go-cache是什么以及应用场景
go-cache 是一个类似Memcached的go库，key:value存储在内存中。适合单机应用调用。
使用
import (
"fmt"
"github.com/patrickmn/go-cache"
"time"
)
func main() {
// Create a cache with a default expiration time of 5 minutes, and which
// purges expired items every 10 minutes
c := cache.New(5*time.Minute, 10*time.Minute)
// Set the value of the key "foo" to "bar", with the default expiration time
c.Set("foo", "bar", cache.DefaultExpiration)
// Set the value of the key "baz" to 42, with no expiration time
// (the item won't be removed until it is re-set, or removed using
// c.Delete("baz")
c.Set("baz", 42, cache.NoExpiration)
// Get the string associated with the key "foo" from the cache
foo, found := c.Get("foo")
if found {
fmt.Println(foo)
}
}go-cache源码分析
从上面的基本使用，用了New，Set，Get方法，去查看的对应的源码。
New()
func New(defaultExpiration, cleanupInterval time.Duration) *Cache {
items := make(map[string]Item)
return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}
func newCacheWithJanitor(de time.Duration, ci time.Duration, m map[string]Item) *Cache {
c := newCache(de, m)
// This trick ensures that the janitor goroutine (which--granted it
// was enabled--is running DeleteExpired on c forever) does not keep
// the returned C object from being garbage collected. When it is
// garbage collected, the finalizer stops the janitor goroutine, after
// which c can be collected.
C := &Cache{c}
if ci > 0 {
runJanitor(c, ci)
runtime.SetFinalizer(C, stopJanitor)
}
return C
}
type Item struct {
Object interface{}
Expiration int64
}
type Cache struct {
*cache
// If this is confusing, see the comment at the bottom of New()
}
type cache struct {
defaultExpiration time.Duration
items map[string]Item
mu sync.RWMutex
onEvicted func(string, interface{})
janitor *janitor
}go-cache对每一个key/value item对设置了过期时间，并加入了定期清理过期的item。对应的是New()函数的两个参数，返回值是名叫Cache的结构体（调用的是newCacheWithJanitor方法，该方法返回Cache的结构体）。newCacheWithJanitor方法做下面几件事：
- 创建cache结构体，包装成Cache结构体。用于返回值
- 若第二个参数，即清理清理时间大于零，则执行定时清理。
func newCache(de time.Duration, m map[string]Item) *cache {
if de == 0 {
de = -1
}
c := &cache{
defaultExpiration: de,
items: m,
}
return c
}type janitor struct {
Interval time.Duration
stop chan bool
}
func (j *janitor) Run(c *cache) {
ticker := time.NewTicker(j.Interval)
for {
select {
case <-ticker.C:
c.DeleteExpired()
case <-j.stop:
ticker.Stop()
return
}
}
}
func stopJanitor(c *Cache) {
c.janitor.stop <- true
}
func runJanitor(c *cache, ci time.Duration) {
j := &janitor{
Interval: ci,
stop: make(chan bool),
}
c.janitor = j
go j.Run(c)
}定时清理包括：创建看门狗结构体janitor，并赋值给cache结构体的成员变量，执行结构体的Run()方法。
Run()方法包括：time.NewTicker创建一个周期性的定时器ticker，定时器时间一到，执行结构体cache的DeleteExpired()方法，除非执行了stopJanitor()方法，否则定时器一直执行下去。
runtime.SetFinalizer不需要代码显式关闭，在检查到没有引用之后，主动终止goroutine，等待gc回收。
// Delete all expired items from the cache.
func (c *cache) DeleteExpired() {
var evictedItems []keyAndValue
now := time.Now().UnixNano()
c.mu.Lock()
for k, v := range c.items {
// "Inlining" of expired
if v.Expiration > 0 && now > v.Expiration {
ov, evicted := c.delete(k)
if evicted {
evictedItems = append(evictedItems, keyAndValue{k, ov})
}
}
}
c.mu.Unlock()
for _, v := range evictedItems {
c.onEvicted(v.key, v.value)
}
}DeleteExpired()方法做的事情是：检查所有items，一旦某个item配置了过期时间（不配置则永远不被定时清除，除非手动清除），并且已过期，则删除该item，若该item配置了删除回调函数，则执行删除回调函数。
func (c *cache) delete(k string) (interface{}, bool) {
if c.onEvicted != nil {
if v, found := c.items[k]; found {
delete(c.items, k)
return v.Object, true
}
}
delete(c.items, k)
return nil, false
}Set()
Set()比较简单，就是配置key/value对，并附上过期时间。有两个类似的方法Add()和replace()。
区别是，Set()不管key是否存在，都配置。Add()只能用于key不存在的情况，否则报错，replace是反的，只能用于key存在的情况，否则报错。
// Add an item to the cache, replacing any existing item. If the duration is 0
// (DefaultExpiration), the cache's default expiration time is used. If it is -1
// (NoExpiration), the item never expires.
