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80 lines
2.6 KiB
80 lines
2.6 KiB
5 years ago
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# async-cache
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Cache your async lookups and don't fetch the same thing more than
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necessary.
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## Example
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Let's say you have to look up stat info from paths. But you are ok
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with only looking up the stat info once every 10 minutes (since it
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doesn't change that often), and you want to limit your cache size to
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1000 objects, and never have two stat calls for the same file
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happening at the same time (since that's silly and unnecessary).
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You can do this:
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```javascript
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var stats = new AsyncCache({
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// options passed directly to the internal lru cache
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max: 1000,
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maxAge: 1000 * 60 * 10,
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// method to load a thing if it's not in the cache.
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// key must be unique in the context of this cache.
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load: function (key, cb) {
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// the key can be something like the path, or fd+path, or whatever.
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// something that will be unique.
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// this method will only be called if it's not already in cache, and will
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// cache the result in the lru.
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getTheStatFromTheKey(key, cb)
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}
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})
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// then later..
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stats.get(fd + ':' + path, function (er, stat) {
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// maybe loaded from cache, maybe just fetched
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})
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```
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Except for the `load` method, all the options are passed unmolested to
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the internal [lru-cache](http://npm.im/lru-cache).
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### Differences from [lru-cache](http://npm.im/lru-cache)
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Since values are fetched asynchronously, the `get` method takes a
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callback, rather than returning the value synchronously.
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While there is a `set(k,v)` method to manually seed the cache,
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typically you'll just call `get` and let the load function fetch the
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key for you.
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Keys must uniquely identify a single object, and must contain all the
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information required to fetch an object, and must be strings.
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### Per key `maxAge`
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If `load` callback is called with 3 arguments, the 3rd is passed to
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the internal [lru-cache](http://npm.im/lru-cache) as a `maxAge` for
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the retrieved key.
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```javascript
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function load (key, cb) {
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getValueFromTheKey(key, function (err, item) {
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cb(err, item.value, item.maxAge)
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})
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}
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```
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## Methods
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* `get(key, cb)` If the key is in the cache, then calls `cb(null,
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cached)` on nextTick. Otherwise, calls the `load` function that was
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supplied in the options object. If it doesn't return an error, then
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cache the result. Multiple `get` calls with the same key will only
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ever have a single `load` call at the same time.
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* `set(key, val, maxAge)` Seed the cache. This doesn't have to be done, but
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can be convenient if you know that something will be fetched soon.
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`maxAge` is optional - it is passed to internal LRU cache
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* `reset()` Drop all the items in the cache.
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