Tuesday, 23 September 2014

The Goal - The Match Bowl Experiment

Recently I've been re-reading The Goal by Eliyahu Goldratt. It's a great little book about manufacturing plants and how to manage them. It introduces the Theory of Constraints and that's relevant for all software developers for an understanding of why our development processes are structured the way they are.

Throughout the book it uses games and metaphors to illustrate faulty thinking about interconnected processes. In this post, I'd like to introduce Goldratt's dice game.

In the dice game there are a number of stations (representing part of a business process). The stations are arranged in a line with the output from one station becoming the input of the next. This arrangement represents a production line. In order to move items through the production line players take it in turn to roll a dice. The number rolled is the maximum you can move to the next station. For example, if you roll a six, but only have three items in your station, then you can only move three to the next station.

Let's imagine a really simple system with 8 stations that starts with 100 units in the left hand bowl with the aim of producing 100 units in the right hand bowl. Each bowl has the same capacity; it'll produce between 1-6 units each work step.

Based on the rules above, what's the flow of work going to look like through the system?

You might expect the flow to be smooth; each workstation has about 0 items at any one time because as soon as they are produced they move onto the next state. The reality is somewhat different.

The movie below shows the system processing a set of 100 units. The bottleneck (that is the work centre with the most items in it) is highlighted in yellow.

What's really interesting is the chaotic nature of the bottleneck. Random fluctuations mean that the bottleneck can appear anywhere. Balancing capacity across each item is clearly not the right answer.

All the diagrams in this page were built using the excellent Diagrams library for Haskell.