groupcache

package module
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 Go to latest Published: Jun 12, 2023 License: Apache-2.0 Imports: 19 Imported by: 56

README

groupcache

A modified version of group cache with support for context.Context, go modules, and explicit key removal and expiration. See the CHANGELOG for a complete list of modifications.

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/mailgun/groupcache/v2

Modifications from original library

  • Support for explicit key removal from a group. Remove() requests are first sent to the peer who owns the key, then the remove request is forwarded to every peer in the groupcache. NOTE: This is a best case design since it is possible a temporary network disruption could occur resulting in remove requests never making it their peers. In practice this scenario is very rare and the system remains very consistent. In case of an inconsistency placing a expiration time on your values will ensure the cluster eventually becomes consistent again.

  • Support for expired values. SetBytes(), SetProto() and SetString() now accept an optional time.Time which represents a time in the future when the value will expire. If you don't want expiration, pass the zero value for time.Time (for instance, time.Time{}). Expiration is handled by the LRU Cache when a Get() on a key is requested. This means no network coordination of expired values is needed. However this does require that time on all nodes in the cluster is synchronized for consistent expiration of values.

  • Now always populating the hotcache. A more complex algorithm is unnecessary when the LRU cache will ensure the most used values remain in the cache. The evict code ensures the hotcache never overcrowds the maincache.

Comparing Groupcache 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".

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).

Example

import (
    "context"
    "fmt"
    "log"
    "time"

    "github.com/mailgun/groupcache/v2"
)

func ExampleUsage() {

    // NOTE: It is important to pass the same peer `http://192.168.1.1:8080` to `NewHTTPPoolOpts`
    // which is provided to `pool.Set()` so the pool can identify which of the peers is our instance.
    // The pool will not operate correctly if it can't identify which peer is our instance.
    
    // Pool keeps track of peers in our cluster and identifies which peer owns a key.
    pool := groupcache.NewHTTPPoolOpts("http://192.168.1.1:8080", &groupcache.HTTPPoolOptions{})

    // Add more peers to the cluster You MUST Ensure our instance is included in this list else
    // determining who owns the key accross the cluster will not be consistent, and the pool won't
    // be able to determine if our instance owns the key.
    pool.Set("http://192.168.1.1:8080", "http://192.168.1.2:8080", "http://192.168.1.3:8080")

    server := http.Server{
        Addr:    "192.168.1.1:8080",
        Handler: pool,
    }

    // Start a HTTP server to listen for peer requests from the groupcache
    go func() {
        log.Printf("Serving....\n")
        if err := server.ListenAndServe(); err != nil {
            log.Fatal(err)
        }
    }()
    defer server.Shutdown(context.Background())

    // Create a new group cache with a max cache size of 3MB
    group := groupcache.NewGroup("users", 3000000, groupcache.GetterFunc(
        func(ctx context.Context, id string, dest groupcache.Sink) error {

            // Returns a protobuf struct `User`
            user, err := fetchUserFromMongo(ctx, id)
            if err != nil {
                return err
            }

            // Set the user in the groupcache to expire after 5 minutes
            return dest.SetProto(&user, time.Now().Add(time.Minute*5))
        },
    ))

    var user User

    ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*500)
    defer cancel()

    if err := group.Get(ctx, "12345", groupcache.ProtoSink(&user)); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("-- User --\n")
    fmt.Printf("Id: %s\n", user.Id)
    fmt.Printf("Name: %s\n", user.Name)
    fmt.Printf("Age: %d\n", user.Age)
    fmt.Printf("IsSuper: %t\n", user.IsSuper)

    // Remove the key from the groupcache
    if err := group.Remove(ctx, "12345"); err != nil {
        log.Fatal(err)
    }
}
Note

The call to groupcache.NewHTTPPoolOpts() is a bit misleading. NewHTTPPoolOpts() creates a new pool internally within the groupcache package where it is uitilized by any groups created. The pool returned is only a pointer to the internallly registered pool so the caller can update the peers in the pool as needed.

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

View Source 
var NowFunc lru.NowFunc = time.Now

NowFunc returns the current time which is used by the LRU to determine if the value has expired. This can be overridden by tests to ensure items are evicted when expired.

Functions

func DeregisterGroup added in v2.2.0 

func DeregisterGroup(name string)

DeregisterGroup removes group from group pool

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. Either RegisterPeerPicker or RegisterPerGroupPeerPicker should be called exactly once, but not both.

func RegisterPerGroupPeerPicker 

func RegisterPerGroupPeerPicker(fn func(groupName string) PeerPicker)

RegisterPerGroupPeerPicker registers the peer initialization function, which takes the groupName, to be used in choosing a PeerPicker. It is called once, when the first group is created. Either RegisterPeerPicker or RegisterPerGroupPeerPicker should be called exactly once, but not both.

func RegisterServerStart 

func RegisterServerStart(fn func())

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

func SetLogger added in v2.2.0 

func SetLogger(log *logrus.Entry)

SetLogger - this is legacy to provide backwards compatibility with logrus.

func SetLoggerFromLogger added in v2.3.3 

func SetLoggerFromLogger(log Logger)

SetLoggerFromLogger - set the logger to an implementation of the Logger interface

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) Store added in v2.2.0 

func (i *AtomicInt) Store(n int64)

Store atomically stores n to 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) Expire 

func (v ByteView) Expire() time.Time

Returns the expire time associated with this view

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.

func (ByteView) WriteTo 

func (v ByteView) WriteTo(w io.Writer) (n int64, err error)

WriteTo implements io.WriterTo on the bytes in v.

