gomemcache/memcache/memcache.go

/*
Copyright 2011 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 memcache provides a client for the memcached cache server.
package memcache

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"net"
)

var _ = log.Printf

// Similar to:
// http://code.google.com/appengine/docs/go/memcache/reference.html

var (
	// ErrCacheMiss means that a Get failed because the item wasn't present.
	ErrCacheMiss = os.NewError("memcache: cache miss")

	// ErrCASConflict means that a CompareAndSwap call failed due to the
	// cached value being modified between the Get and the CompareAndSwap.
	// If the cached value was simply evicted rather than replaced,
	// ErrNotStored will be returned instead.
	ErrCASConflict = os.NewError("memcache: compare-and-swap conflict")

	// ErrNotStored means that a conditional write operation (i.e. Add or
	// CompareAndSwap) failed because the condition was not satisfied.
	ErrNotStored = os.NewError("memcache: item not stored")

	// ErrServer means that a server error occurred.
	ErrServerError = os.NewError("memcache: server error")

	// ErrNoStats means that no statistics were available.
	ErrNoStats = os.NewError("memcache: no statistics available")

	// ErrMalformedKey is returned when an invalid key is used.
	// Keys must be at maximum 250 bytes long, ASCII, and not
	// contain whitespace or control characters.
	ErrMalformedKey = os.NewError("malformed: key is too long or contains invalid characters")

	// ErrNoServers is returned when no servers are configured or available.
	ErrNoServers = os.NewError("memcache: no servers configured or available")
)

// resumableError returns true if err is only a protocol-level cache error.
// This is used to determine whether or not a server connection should
// be re-used or not. If an error occurs, by default we don't reuse the
// connection, unless it was just a cache error.
func resumableError(err os.Error) bool {
	switch err {
	case ErrCacheMiss, ErrCASConflict, ErrNotStored, ErrMalformedKey:
		return true
	}
	return false
}

func legalKey(key string) bool {
	if len(key) > 250 {
		return false
	}
	for i := 0; i < len(key); i++ {
		if key[i] <= ' ' || key[i] > 0x7e {
			return false
		}
	}
	return true
}

var (
	crlf            = []byte("\r\n")
	resultStored    = []byte("STORED\r\n")
	resultNotStored = []byte("NOT_STORED\r\n")
	resultExists    = []byte("EXISTS\r\n")
	resultNotFound  = []byte("NOT_FOUND\r\n")
	end             = []byte("END\r\n")
)

// New returns a memcache client using the provided server(s)
// with equal weight. If a server is listed multiple times,
// it gets a proportional amount of weight.
func New(server ...string) *Client {
	ss := new(ServerList)
	ss.SetServers(server...)
	return NewFromSelector(ss)
}

// NewFromSelector returns a new Client using the provided ServerSelector.
func NewFromSelector(ss ServerSelector) *Client {
	return &Client{selector: ss}
}

// Client is a memcache client.
// It is safe for unlocked use by multiple concurrent goroutines.
type Client struct {
	selector ServerSelector
}

// Item is an item to be got or stored in a memcached server.
type Item struct {
	// Key is the Item's key (250 bytes maximum).
	Key string

	// Value is the Item's value.
	Value []byte

	// Object is the Item's value for use with a Codec.
	Object interface{}

	// Flags are server-opaque flags whose semantics are entirely up to the
	// App Engine app.
	Flags uint32

	// Expiration is the cache expiration time, in seconds: either a relative
	// time from now (up to 1 month), or an absolute Unix epoch time.
	// Zero means the Item has no expiration time.
	Expiration int32

	// Compare and swap ID.
	casid uint64
}

// conn is a connection to a server.
type conn struct {
	nc   net.Conn
	rw   *bufio.ReadWriter
	addr net.Addr
	c    *Client
}

// release returns this connection back to the client's free pool
func (cn *conn) release() {
	// TODO: return to client's free pool
}

// condRelease releases this connection if the error pointed to by err
// is is nil (not an error) or is only a protocol level error (e.g. a
// cache miss).  The purpose is to not recycle TCP connections that
// are bad.
func (cn *conn) condRelease(err *os.Error) {
	if *err == nil || resumableError(*err) {
		cn.release()
	}
}

func (c *Client) getConn(addr net.Addr) (*conn, os.Error) {
	// TODO(bradfitz): get from a free pool
	nc, err := net.Dial(addr.Network(), addr.String())
	if err != nil {
		return nil, err
	}
	// TODO: set read/write timeouts
	return &conn{
		nc:   nc,
		addr: addr,
		rw:   bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc)),
		c:    c,
	}, nil
}

func (c *Client) onItem(item *Item, fn func(*Client, *bufio.ReadWriter, *Item) os.Error) os.Error {
	addr, err := c.selector.PickServer(item.Key)
	if err != nil {
		return err
	}
	cn, err := c.getConn(addr)
	if err != nil {
		return err
	}
	defer cn.condRelease(&err)
	if err := fn(c, cn.rw, item); err != nil {
		return err
	}
	return nil
}

