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cache2go-源码阅读

作者:互联网

简介

cache2go 是非常简短的 go 开源项目了,很适合作为第一个读源码项目。

如果你有一定的 go 开发经验,读起来会感觉到比较容易。

如果你刚刚接触 go 语音,基础知识还不完全了解,希望阅读本文时,遇到一个不会的知识点,去攻克一个,带着为了看懂本文源码的目的去学习基础知识。比如:

作者这样介绍:Concurrency-safe golang caching library with expiration capabilities,简单来说就是具有过期功能的并发安全 golang 缓存库,因此它具有两大特性:

项目结构

该项目非常简单,全部逻辑由三个文件实现:

数据结构图:

接下来会自下而上地分析源码。

cacheitem.go

该文件中包含两块重要内容:

CacheItem

CacheItem 用来缓存表中的一个条目,属性解释:

源码如下:

// CacheItem is an individual cache item
// Parameter data contains the user-set value in the cache.
type CacheItem struct {
	sync.RWMutex

	// The item's key.
	key interface{}
	// The item's data.
	data interface{}
	// How long will the item live in the cache when not being accessed/kept alive.
	lifeSpan time.Duration

	// Creation timestamp.
	createdOn time.Time
	// Last access timestamp.
	accessedOn time.Time
	// How often the item was accessed.
	accessCount int64

	// Callback method triggered right before removing the item from the cache
	aboutToExpire []func(key interface{})
}

Get 方法

下面是一些比较简单的 Get 方法,一些有写场景的属性会多两行获取锁与释放锁的代码。

// LifeSpan returns this item's expiration duration.
func (item *CacheItem) LifeSpan() time.Duration {
	// immutable
	return item.lifeSpan
}

// AccessedOn returns when this item was last accessed.
func (item *CacheItem) AccessedOn() time.Time {
	item.RLock()
	defer item.RUnlock()
	return item.accessedOn
}

// CreatedOn returns when this item was added to the cache.
func (item *CacheItem) CreatedOn() time.Time {
	// immutable
	return item.createdOn
}

// AccessCount returns how often this item has been accessed.
func (item *CacheItem) AccessCount() int64 {
	item.RLock()
	defer item.RUnlock()
	return item.accessCount
}

// Key returns the key of this cached item.
func (item *CacheItem) Key() interface{} {
	// immutable
	return item.key
}

// Data returns the value of this cached item.
func (item *CacheItem) Data() interface{} {
	// immutable
	return item.data
}

KeepAlive

保活函数:

// KeepAlive marks an item to be kept for another expireDuration period.
func (item *CacheItem) KeepAlive() {
	item.Lock()
	defer item.Unlock()
	item.accessedOn = time.Now()
	item.accessCount++
}

AddAboutToExpireCallback

新增回调函数,回调函数无返回值,仅有一个参数 interface{},即支持任意的参数。

// AddAboutToExpireCallback appends a new callback to the AboutToExpire queue
func (item *CacheItem) AddAboutToExpireCallback(f func(interface{})) {
	item.Lock()
	defer item.Unlock()
	item.aboutToExpire = append(item.aboutToExpire, f)
}

SetAboutToExpireCallback

设置回调函数需要完全替代,不同于新增,需要先清空,再覆盖。

// SetAboutToExpireCallback configures a callback, which will be called right
// before the item is about to be removed from the cache.
func (item *CacheItem) SetAboutToExpireCallback(f func(interface{})) {
	if len(item.aboutToExpire) > 0 {
		item.RemoveAboutToExpireCallback()
	}
	item.Lock()
	defer item.Unlock()
	item.aboutToExpire = append(item.aboutToExpire, f)
}

RemoveAboutToExpireCallback

通过直接置空,删除所有的回调函数。

// RemoveAboutToExpireCallback empties the about to expire callback queue
func (item *CacheItem) RemoveAboutToExpireCallback() {
	item.Lock()
	defer item.Unlock()
	item.aboutToExpire = nil
}

NewCacheItem

创建 CacheItem 实例

func NewCacheItem(key interface{}, lifeSpan time.Duration, data interface{}) *CacheItem {
	t := time.Now()
	return &CacheItem{
		key:           key,
		lifeSpan:      lifeSpan,
		createdOn:     t,
		accessedOn:    t,
		accessCount:   0,
		aboutToExpire: nil,
		data:          data,
	}
}

cachetable.go

该文件中总共有 3 个类:CacheTable、CacheItemPair 和 CacheItemPairList。

下面由简单到复杂逐个分析。

CacheItemPair

CacheItemPair 用来记录缓存访问的次数。

// CacheItemPair maps key to access counter
type CacheItemPair struct {
	Key         interface{}
	AccessCount int64
}

