The following is derived from the redis list operations. The data model is that a key maps to an list of items. The operation is to push a new item into the front, and discard any items from the end above a threshold number of items.
of course, this can be done by reading a value, fiddling with it, and writing it back. I write this email to wonder if there's any native trickery to avoid having to read the value, but rather permitting some sort of 'push' operation.