// Get gets the item for the given key. ErrCacheMiss is returned for a
// memcache cache miss. The key must be at most 250 bytes in length.
func (c *Client) Get(key string) (item *Item, err os.Error) {
	if !legalKey(key) {
		return nil, ErrMalformedKey
	}
	addr, err := c.selector.PickServer(key)
	if err != nil {
		return nil, err
	}
	cn, err := c.getConn(addr)
	if err != nil {
		return nil, err
	}
	defer cn.condRelease(&err)

	if _, err = fmt.Fprintf(cn.rw, "gets %s\r\n", key); err != nil {
		return
	}
	if err = cn.rw.Flush(); err != nil {
		return
	}
	if err = parseGetResponse(cn.rw.Reader, func(it *Item) { item = it }); err != nil {
		return
	}
	if item == nil {
		err = ErrCacheMiss
	}
	return
}

// parseGetResponse reads a GET response from r and calls cb for each
// read and allocated Item
func parseGetResponse(r *bufio.Reader, cb func(*Item)) os.Error {
	for {
		line, err := r.ReadSlice('\n')
		if err != nil {
			return err
		}
		if bytes.Equal(line, end) {
			return nil
		}
		it := new(Item)
		var size int
		n, err := fmt.Sscanf(string(line), "VALUE %s %d %d %d\r\n",
			&it.Key, &it.Flags, &size, &it.casid)
		if err != nil {
			return err
		}
		if n != 4 {
			return fmt.Errorf("memcache: unexpected line in get response: %q", string(line))
		}
		it.Value, err = ioutil.ReadAll(io.LimitReader(r, int64(size)+2))
		if err != nil {
			return err
		}
		if !bytes.HasSuffix(it.Value, crlf) {
			return fmt.Errorf("memcache: corrupt get result read")
		}
		it.Value = it.Value[:size]
		cb(it)
	}
	panic("unreached")
}

// Set writes the given item, unconditionally.
func (c *Client) Set(item *Item) os.Error {
	return c.onItem(item, (*Client).set)
}

func (c *Client) set(rw *bufio.ReadWriter, item *Item) os.Error {
	return c.populateOne(rw, "set", item)
}

// Add writes the given item, if no value already exists for its
// key. ErrNotStored is returned if that condition is not met.
func (c *Client) Add(item *Item) os.Error {
	return c.onItem(item, (*Client).add)
}

func (c *Client) add(rw *bufio.ReadWriter, item *Item) os.Error {
	return c.populateOne(rw, "add", item)
}

// CompareAndSwap writes the given item that was previously returned
// by Get, if the value was neither modified or evicted between the
// Get and the CompareAndSwap calls. The item's Key should not change
// between calls but all other item fields may differ. ErrCASConflict
// is returned if the value was modified in between the
// calls. ErrNotStored is returned if the value was evicted in between
// the calls.
func (c *Client) CompareAndSwap(item *Item) os.Error {
	return c.onItem(item, (*Client).cas)
}

func (c *Client) cas(rw *bufio.ReadWriter, item *Item) os.Error {
	return c.populateOne(rw, "cas", item)
}


func (c *Client) populateOne(rw *bufio.ReadWriter, verb string, item *Item) os.Error {
	if !legalKey(item.Key) {
		return ErrMalformedKey
	}
	var err os.Error
	if verb == "cas" {
		_, err = fmt.Fprintf(rw, "%s %s %d %d %d %d %d\r\n",
			verb, item.Key, item.Flags, item.Expiration, len(item.Value), item.casid)
	} else {
		_, err = fmt.Fprintf(rw, "%s %s %d %d %d %d\r\n",
			verb, item.Key, item.Flags, item.Expiration, len(item.Value))
	}
	if err != nil {
		return err
	}
	if _, err = rw.Write(item.Value); err != nil {
		return err
	}
	if _, err := rw.Write(crlf); err != nil {
		return err
	}
	if err := rw.Flush(); err != nil {
		return err
	}
	line, err := rw.ReadSlice('\n')
	if err != nil {
		return err
	}
	switch {
	case bytes.Equal(line, resultStored):
		return nil
	case bytes.Equal(line, resultNotStored):
		return ErrNotStored
	case bytes.Equal(line, resultExists):
		return ErrCASConflict
	case bytes.Equal(line, resultNotFound):
		return ErrCacheMiss
	}
	return fmt.Errorf("memcache: unexpected response line from %q: %q", verb, string(line))
}