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Published: Jan 25, 2015 License: Apache-2.0 Imports: 16 Imported by: 0




groupcache is a caching and cache-filling library, intended as a replacement for memcached in many cases.

For API docs and examples, see

Comparison to memcached

Like memcached, groupcache:
  • shards by key to select which peer is responsible for that key
Unlike memcached, groupcache:
  • does not require running a separate set of servers, thus massively reducing deployment/configuration pain. groupcache is a client library as well as a server. It connects to its own peers.

  • comes with a cache filling mechanism. Whereas memcached just says "Sorry, cache miss", often resulting in a thundering herd of database (or whatever) loads from an unbounded number of clients (which has resulted in several fun outages), groupcache coordinates cache fills such that only one load in one process of an entire replicated set of processes populates the cache, then multiplexes the loaded value to all callers.

  • does not support versioned values. If key "foo" is value "bar", key "foo" must always be "bar". There are neither cache expiration times, nor explicit cache evictions. Thus there is also no CAS, nor Increment/Decrement. This also means that groupcache....

  • ... supports automatic mirroring of super-hot items to multiple processes. This prevents memcached hot spotting where a machine's CPU and/or NIC are overloaded by very popular keys/values.

  • is currently only available for Go. It's very unlikely that I (bradfitz@) will port the code to any other language.

Loading process

In a nutshell, a groupcache lookup of Get("foo") looks like:

(On machine #5 of a set of N machines running the same code)

  1. Is the value of "foo" in local memory because it's super hot? If so, use it.

  2. Is the value of "foo" in local memory because peer #5 (the current peer) is the owner of it? If so, use it.

  3. Amongst all the peers in my set of N, am I the owner of the key "foo"? (e.g. does it consistent hash to 5?) If so, load it. If other callers come in, via the same process or via RPC requests from peers, they block waiting for the load to finish and get the same answer. If not, RPC to the peer that's the owner and get the answer. If the RPC fails, just load it locally (still with local dup suppression).


groupcache is in production use by (its original user), parts of Blogger, parts of Google Code, parts of Google Fiber, parts of Google production monitoring systems, etc.




Use the golang-nuts mailing list for any discussion or questions.



Package groupcache provides a data loading mechanism with caching and de-duplication that works across a set of peer processes.

Each data Get first consults its local cache, otherwise delegates to the requested key's canonical owner, which then checks its cache or finally gets the data. In the common case, many concurrent cache misses across a set of peers for the same key result in just one cache fill.



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func RegisterNewGroupHook

func RegisterNewGroupHook(fn func(*Group))

RegisterNewGroupHook registers a hook that is run each time a group is created.

func RegisterPeerPicker

func RegisterPeerPicker(fn func() PeerPicker)

RegisterPeerPicker registers the peer initialization function. It is called once, when the first group is created.

func RegisterServerStart

func RegisterServerStart(fn func())

RegisterServerStart registers a hook that is run when the first group is created.


type AtomicInt

type AtomicInt int64

An AtomicInt is an int64 to be accessed atomically.

func (*AtomicInt) Add

func (i *AtomicInt) Add(n int64)

Add atomically adds n to i.

func (*AtomicInt) Get

func (i *AtomicInt) Get() int64

Get atomically gets the value of i.

func (*AtomicInt) String

func (i *AtomicInt) String() string

type ByteView

type ByteView struct {
	// contains filtered or unexported fields

A ByteView holds an immutable view of bytes. Internally it wraps either a []byte or a string, but that detail is invisible to callers.

A ByteView is meant to be used as a value type, not a pointer (like a time.Time).

func (ByteView) At

func (v ByteView) At(i int) byte

At returns the byte at index i.

func (ByteView) ByteSlice

func (v ByteView) ByteSlice() []byte

ByteSlice returns a copy of the data as a byte slice.

func (ByteView) Copy

func (v ByteView) Copy(dest []byte) int

Copy copies b into dest and returns the number of bytes copied.

func (ByteView) Equal

func (v ByteView) Equal(b2 ByteView) bool

Equal returns whether the bytes in b are the same as the bytes in b2.

func (ByteView) EqualBytes

func (v ByteView) EqualBytes(b2 []byte) bool

EqualBytes returns whether the bytes in b are the same as the bytes in b2.

func (ByteView) EqualString

func (v ByteView) EqualString(s string) bool

EqualString returns whether the bytes in b are the same as the bytes in s.

func (ByteView) Len

func (v ByteView) Len() int

Len returns the view's length.

func (ByteView) ReadAt

func (v ByteView) ReadAt(p []byte, off int64) (n int, err error)

ReadAt implements io.ReaderAt on the bytes in v.

func (ByteView) Reader

func (v ByteView) Reader() io.ReadSeeker

Reader returns an io.ReadSeeker for the bytes in v.

func (ByteView) Slice

func (v ByteView) Slice(from, to int) ByteView

Slice slices the view between the provided from and to indices.

func (ByteView) SliceFrom

func (v ByteView) SliceFrom(from int) ByteView

SliceFrom slices the view from the provided index until the end.

func (ByteView) String

func (v ByteView) String() string

String returns the data as a string, making a copy if necessary.

type CacheStats

type CacheStats struct {
	Bytes     int64
	Items     int64
	Gets      int64
	Hits      int64
	Evictions int64

CacheStats are returned by stats accessors on Group.

type CacheType

type CacheType int

CacheType represents a type of cache.

const (
	// The MainCache is the cache for items that this peer is the
	// owner for.
	MainCache CacheType = iota + 1

	// The HotCache is the cache for items that seem popular
	// enough to replicate to this node, even though it's not the
	// owner.