CacheItemPairList

CacheItemPairList 是 CacheItemPair 的切片,通过实现方法 Swap、Len 和 Less 实现了 sort.Interface,支持排序。

需要注意方法 Less 的实现,是元素 i 大于元素 j,这种实现是为了降序排序。降序排序是为了方法 CacheTable.MostAccessed 返回访问次数最多的条目列表。

// CacheItemPairList is a slice of CacheItemPairs that implements sort.
// Interface to sort by AccessCount.
type CacheItemPairList []CacheItemPair

func (p CacheItemPairList) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
func (p CacheItemPairList) Len() int           { return len(p) }
func (p CacheItemPairList) Less(i, j int) bool { return p[i].AccessCount > p[j].AccessCount }

CacheTable

CacheTable 用来缓存一个表,属性解释:

源码如下:

// CacheTable is a table within the cache
type CacheTable struct {
	sync.RWMutex

	// The table's name.
	name string
	// All cached items.
	items map[interface{}]*CacheItem

	// Timer responsible for triggering cleanup.
	cleanupTimer *time.Timer
	// Current timer duration.
	cleanupInterval time.Duration

	// The logger used for this table.
	logger *log.Logger

	// Callback method triggered when trying to load a non-existing key.
	loadData func(key interface{}, args ...interface{}) *CacheItem
	// Callback method triggered when adding a new item to the cache.
	addedItem []func(item *CacheItem)
	// Callback method triggered before deleting an item from the cache.
	aboutToDeleteItem []func(item *CacheItem)
}

下面会先介绍核心方法,再看简单的方法。

Add 新增条目

代码逻辑通过流程图描述了一下,其中的「过期检查」单独抽出来后面分析。

NotFoundAdd 和 Add 核心逻辑是一样的,具体区别不做额外描述,源代码如下:

// Add adds a key/value pair to the cache.
// Parameter key is the item's cache-key.
// Parameter lifeSpan determines after which time period without an access the item
// will get removed from the cache.
// Parameter data is the item's value.
func (table *CacheTable) Add(key interface{}, lifeSpan time.Duration, data interface{}) *CacheItem {
	item := NewCacheItem(key, lifeSpan, data)

	// Add item to cache.
	table.Lock()
	table.addInternal(item)

	return item
}

func (table *CacheTable) addInternal(item *CacheItem) {
	// Careful: do not run this method unless the table-mutex is locked!
	// It will unlock it for the caller before running the callbacks and checks
	table.log("Adding item with key", item.key, "and lifespan of", item.lifeSpan, "to table", table.name)
	table.items[item.key] = item

	// Cache values so we don't keep blocking the mutex.
	expDur := table.cleanupInterval
	addedItem := table.addedItem
	table.Unlock()

	// Trigger callback after adding an item to cache.
	if addedItem != nil {
		for _, callback := range addedItem {
			callback(item)
		}
	}

	// If we haven't set up any expiration check timer or found a more imminent item.
	if item.lifeSpan > 0 && (expDur == 0 || item.lifeSpan < expDur) {
		table.expirationCheck()
	}
}

// NotFoundAdd checks whether an item is not yet cached. Unlike the Exists
// method this also adds data if the key could not be found.
func (table *CacheTable) NotFoundAdd(key interface{}, lifeSpan time.Duration, data interface{}) bool {
	table.Lock()

	if _, ok := table.items[key]; ok {
		table.Unlock()
		return false
	}

	item := NewCacheItem(key, lifeSpan, data)
	table.addInternal(item)

	return true
}

expirationCheck 过期检查

过期检查的处理,是一个值得学习的点,这里并不是我们印象中用循环定期扫描哪些 key 过期了,也不是给每个条目分别定义一个定时器。

每次新增条目时,扫描得到最近过期条目的过期时间,仅定义一个定时器。该定时器触发时清除缓存,并生成下一个定时器,如此接力处理。

过期检查中会调用方法 table.deleteInternal 来清除过期的 key,这块儿在讲 Delete 方法时会再详细分析。

// Expiration check loop, triggered by a self-adjusting timer.
func (table *CacheTable) expirationCheck() {
	table.Lock()
	if table.cleanupTimer != nil {
		table.cleanupTimer.Stop()
	}
	if table.cleanupInterval > 0 {
		table.log("Expiration check triggered after", table.cleanupInterval, "for table", table.name)
	} else {
		table.log("Expiration check installed for table", table.name)
	}

