go-cache/sharded.go

package cache

import (
	"encoding/binary"
	"hash/fnv"
	"runtime"
	"time"
)

// This is an experimental and unexported (for now) attempt at making a cache
// with better algorithmic complexity than the standard one, namely by
// preventing write locks of the entire cache when an item is added. As of the
// time of writing, the overhead of selecting buckets is much too great to be
// worth it except perhaps with extremely large caches.
//
// See cache_test.go for a few benchmarks.

type unexportedShardedCache struct {
	*shardedCache
}

type shardedCache struct {
	m       uint32
	cs      []*cache
	janitor *shardedJanitor
}

func (sc *shardedCache) bucket(k string) *cache {
	h := fnv.New32()
	h.Write([]byte(k))
	n := binary.BigEndian.Uint32(h.Sum(nil))
	return sc.cs[n%sc.m]
}

func (sc *shardedCache) Set(k string, x interface{}, d time.Duration) {
	sc.bucket(k).Set(k, x, d)
}

func (sc *shardedCache) Add(k string, x interface{}, d time.Duration) error {
	return sc.bucket(k).Add(k, x, d)
}

func (sc *shardedCache) Replace(k string, x interface{}, d time.Duration) error {
	return sc.bucket(k).Replace(k, x, d)
}

func (sc *shardedCache) Get(k string) (interface{}, bool) {
	return sc.bucket(k).Get(k)
}

func (sc *shardedCache) Increment(k string, n int64) error {
	return sc.bucket(k).Increment(k, n)
}

func (sc *shardedCache) IncrementFloat(k string, n float64) error {
	return sc.bucket(k).IncrementFloat(k, n)
}

func (sc *shardedCache) Decrement(k string, n int64) error {
	return sc.bucket(k).Decrement(k, n)
}

func (sc *shardedCache) Delete(k string) {
	sc.bucket(k).Delete(k)
}

func (sc *shardedCache) DeleteExpired() {
	for _, v := range sc.cs {
		v.DeleteExpired()
	}
}

func (sc *shardedCache) Flush() {
	for _, v := range sc.cs {
		v.Flush()
	}
}

type shardedJanitor struct {
	Interval time.Duration
	stop     chan bool
}

func (j *shardedJanitor) Run(sc *shardedCache) {
	j.stop = make(chan bool)
	tick := time.Tick(j.Interval)
	for {
		select {
		case <-tick:
			sc.DeleteExpired()
		case <-j.stop:
			return
		}
	}
}

func stopShardedJanitor(sc *unexportedShardedCache) {
	sc.janitor.stop <- true
}

func runShardedJanitor(sc *shardedCache, ci time.Duration) {
	j := &shardedJanitor{
		Interval: ci,
	}
	sc.janitor = j
	go j.Run(sc)
}

func newShardedCache(n int, de time.Duration) *shardedCache {
	sc := &shardedCache{
		m:  uint32(n - 1),
		cs: make([]*cache, n),
	}
	for i := 0; i < n; i++ {
		c := &cache{
			defaultExpiration: de,
			items:             map[string]*Item{},
		}
		sc.cs[i] = c
	}
	return sc
}

func unexportedNewSharded(shards int, defaultExpiration, cleanupInterval time.Duration) *unexportedShardedCache {
	if defaultExpiration == 0 {
		defaultExpiration = -1
	}
	sc := newShardedCache(shards, defaultExpiration)
	SC := &unexportedShardedCache{sc}
	if cleanupInterval > 0 {
		runShardedJanitor(sc, cleanupInterval)
		runtime.SetFinalizer(SC, stopShardedJanitor)
	}
	return SC
}