groupcache

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 Go to latest Published: Sep 24, 2013 License: Apache-2.0 Imports: 19 Imported by: 1

README

groupcache

Summary

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

For API docs and examples, see http://godoc.org/github.com/golang/groupcache

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).

Users

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

Presentations

See http://talks.golang.org/2013/oscon-dl.slide

Help

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

Documentation

Overview

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.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

func GetTime 

func GetTime() int64

GetTime should be called to produce a new timestamp to supply for Group.SetTimeStampBytes()

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.

func UnpackTimestamp 

func UnpackTimestamp(b []byte) (result []byte, timestamp int64, err error)

UnpackTimestamp unpacks the original data and the timestamp encoded with Timestamp() or PackTimestamp(). Used when cache expiration functionality is needed. See Group.SetExpiration() for more.

Types

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.
	HotCache
)

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.

func (*Group) SetDisableHotCache 

func (g *Group) SetDisableHotCache(disable bool) *Group

SetDisableHotCache controls disabling of writes to the hotCache. May be used with expiration functionality to prevent clients from getting inconsistent data freshness due to unsynchronized clocks in a multi-peer setup. With the hotCache disabled, there is only ever one server in a system where an item may be cached (the authorative server for a given key). With the hotCache disabled, if the authoritative server for a given key goes down, its peers will generate the value locally, but will not cache it (otherwise the value would be generated locally and cached locally). Writes to the hotCache are enabled by default.

func (*Group) SetExpiration 

func (g *Group) SetExpiration(d time.Duration) *Group

SetExpiration sets the cache expiration time. Internally the duration is truncated to seconds, so it's not useful to set with greater precision. If unset or set to 0, will cache items forever. To effectively disable caching, set to 1ns.

In getter, set timestamp using SetTimestampBytes(): 

func(gctx groupcache.Context, key string, dest groupcache.Sink) error {
   content = MakeMyContent(key)
   return dest.SetTimestampBytes(content, groupcache.GetTime())
 }

In call to Get() use AllocatingByteSliceSink, StringSink, or ByteViewSink, then unpack returned result. Using an unsupported sink implementation will produce an error when calling Get().

var packedContent []byte var timestamp int64 

if err := gcache.Get(nil, key, groupcache.AllocatingByteSliceSink(&packedContent)); err != nil {
	handleErr()
}

content, timestamp, err := groupcache.UnpackTimestamp(packedContent)
if err != nil {
  handleErr()
}

func (*Group) SetStaleDeadline 

func (g *Group) SetStaleDeadline(d time.Duration) *Group

SetStaleDeadline sets the deadline during the stale period for a background reload after which stale data will be returned if fresh data is not ready.

func (*Group) SetStalePeriod 

func (g *Group) SetStalePeriod(d time.Duration) *Group

SetStalePeriod sets the duration after expiration in which stale data may be served. See SetExpiration() for details. Internally this is truncated to seconds, so it's not useful to set with greater precision.

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. It registers itself as a PeerPicker and as an HTTP handler with the http.DefaultServeMux. The self argument be a valid base URL that points to the current server, for example "http://example.net:8000".

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 "http://example.net:8000".

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

	// SetTimestampBytes sets the value to the contents of v, appending the
	// supplied Unix epoch timestamp in seconds.
	SetTimestampBytes(v []byte, timestamp int64) 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.

type TimeProvider 

type TimeProvider interface {
	// Return the current Unix epoch time in seconds.
	NowUnixSeconds() int64

	// Return the time on a channel after delay d.
	After(d time.Duration) <-chan time.Time
}

Source Files

View all Source files

Directories

Path Synopsis
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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