	// To be more accurate with timers, we would need to update 'now' on every
	// loop iteration. Not sure it's really efficient though.
	now := time.Now()
	smallestDuration := 0 * time.Second
	for key, item := range table.items {
		// Cache values so we don't keep blocking the mutex.
		item.RLock()
		lifeSpan := item.lifeSpan
		accessedOn := item.accessedOn
		item.RUnlock()

		if lifeSpan == 0 {
			continue
		}
		if now.Sub(accessedOn) >= lifeSpan {
			// Item has excessed its lifespan.
			table.deleteInternal(key)
		} else {
			// Find the item chronologically closest to its end-of-lifespan.
			if smallestDuration == 0 || lifeSpan-now.Sub(accessedOn) < smallestDuration {
				smallestDuration = lifeSpan - now.Sub(accessedOn)
			}
		}
	}

	// Setup the interval for the next cleanup run.
	table.cleanupInterval = smallestDuration
	if smallestDuration > 0 {
		table.cleanupTimer = time.AfterFunc(smallestDuration, func() {
			go table.expirationCheck()
		})
	}
	table.Unlock()
}

Delete 方法

从流程图可以看出,这块儿大部分逻辑是在加锁、释放锁,有这么多锁主要是有如下几个原因:

// Delete an item from the cache.
func (table *CacheTable) Delete(key interface{}) (*CacheItem, error) {
	table.Lock()
	defer table.Unlock()

	return table.deleteInternal(key)
}

func (table *CacheTable) deleteInternal(key interface{}) (*CacheItem, error) {
	r, ok := table.items[key]
	if !ok {
		return nil, ErrKeyNotFound
	}

	// Cache value so we don't keep blocking the mutex.
	aboutToDeleteItem := table.aboutToDeleteItem
	table.Unlock()

	// Trigger callbacks before deleting an item from cache.
	if aboutToDeleteItem != nil {
		for _, callback := range aboutToDeleteItem {
			callback(r)
		}
	}

	r.RLock()
	defer r.RUnlock()
	if r.aboutToExpire != nil {
		for _, callback := range r.aboutToExpire {
			callback(key)
		}
	}

	table.Lock()
	table.log("Deleting item with key", key, "created on", r.createdOn, "and hit", r.accessCount, "times from table", table.name)
	delete(table.items, key)

	return r, nil
}

Value 取值

取值本身是比较简单的,只不过这里要进行一些额外处理:

// Value returns an item from the cache and marks it to be kept alive. You can
// pass additional arguments to your DataLoader callback function.
func (table *CacheTable) Value(key interface{}, args ...interface{}) (*CacheItem, error) {
	table.RLock()
	r, ok := table.items[key]
	loadData := table.loadData
	table.RUnlock()

	if ok {
		// Update access counter and timestamp.
		r.KeepAlive()
		return r, nil
	}

	// Item doesn't exist in cache. Try and fetch it with a data-loader.
	if loadData != nil {
		item := loadData(key, args...)
		if item != nil {
			table.Add(key, item.lifeSpan, item.data)
			return item, nil
		}

		return nil, ErrKeyNotFoundOrLoadable
	}

	return nil, ErrKeyNotFound
}

MostAccessed 最常访问的条目

这个方法用到了前文提到的 CacheItemPair 和 CacheItemPairList。

// MostAccessed returns the most accessed items in this cache table
func (table *CacheTable) MostAccessed(count int64) []*CacheItem {
	table.RLock()
	defer table.RUnlock()

	p := make(CacheItemPairList, len(table.items))
	i := 0
	for k, v := range table.items {
		p[i] = CacheItemPair{k, v.accessCount}
		i++
	}
	sort.Sort(p)

	var r []*CacheItem
	c := int64(0)
	for _, v := range p {
		if c >= count {
			break
		}

		item, ok := table.items[v.Key]
		if ok {
			r = append(r, item)
		}
		c++
	}

	return r
}

Foreach 方法

为开发者提供更加丰富的自定义操作。

// Foreach all items
func (table *CacheTable) Foreach(trans func(key interface{}, item *CacheItem)) {
	table.RLock()
	defer table.RUnlock()

	for k, v := range table.items {
		trans(k, v)
	}
}

清空缓存

清空缓存的方法比较简单,一方面是数据的清空,另一方面是定时器的清空。

// Flush deletes all items from this cache table.
func (table *CacheTable) Flush() {
	table.Lock()
	defer table.Unlock()

	table.log("Flushing table", table.name)

	table.items = make(map[interface{}]*CacheItem)
	table.cleanupInterval = 0
	if table.cleanupTimer != nil {
		table.cleanupTimer.Stop()
	}
}

