Little’s law and stable systems

Little’s law is defined in terms of stable systems. On my previous post, I have described my bar as a stable system: “Whenever a bottle finishes, I remove it from the bar. Then I open a new one, and add it to the bar. My bar is a stable system: the rate at which whiskey bottles enter the bar is the rate at which they exit.”

This describes the mechanics for inbound and outbound work items (whiskey bottles); but what is guiding these decisions?

Basically two guiding rules make my bar a stable system: (1) WIP limit and (2) Pull System.

WIP limit
WIP, or work in progress, is the number of work items in my system. In my bar example it is the number of whiskey bottles on the bar. Bottles that have been opened, but are not finished yet. The WIP limit on my bar is 12 bottles because that's all I have space for.

Pull System
Pull System describes the movement of work items driven by actual demand. In my bar example, a bottle that is finished opens a spot on my bar. Therefore creating a demand for a new bottle to be opened and placed at the bar. Essentially, the movement of work items (whiskey bottles) is driven by actual demand: a finished bottle is removed from the bar, opening space for a new one that is promptly added to the bar, occupying the vacant space..