type Context

type Context interface{}

Context is an opaque value passed through calls to the ProtoGetter. It may be nil if your ProtoGetter implementation does not require a context.

type Getter

type Getter interface {
	// Get returns the value identified by key, populating dest.
	// The returned data must be unversioned. That is, key must
	// uniquely describe the loaded data, without an implicit
	// current time, and without relying on cache expiration
	// mechanisms.
	Get(ctx Context, key string, dest Sink) error

A Getter loads data for a key.

type GetterFunc

type GetterFunc func(ctx Context, key string, dest Sink) error

A GetterFunc implements Getter with a function.

func (GetterFunc) Get

func (f GetterFunc) Get(ctx Context, key string, dest Sink) error

type Group

type Group struct {

	// Stats are statistics on the group.
	Stats Stats
	// contains filtered or unexported fields

A Group is a cache namespace and associated data loaded spread over a group of 1 or more machines.

func GetGroup

func GetGroup(name string) *Group

GetGroup returns the named group previously created with NewGroup, or nil if there's no such group.

func NewGroup

func NewGroup(name string, cacheBytes int64, getter Getter) *Group

NewGroup creates a coordinated group-aware Getter from a Getter.

The returned Getter tries (but does not guarantee) to run only one Get call at once for a given key across an entire set of peer processes. Concurrent callers both in the local process and in other processes receive copies of the answer once the original Get completes.

The group name must be unique for each getter.

func (*Group) CacheStats

func (g *Group) CacheStats(which CacheType) CacheStats

CacheStats returns stats about the provided cache within the group.

func (*Group) Get

func (g *Group) Get(ctx Context, key string, dest Sink) error

func (*Group) Name

func (g *Group) Name() string

Name returns the name of the group.

type HTTPPool

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
	// contains filtered or unexported fields

HTTPPool implements PeerPicker for a pool of HTTP peers.

func NewHTTPPool

func NewHTTPPool(self string) *HTTPPool

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 "".

func NewHTTPPoolOpts

func NewHTTPPoolOpts(self string, o *HTTPPoolOptions) *HTTPPool

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 (*HTTPPool) PickPeer

func (p *HTTPPool) PickPeer(key string) (ProtoGetter, bool)

func (*HTTPPool) ServeHTTP

func (p *HTTPPool) ServeHTTP(w http.ResponseWriter, r *http.Request)

func (*HTTPPool) Set

func (p *HTTPPool) Set(peers ...string)

Set updates the pool's list of peers. Each peer value should be a valid base URL, for example "".

type HTTPPoolOptions

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

HTTPPoolOptions are the configurations of a HTTPPool.

type NoPeers

type NoPeers struct{}

NoPeers is an implementation of PeerPicker that never finds a peer.

func (NoPeers) PickPeer

func (NoPeers) PickPeer(key string) (peer ProtoGetter, ok bool)

type PeerPicker

type PeerPicker interface {
	// PickPeer returns the peer that owns the specific key
	// and true to indicate that a remote peer was nominated.
	// It returns nil, false if the key owner is the current peer.
	PickPeer(key string) (peer ProtoGetter, ok bool)

PeerPicker is the interface that must be implemented to locate the peer that owns a specific key.

type ProtoGetter

type ProtoGetter interface {
	Get(context Context, in *pb.GetRequest, out *pb.GetResponse) error

ProtoGetter is the interface that must be implemented by a peer.

type Sink

type Sink interface {
	// SetString sets the value to s.
	SetString(s string) error

	// SetBytes sets the value to the contents of v.
	// The caller retains ownership of v.
	SetBytes(v []byte) error

	// SetProto sets the value to the encoded version of m.
	// The caller retains ownership of m.
	SetProto(m proto.Message) error
	// contains filtered or unexported methods

A Sink receives data from a Get call.

Implementation of Getter must call exactly one of the Set methods on success.

func AllocatingByteSliceSink

func AllocatingByteSliceSink(dst *[]byte) Sink

AllocatingByteSliceSink returns a Sink that allocates a byte slice to hold the received value and assigns it to *dst. The memory is not retained by groupcache.

func ByteViewSink

func ByteViewSink(dst *ByteView) Sink

ByteViewSink returns a Sink that populates a ByteView.

func ProtoSink

func ProtoSink(m proto.Message) Sink

ProtoSink returns a sink that unmarshals binary proto values into m.

func StringSink

func StringSink(sp *string) Sink

StringSink returns a Sink that populates the provided string pointer.

func TruncatingByteSliceSink

func TruncatingByteSliceSink(dst *[]byte) Sink

TruncatingByteSliceSink returns a Sink that writes up to len(*dst) bytes to *dst. If more bytes are available, they're silently truncated. If fewer bytes are available than len(*dst), *dst is shrunk to fit the number of bytes available.

type Stats

type Stats struct {
	Gets           AtomicInt // any Get request, including from peers
	CacheHits      AtomicInt // either cache was good
	PeerLoads      AtomicInt // either remote load or remote cache hit (not an error)
	PeerErrors     AtomicInt
	Loads          AtomicInt // (gets - cacheHits)
	LoadsDeduped   AtomicInt // after singleflight
	LocalLoads     AtomicInt // total good local loads
	LocalLoadErrs  AtomicInt // total bad local loads
	ServerRequests AtomicInt // gets that came over the network from peers

Stats are per-group statistics.


Path Synopsis
Package consistenthash provides an implementation of a ring hash.
Package consistenthash provides an implementation of a ring hash.
Package lru implements an LRU cache.
Package lru implements an LRU cache.
Package singleflight provides a duplicate function call suppression mechanism.
Package singleflight provides a duplicate function call suppression mechanism.

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