cache stack

See:https://github.com/gaiaops/gaia_core_php/blob/master/examples/cache/stack.t

Developers often need to make temporary lists of data. Sometimes those lists can be lossy, and impermanent. An example might be a shoutbox function on a page, where we want to allow people to post quick rants around a topic similar to a group chat function on instant message. We have no interest in keeping those comments around long, and if we lose a few, no big loss. The Cache\Stack is a very inexpensive solution to this problem.

The tough thing about implementing a shoutbox is that it has a high degree of contention over a single key. I saw a developer implement this feature by serializing a list of comments to a single cache key. Every time someone posted, he would pull down the cached value, append the item to the list, and then write the list back to the cache. This introduces a race condition. If two clients read and write at the same time, one of their comments will be lost.

Cache\Stack works by incrementing a counter in the cache and using the most recent value from the counter as the name of a new cache key, to store that item in the list. Since all of the items in the list are in order, The Cache\Stack class just needs to know the value of the counter to predict the key names of all the previous items in the list. It then fetches back the range of items it needs based on that value using a multi-get.