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 "".
func (p *HTTPPool) Set(peers ...string) {
p.peers = consistenthash.New(p.opts.Replicas, p.opts.HashFn)
p.httpGetters = make(map[string]*httpGetter, len(peers))
for _, peer := range peers {
p.httpGetters[peer] = &httpGetter{transport: p.Transport, baseURL: peer + p.opts.BasePath}
func (p *HTTPPool) PickPeer(key string) (ProtoGetter, bool) {
if p.peers.IsEmpty() {
return nil, false
if peer := p.peers.Get(key); peer != p.self {
return p.httpGetters[peer], true
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)
groupName := parts[0]
key := parts[1]
// Fetch the value for this group/key.
group := GetGroup(groupName)
if group == nil {
http.Error(w, "no such group: "+groupName, http.StatusNotFound)
var ctx Context
if p.Context != nil {
ctx = p.Context(r)
var value []byte
err := group.Get(ctx, key, AllocatingByteSliceSink(&value))
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
// 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)
w.Header().Set("Content-Type", "application/x-protobuf")
type httpGetter struct {
transport func(Context) http.RoundTripper
baseURL string
var bufferPool = sync.Pool{
New: func() interface{} { return new(bytes.Buffer) },
func (h *httpGetter) Get(context Context, in *pb.GetRequest, out *pb.GetResponse) error {
u := fmt.Sprintf(
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)
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
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