lru_cache最近最少使用缓存处理

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LRU是Least Recently Used 的缩写，翻译过来就是“最近最少使用”。

LRU缓存的处理方式如其命名，在固定的缓存空间下，把最近最少使用的数据移除，让给最新读取的数据。算法假设最常读取的数据一般也是访问次数最多的，尽力保留这部分数据可以提高cache的命中。

lru-cache

例子

在保持几个从MongoDB中取出来的数据

使用

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var LRU = require("lru-cache")
//默认接受max,下面的参数都是可选的
otherCache = LRU(50) // sets just the max size

options = {
    max: 500,
    length: function (n, key) {
        return n * 2 + key.length
        },
    dispose: function (key, n) { 
        n.close() 
    },
    maxAge: 1000 * 60 * 60
}
cache = LRU(options)

cache.set("key", "value")
cache.get("key") // "value"

// non-string keys ARE fully supported
var someObject = {}
cache.set(someObject, 'a value')
cache.set('[object Object]', 'a different value')
assert.equal(cache.get(someObject), 'a value')

cache.reset()    // empty the cache

当然要注意数据不能放太多,不然溢出

Options

  • max cache的最大长度,使用length函数来对所有值检查合法性.必须设置,不然默认无穷大.
  • maxAge 以ms为单位的最大逗留时间.
  • length 这是一个Function用来计算 the length of stored items.

    你在存储 strings or buffers时, 你可能想要做如下 function(n, key){return n.length}. The default is function(){return 1}, 表示你存储大小规模是 max 那设置就没事. The item is passed as the first argument, and the key is passed as the second argumnet.

  • dispose 这个Function 是用来将items从cache中drop的. 当你关闭文件或做其他清除cache工作时,不需要用到这些items时可以使用. Called with key, value. 实际上是先call再把这个item是从内部cache中remove,如果你想立即put it back in, you’ll have to do that in a nextTick or setTimeout callback or it won’t do anything.
  • stale By default, if you set a maxAge, 只有当你使用get(key) 他才会真正把 stale items 拉出cache . (也就是说在做setTimeout or anything前并不能做其他事.)
    1. 如果你设置stale:true, 在你删除这个stale value前,他会返回给你.
    2. 如果你没有设置这个值, 那么当你试图get a stale entry, 他会返回undefined , 因为他已经被删除了
  • noDisposeOnSet By default, 如果你设置了 dispose() 方法,当你使用 set() 操作来重写一个已经存在的key时,他会被调用 If you set this option, dispose() will only be called when a key falls out of the cache, not when it is overwritten.

API

  • set(key, value, maxAge)

  • get(key) => value

Both of these will update the “recently used”-ness of the key. They do what you think. maxAge is optional and overrides the cache maxAge option if provided.

If the key is not found, get() will return undefined.

The key and val can be any value.

  • peek(key)

Returns the key value (or undefined if not found) without updating the “recently used”-ness of the key.

(If you find yourself using this a lot, you might be using the wrong sort of data structure, but there are some use cases where it’s handy.)

  • del(key)

Deletes a key out of the cache.

  • reset()

Clear the cache entirely, throwing away all values.

  • has(key)

Check if a key is in the cache, without updating the recent-ness or deleting it for being stale.

  • forEach(function(value,key,cache), [thisp])

Just like Array.prototype.forEach. Iterates over all the keys in the cache, in order of recent-ness. (Ie, more recently used items are iterated over first.)

  • rforEach(function(value,key,cache), [thisp])

The same as cache.forEach(...) but items are iterated over in reverse order. (ie, less recently used items are iterated over first.)

  • keys()

Return an array of the keys in the cache.

  • values()

Return an array of the values in the cache.

  • length

Return total length of objects in cache taking into account length options function.

  • itemCount

Return total quantity of objects currently in cache. Note, that stale (see options) items are returned as part of this item count.

  • dump()

Return an array of the cache entries ready for serialization and usage with ‘destinationCache.load(arr)`.

  • load(cacheEntriesArray)

Loads another cache entries array, obtained with sourceCache.dump(), into the cache. The destination cache is reset before loading new entries

  • prune()

Manually iterates over the entire cache proactively pruning old entries