func (c *cache) Set(k string, x interface{}, d time.Duration) {
// "Inlining" of set
var e int64
if d == DefaultExpiration {
d = c.defaultExpiration
}
if d > 0 {
e = time.Now().Add(d).UnixNano()
}
c.mu.Lock()
c.items[k] = Item{
Object: x,
Expiration: e,
}
// TODO: Calls to mu.Unlock are currently not deferred because defer
// adds ~200 ns (as of go1.)
c.mu.Unlock()
}// Add an item to the cache only if an item doesn't already exist for the given
// key, or if the existing item has expired. Returns an error otherwise.
func (c *cache) Add(k string, x interface{}, d time.Duration) error {
c.mu.Lock()
_, found := c.get(k)
if found {
c.mu.Unlock()
return fmt.Errorf("Item %s already exists", k)
}
c.set(k, x, d)
c.mu.Unlock()
return nil
}
// Set a new value for the cache key only if it already exists, and the existing
// item hasn't expired. Returns an error otherwise.
func (c *cache) Replace(k string, x interface{}, d time.Duration) error {
c.mu.Lock()
_, found := c.get(k)
if !found {
c.mu.Unlock()
return fmt.Errorf("Item %s doesn't exist", k)
}
c.set(k, x, d)
c.mu.Unlock()
return nil
}Get()
Get()方法比较简单，获取key/value对，并返回是key是否存在且未过期。GetWithExpiration()相较Get()方法多了个返回过期时间的参数。
// Get an item from the cache. Returns the item or nil, and a bool indicating
// whether the key was found.
func (c *cache) Get(k string) (interface{}, bool) {
c.mu.RLock()
// "Inlining" of get and Expired
item, found := c.items[k]
if !found {
c.mu.RUnlock()
return nil, false
}
if item.Expiration > 0 {
if time.Now().UnixNano() > item.Expiration {
c.mu.RUnlock()
return nil, false
}
}
c.mu.RUnlock()
return item.Object, true
}// GetWithExpiration returns an item and its expiration time from the cache.
// It returns the item or nil, the expiration time if one is set (if the item
// never expires a zero value for time.Time is returned), and a bool indicating
// whether the key was found.
func (c *cache) GetWithExpiration(k string) (interface{}, time.Time, bool) {
c.mu.RLock()
// "Inlining" of get and Expired
item, found := c.items[k]
if !found {
c.mu.RUnlock()
return nil, time.Time{}, false
}
if item.Expiration > 0 {
if time.Now().UnixNano() > item.Expiration {
c.mu.RUnlock()
return nil, time.Time{}, false
}
// Return the item and the expiration time
c.mu.RUnlock()
return item.Object, time.Unix(0, item.Expiration), true
}
// If expiration <= 0 (i.e. no expiration time set) then return the item
// and a zeroed time.Time
c.mu.RUnlock()
return item.Object, time.Time{}, true
}其他
Flush()清空所有的items。ItemCount()获取item数量。还有将所有items保存到文件，从文件中恢复items的方法，逻辑比较简单，这里就不详细分析了。
// Delete all items from the cache.
func (c *cache) Flush() {
c.mu.Lock()
c.items = map[string]Item{}
c.mu.Unlock()
}// Returns the number of items in the cache. This may include items that have
// expired, but have not yet been cleaned up.
func (c *cache) ItemCount() int {
c.mu.RLock()
n := len(c.items)
c.mu.RUnlock()
return n
}转载请注明来源，欢迎指出任何有错误或不够清晰的表达。可以邮件至 backendcloud@gmail.com
