memcache

Documentation

Overview

Package memcache provides a client for App Engine's distributed in-memory key-value store for small chunks of arbitrary data.

The fundamental operations get and set items, keyed by a string.

item0, err := memcache.Get(c, "key")
if err != nil && err != memcache.ErrCacheMiss {
	return err
}
if err == nil {
	fmt.Fprintf(w, "memcache hit: Key=%q Val=[% x]\n", item0.Key, item0.Value)
} else {
	fmt.Fprintf(w, "memcache miss\n")
}

and

item1 := &memcache.Item{
	Key:   "foo",
	Value: []byte("bar"),
}
if err := memcache.Set(c, item1); err != nil {
	return err
}

Index

• Variables
• func Add(c appengine.Context, item *Item) error
• func AddMulti(c appengine.Context, item []*Item) error
• func CompareAndSwap(c appengine.Context, item *Item) error
• func CompareAndSwapMulti(c appengine.Context, item []*Item) error
• func Delete(c appengine.Context, key string) error
• func DeleteMulti(c appengine.Context, key []string) error
• func Flush(c appengine.Context) error
• func GetMulti(c appengine.Context, key []string) (map[string]*Item, error)
• func Increment(c appengine.Context, key string, delta int64, initialValue uint64) (newValue uint64, err error)
• func IncrementExisting(c appengine.Context, key string, delta int64) (newValue uint64, err error)
• func Set(c appengine.Context, item *Item) error
• func SetMulti(c appengine.Context, item []*Item) error
• type Codec
  • func (cd Codec) Add(c appengine.Context, item *Item) error
  • func (cd Codec) AddMulti(c appengine.Context, items []*Item) error
  • func (cd Codec) CompareAndSwap(c appengine.Context, item *Item) error
  • func (cd Codec) CompareAndSwapMulti(c appengine.Context, items []*Item) error
  • func (cd Codec) Get(c appengine.Context, key string, v interface{}) (*Item, error)
  • func (cd Codec) Set(c appengine.Context, item *Item) error
  • func (cd Codec) SetMulti(c appengine.Context, items []*Item) error
• type Item
  • func Get(c appengine.Context, key string) (*Item, error)
• type Statistics
  • func Stats(c appengine.Context) (*Statistics, error)

Variables

var (
	// ErrCacheMiss means that an operation failed
	// because the item wasn't present.
	ErrCacheMiss = errors.New("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 = errors.New("memcache: compare-and-swap conflict")
	// ErrNoStats means that no statistics were available.
	ErrNoStats = errors.New("memcache: no statistics available")
	// ErrNotStored means that a conditional write operation (i.e. Add or
	// CompareAndSwap) failed because the condition was not satisfied.
	ErrNotStored = errors.New("memcache: item not stored")
	// ErrServerError means that a server error occurred.
	ErrServerError = errors.New("memcache: server error")
)

var (
	// Gob is a Codec that uses the gob package.
	Gob = Codec{gobMarshal, gobUnmarshal}
	// JSON is a Codec that uses the json package.
	JSON = Codec{json.Marshal, json.Unmarshal}
)

Functions

func Add

func Add(c appengine.Context, item *Item) error

Add writes the given item, if no value already exists for its key. ErrNotStored is returned if that condition is not met.

func AddMulti

func AddMulti(c appengine.Context, item []*Item) error

AddMulti is a batch version of Add. appengine.MultiError may be returned.

func CompareAndSwap

func CompareAndSwap(c appengine.Context, item *Item) error

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 CompareAndSwapMulti

func CompareAndSwapMulti(c appengine.Context, item []*Item) error

CompareAndSwapMulti is a batch version of CompareAndSwap. appengine.MultiError may be returned.

func Delete

func Delete(c appengine.Context, key string) error

Delete deletes the item for the given key. ErrCacheMiss is returned if the specified item can not be found. The key must be at most 250 bytes in length.

func DeleteMulti

func DeleteMulti(c appengine.Context, key []string) error

DeleteMulti is a batch version of Delete. If any keys cannot be found, an appengine.MultiError is returned. Each key must be at most 250 bytes in length.