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 ErrNotFound added in v2.5.0 

type ErrNotFound struct {
	Msg string
}

ErrNotFound should be returned from an implementation of `GetterFunc` to indicate the requested value is not available. When remote HTTP calls are made to retrieve values from other groupcache instances, returning this error will indicate to groupcache that the value requested is not available, and it should NOT attempt to call `GetterFunc` locally.

func (*ErrNotFound) Error added in v2.5.0 

func (e *ErrNotFound) Error() string

func (*ErrNotFound) Is added in v2.5.0 

func (e *ErrNotFound) Is(target error) bool

type ErrRemoteCall added in v2.5.0 

type ErrRemoteCall struct {
	Msg string
}

ErrRemoteCall is returned from `group.Get()` when an error that is not a `ErrNotFound` is returned during a remote HTTP instance call

func (*ErrRemoteCall) Error added in v2.5.0 

func (e *ErrRemoteCall) Error() string

func (*ErrRemoteCall) Is added in v2.5.0 

func (e *ErrRemoteCall) Is(target error) bool

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.Context, key string, dest Sink) error
}

A Getter loads data for a key.

type GetterFunc 

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

A GetterFunc implements Getter with a function.

func (GetterFunc) Get 

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

func (*Group) Name 

func (g *Group) Name() string

Name returns the name of the group.

func (*Group) Remove 

func (g *Group) Remove(ctx context.Context, key string) error

Remove clears the key from our cache then forwards the remove request to all peers.

func (*Group) Set added in v2.3.0 

func (g *Group) Set(ctx context.Context, key string, value []byte, expire time.Time, hotCache bool) error

type HTTPPool 

type HTTPPool struct {
	// 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 should be a valid base URL that points to the current server, for example "http://example.net:8000".

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) GetAll 

func (p *HTTPPool) GetAll() []ProtoGetter

GetAll returns all the peers in the pool

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 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

	// 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.Context) http.RoundTripper

	// Context optionally specifies a context for the server to use when it
	// receives a request.
	// If nil, uses the http.Request.Context()
	Context func(*http.Request) context.Context
}

HTTPPoolOptions are the configurations of a HTTPPool.

type Logger added in v2.3.3 

type Logger interface {
	// Error logging level
	Error() Logger

	// Warn logging level
	Warn() Logger

	// Info logging level
	Info() Logger

	// Debug logging level
	Debug() Logger

	// ErrorField is a field with an error value
	ErrorField(label string, err error) Logger

	// StringField is a field with a string value
	StringField(label string, val string) Logger

	// WithFields is willy-nilly key value pairs.
	WithFields(fields map[string]interface{}) Logger

	// Printf is called last to emit the log at the given
	// level.
	Printf(format string, args ...interface{})
}

Logger is a minimal interface that will allow us to use structured loggers, including (but not limited to) logrus.

type LogrusLogger added in v2.3.3 

type LogrusLogger struct {
	Entry *logrus.Entry
	// contains filtered or unexported fields
}

LogrusLogger is an implementation of Logger that wraps logrus... who knew?

func (LogrusLogger) Debug added in v2.3.3 

func (l LogrusLogger) Debug() Logger

func (LogrusLogger) Error added in v2.3.3 

func (l LogrusLogger) Error() Logger

func (LogrusLogger) ErrorField added in v2.3.3 

func (l LogrusLogger) ErrorField(label string, err error) Logger

ErrorField - create a field for an error

func (LogrusLogger) Info added in v2.3.3 

func (l LogrusLogger) Info() Logger

func (LogrusLogger) Printf added in v2.3.3 

func (l LogrusLogger) Printf(format string, args ...interface{})

func (LogrusLogger) StringField added in v2.3.3 

func (l LogrusLogger) StringField(label string, val string) Logger

StringField - create a field for a string.

func (LogrusLogger) Warn added in v2.3.3 

func (l LogrusLogger) Warn() Logger

func (LogrusLogger) WithFields added in v2.3.3 

func (l LogrusLogger) WithFields(fields map[string]interface{}) Logger

type NoPeers 

type NoPeers struct{}

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

func (NoPeers) GetAll 

func (NoPeers) GetAll() []ProtoGetter

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)
	// GetAll returns all the peers in the group
	GetAll() []ProtoGetter
}

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.Context, in *pb.GetRequest, out *pb.GetResponse) error
	Remove(context context.Context, in *pb.GetRequest) error
	Set(context context.Context, in *pb.SetRequest) error
	// GetURL returns the peer URL
	GetURL() string
}

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, e time.Time) error

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

	// SetProto sets the value to the encoded version of m.
	// The caller retains ownership of m.
	SetProto(m proto.Message, e time.Time) 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.

`e` sets an optional time in the future when the value will expire. If you don't want expiration, pass the zero value for `time.Time` (for instance, `time.Time{}`).

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
	GetFromPeersLatencyLower AtomicInt // slowest duration to request value from peers
	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.

Source Files

View all Source files

Directories

Path Synopsis
cmd
server
Package consistenthash provides an implementation of a ring hash.
 Package consistenthash provides an implementation of a ring hash.
Package groupcachepb is a generated protocol buffer package.
 Package groupcachepb is a generated protocol buffer package.
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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