cache-helpers is a Node.JS module containing several generic caching
functions. It was written as a way of abstracting out a number of commonly used
patterns in the Dark Sky API.
cacheHelpers.once: Takes a (presumably expensive) function which takes a single callback and calls it with an optional error argument and a data argument. It returns a function that you may use as a proxy for the passed function. The first time it is called, it will call the function and cache its data. Subsequent calls will return the cached data.
(It is smart enough to only call the backing function once even if it is called many times simultaneously.)
cacheHelpers.timeBasedWithGrace: Takes a (presumably expensive) function, like
once, above, and two numbers: a soft cache timeout and a hard cache timeout. These timeouts may be in any arbitrary units you prefer, though either seconds or milliseconds is most convenient. Like
once, it returns a proxy function, which is called with the current time (in whatever units you specified for the timeouts), and a callback which is called when the data is available.
The first time this function is called, it will call the callback once the backing function returns. Subsequent calls do different things depending on how much time has elapsed since the data was cached: if the soft timeout has not elapsed, then the cached data is returned immediately. If the soft timeout has elapsed but the hard timeout has not, the cached data is returned immediately, but the backing function is called in the background to update the cache, so as not to impede whatever processing you want to do. Finally, if the hard timeout has elapsed, the cache is updated before calling the callback.
You may think of these two limits as "the time until data is out of date" and "the time until data is unnacceptably out of date".
once, this function is smart enough to not call the backing function more often than necessary.)
cacheHelpers.sizeBasedKeyValue: Similar to the others. Takes a function and a maximum cache size. The passed function is expected to take a key parameter and a callback. The function returned by this method will take the same.
If the cache gets full and a new item is requested, the least-recently-used item is purged.
(Like the other functions, this function is also smart enough to not call the backing function more often than necessary.)
These are kind of advanced, and may be fairly hard to understand without a concrete use case. Checking out the tests may help.