async-cache

Cache your async lookups and don't fetch the same thing more than necessary.

Example

Let's say you have to look up stat info from paths. But you are ok with only looking up the stat info once every 10 minutes (since it doesn't change that often), and you want to limit your cache size to 1000 objects, and never have two stat calls for the same file happening at the same time (since that's silly and unnecessary).

You can do this:

var stats = new AsyncCache({
  // options passed directly to the internal lru cache
  max: 1000,
  maxAge: 1000 * 60 * 10,
  // method to load a thing if it's not in the cache.
  // key must be unique in the context of this cache.
  load: function (key, cb) {
    // the key can be something like the path, or fd+path, or whatever.
    // something that will be unique.
    // this method will only be called if it's not already in cache, and will
    // cache the result in the lru.
    getTheStatFromTheKey(key, cb)
  }
})

// then later..
stats.get(fd + ':' + path, function (er, stat) {
  // maybe loaded from cache, maybe just fetched
})

Except for the load method, all the options are passed unmolested to the internal lru-cache.

Differences from lru-cache

Since values are fetched asynchronously, the get method takes a callback, rather than returning the value synchronously.

While there is a set(k,v) method to manually seed the cache, typically you'll just call get and let the load function fetch the key for you.

Keys must uniquely identify a single object, and must contain all the information required to fetch an object, and must be strings.

Per key maxAge

If load callback is called with 3 arguments, the 3rd is passed to the internal lru-cache as a maxAge for the retrieved key.

  function load (key, cb) {
    getValueFromTheKey(key, function (err, item) {
      cb(err, item.value, item.maxAge)
    })
  }

Methods

• get(key, cb) If the key is in the cache, then calls cb(null, cached) on nextTick. Otherwise, calls the load function that was supplied in the options object. If it doesn't return an error, then cache the result. Multiple get calls with the same key will only ever have a single load call at the same time.

• set(key, val, maxAge) Seed the cache. This doesn't have to be done, but can be convenient if you know that something will be fetched soon. maxAge is optional - it is passed to internal LRU cache

• reset() Drop all the items in the cache.