func Flush

func Flush(c appengine.Context) error

Flush flushes all items from memcache.

func GetMulti

func GetMulti(c appengine.Context, key []string) (map[string]*Item, error)

GetMulti is a batch version of Get. The returned map from keys to items may have fewer elements than the input slice, due to memcache cache misses. Each key must be at most 250 bytes in length.

func Increment

func Increment(c appengine.Context, key string, delta int64, initialValue uint64) (newValue uint64, err error)

Increment atomically increments the decimal value in the given key by delta and returns the new value. The value must fit in a uint64. Overflow wraps around, and underflow is capped to zero. The provided delta may be negative. If the key doesn't exist in memcacheg, the provided initial value is used to atomically populate it before the delta is applied. The key must be at most 250 bytes in length.

func IncrementExisting

func IncrementExisting(c appengine.Context, key string, delta int64) (newValue uint64, err error)

IncrementExisting works like Increment but assumes that the key already exists in memcache and doesn't take an initial value. IncrementExisting can save work if calculating the initial value is expensive. An error is returned if the specified item can not be found.

func Set

func Set(c appengine.Context, item *Item) error

Set writes the given item, unconditionally.

func SetMulti

func SetMulti(c appengine.Context, item []*Item) error

SetMulti is a batch version of Set. appengine.MultiError may be returned.

Types

type Codec

type Codec struct {
	Marshal   func(interface{}) ([]byte, error)
	Unmarshal func([]byte, interface{}) error
}

Codec represents a symmetric pair of functions that implement a codec. Items stored into or retrieved from memcache using a Codec have their values marshaled or unmarshaled.

All the methods provided for Codec behave analogously to the package level function with same name.

func (Codec) Add

func (cd Codec) Add(c appengine.Context, item *Item) error

Add writes the given item, if no value already exists for its key. ErrNotStored is returned if that condition is not met.

func (Codec) AddMulti

func (cd Codec) AddMulti(c appengine.Context, items []*Item) error

AddMulti is a batch version of Add. appengine.MultiError may be returned.

func (Codec) CompareAndSwap

func (cd Codec) CompareAndSwap(c appengine.Context, item *Item) error

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 (Codec) CompareAndSwapMulti

func (cd Codec) CompareAndSwapMulti(c appengine.Context, items []*Item) error

CompareAndSwapMulti is a batch version of CompareAndSwap. appengine.MultiError may be returned.

func (Codec) Get

func (cd Codec) Get(c appengine.Context, key string, v interface{}) (*Item, error)

Get gets the item for the given key and decodes the obtained value into v. ErrCacheMiss is returned for a memcache cache miss. The key must be at most 250 bytes in length.

func (Codec) Set

func (cd Codec) Set(c appengine.Context, item *Item) error

Set writes the given item, unconditionally.

func (Codec) SetMulti

func (cd Codec) SetMulti(c appengine.Context, items []*Item) error

SetMulti is a batch version of Set. appengine.MultiError may be returned.

type Item

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 maximum duration that the item will stay
	// in the cache.
	// The zero value means the Item has no expiration time.
	// Subsecond precision is ignored.
	// This is not set when getting items.
	Expiration time.Duration
	// contains filtered or unexported fields
}

Item is the unit of memcache gets and sets.

func Get

func Get(c appengine.Context, key string) (*Item, error)

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.

type Statistics

type Statistics struct {
	Hits     uint64 // Counter of cache hits
	Misses   uint64 // Counter of cache misses
	ByteHits uint64 // Counter of bytes transferred for gets

	Items uint64 // Items currently in the cache
	Bytes uint64 // Size of all items currently in the cache

	Oldest int64 // Age of access of the oldest item, in seconds
}

Statistics represents a set of statistics about the memcache cache. This may include items that have expired but have not yet been removed from the cache.

func Stats

func Stats(c appengine.Context) (*Statistics, error)

Stats retrieves the current memcache statistics.