groupcache/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 (
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"
	"strings"
	"sync"

	"code.google.com/p/goprotobuf/proto"
	"github.com/golang/groupcache/consistenthash"
	pb "github.com/golang/groupcache/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

	// base path including leading and trailing slash, e.g. "/_groupcache/"
	basePath string

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

	mu    sync.Mutex
	peers *consistenthash.Map
}

// 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 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.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

	opts := HTTPPoolOptions{}
	if o != nil {
		opts = *o
	}
	if opts.BasePath == "" {
		opts.BasePath = defaultBasePath
	}
	if opts.Replicas == 0 {
		opts.Replicas = defaultReplicas
	}

	p := &HTTPPool{
		basePath: opts.BasePath,
		self:     self,
		peers:    consistenthash.New(opts.Replicas, 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(defaultReplicas, nil)
	p.peers.Add(peers...)
}

func (p *HTTPPool) PickPeer(key string) (ProtoGetter, bool) {
	p.mu.Lock()
	defer p.mu.Unlock()
	if p.peers.IsEmpty() {
		return nil, false
	}
	if peer := p.peers.Get(key); peer != p.self {
		// TODO: pre-build a slice of *httpGetter when Set()
		// is called to avoid these two allocations.
		return &httpGetter{p.Transport, peer + p.basePath}, true
	}
	return nil, false
}

func (p *HTTPPool) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Parse request.
	if !strings.HasPrefix(r.URL.Path, p.basePath) {
		panic("HTTPPool serving unexpected path: " + r.URL.Path)
	}
	parts := strings.SplitN(r.URL.Path[len(p.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)
	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)
	var value []byte
	err := group.Get(ctx, key, AllocatingByteSliceSink(&value))
	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)
}

type httpGetter struct {
	transport func(Context) http.RoundTripper
	baseURL   string
}

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()),
	)
	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)
	}
	// TODO: avoid this garbage.
	b, err := ioutil.ReadAll(res.Body)
	if err != nil {
		return fmt.Errorf("reading response body: %v", err)
	}
	err = proto.Unmarshal(b, out)
	if err != nil {
		return fmt.Errorf("decoding response body: %v", err)
	}
	return nil
}