查询相关方法

Count 和 Exists 方法是比较简单的,不用多说。

// Count returns how many items are currently stored in the cache.
func (table *CacheTable) Count() int {
	table.RLock()
	defer table.RUnlock()
	return len(table.items)
}

// Exists returns whether an item exists in the cache. Unlike the Value method
// Exists neither tries to fetch data via the loadData callback nor does it
// keep the item alive in the cache.
func (table *CacheTable) Exists(key interface{}) bool {
	table.RLock()
	defer table.RUnlock()
	_, ok := table.items[key]

	return ok
}

Set 相关方法

下面这些 Set 方法比较简单,也不多做赘述。

// SetDataLoader configures a data-loader callback, which will be called when
// trying to access a non-existing key. The key and 0...n additional arguments
// are passed to the callback function.
func (table *CacheTable) SetDataLoader(f func(interface{}, ...interface{}) *CacheItem) {
	table.Lock()
	defer table.Unlock()
	table.loadData = f
}

// SetAddedItemCallback configures a callback, which will be called every time
// a new item is added to the cache.
func (table *CacheTable) SetAddedItemCallback(f func(*CacheItem)) {
	if len(table.addedItem) > 0 {
		table.RemoveAddedItemCallbacks()
	}
	table.Lock()
	defer table.Unlock()
	table.addedItem = append(table.addedItem, f)
}

//AddAddedItemCallback appends a new callback to the addedItem queue
func (table *CacheTable) AddAddedItemCallback(f func(*CacheItem)) {
	table.Lock()
	defer table.Unlock()
	table.addedItem = append(table.addedItem, f)
}

// SetAboutToDeleteItemCallback configures a callback, which will be called
// every time an item is about to be removed from the cache.
func (table *CacheTable) SetAboutToDeleteItemCallback(f func(*CacheItem)) {
	if len(table.aboutToDeleteItem) > 0 {
		table.RemoveAboutToDeleteItemCallback()
	}
	table.Lock()
	defer table.Unlock()
	table.aboutToDeleteItem = append(table.aboutToDeleteItem, f)
}

// AddAboutToDeleteItemCallback appends a new callback to the AboutToDeleteItem queue
func (table *CacheTable) AddAboutToDeleteItemCallback(f func(*CacheItem)) {
	table.Lock()
	defer table.Unlock()
	table.aboutToDeleteItem = append(table.aboutToDeleteItem, f)
}

// SetLogger sets the logger to be used by this cache table.
func (table *CacheTable) SetLogger(logger *log.Logger) {
	table.Lock()
	defer table.Unlock()
	table.logger = logger
}

删除相关方法

过于简单,不做赘述

// RemoveAddedItemCallbacks empties the added item callback queue
func (table *CacheTable) RemoveAddedItemCallbacks() {
	table.Lock()
	defer table.Unlock()
	table.addedItem = nil
}

// RemoveAboutToDeleteItemCallback empties the about to delete item callback queue
func (table *CacheTable) RemoveAboutToDeleteItemCallback() {
	table.Lock()
	defer table.Unlock()
	table.aboutToDeleteItem = nil
}

cache.go

Cache 函数是该缓存库的入口函数,该函数存在一段双检逻辑,需要特别了解下原因:

// Cache returns the existing cache table with given name or creates a new one
// if the table does not exist yet.
func Cache(table string) *CacheTable {
	mutex.RLock()
	t, ok := cache[table]
	mutex.RUnlock()

	if !ok {
		mutex.Lock()
		t, ok = cache[table]
		// Double check whether the table exists or not.
		if !ok {
			t = &CacheTable{
				name:  table,
				items: make(map[interface{}]*CacheItem),
			}
			cache[table] = t
		}
		mutex.Unlock()
	}

	return t
}

examples

样例也比较简单,读者可以自行阅读下。

引用

  1. https://github.com/muesli/cache2go
  2. https://mp.weixin.qq.com/s/6JjL0KVccW7nAQiKuDAl-w
  3. https://mp.weixin.qq.com/s/gIvNjn7GdOQUwg1pKtDTEQ
  4. https://mp.weixin.qq.com/s/898HtDyFTykvMu2-vvMy-A

标签:key,阅读,cache2go,cache,item,CacheItem,源码,func,table
来源: https://www.cnblogs.com/zhouweixin/p/16538769.html