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Shmula Goes Camping: Drum-Buffer-Rope

Posted by psabilla

July 2, 2006

My family and I went camping with my brother-in-law and his family. We went to a place in Utah

called Uinta National Forest — it was beautiful. We prepared well, got the tent and camping stuff 

ready, then headed to the camp site on Friday. When we arrived, we set-up camp, then we went on

a hike.

That’s us above on a hike — that’s me with the blue shirt holding a baby; yeah, the Asian guy,

that’s me. It was just absolutely beautiful up there. It was nice to be in nature.

During our hike, I couldn’t help but think of the Boy Scout chapter in Goldratt’s “The Goal”. You

know, the chapter where Goldratt introduces the Drum-Buffer-Rope system.

Before explaining the Drum-Buffer-Rope (DBR) system, let me first explain some basic

characteristics of systems, in general:

1. Every system has a bottleneck.

2. A bottleneck is a state of affairs where demand for service exceeds the capacity to serve.

3. The Throughput of a system is dependent on the Throughput of the Bottleneck.

4. Given (1), (2), & (3), for maximum output, a system ought to keep the bottleneck working

at 100% capacity with little or no defects (scrap, waste (muda), time-traps).5. Given (4), Non-bottleneck processes should be working at less than 100% capacity, so as to

not over-burden the bottleneck with large batches of work-in-process (WIP).

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The characteristics above are basic to any system — manufacturing, clinical, software, or 

otherwise. Put on your Systems-wide Hat and think about your experience thus far, regardless of 

industry: bottlenecks are everywhere. That’s not the problem, but managing it is. Hence, the

Drum-Buffer-Rope System.

Managing the Constraint is mostly about managing the non-bottleneck systems and making them

“aware” how fast they should work — when they should slow down, when they should stop, or 

when they should increase pace and by how much. The Drum-Buffer-Rope system allows for a

systems-wide awareness.

The Drum

The Bottleneck or Constraint, acts as a Drum — it sets the rythm that the whole system should

follow. In Lean Manufacturing, this is also called “Takt Time.”

The Buffer

There are situations when when upstream processes can’t produce as much as is needed by the

Bottleneck; the result: the Constraint is starved and overall system output is compromised. So, we

must have a buffer of inventory that is the size of the accounted-for variation is demand. This will

help to level-out variation. A Buffer will assure that the Constraint never has to wait and, waiting 

is a form of waste.

Similarly, if upstream processes are producing more than the Constraint has the capacity to handle,

then there’s going to be excess inventory sitting in front of the Constraint and, hence, a feast.

Put another way, the Buffer is the inventory and inventory is directly related to Lead Time (I

explain this in 2 previous posts on Little’s Law here and here.

This phenomena is sometimes called the Feast-of-Famine Syndrome.

DBR is used to avoid either of these scenarios — the Feast or the Famine — by dictating the batch

size and frequency of the inputs into the Buffer.

The Rope

The Rope is a method by which the Constraint can signal to the upstream processes (non- bottleneck processes) when to slow down, when to stop, or when to produce faster and the

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quantity. This is called “Pull Scheduling” in Lean Manufacturing terms. In software, this can be

implemented as a data structure called a Stack , with “Push” and “Pop” as the methods for pulling

from the Stack.

Applications of DBR 

When I was with Amazon, I led a project where I investigated a production line that was

experiencing a Feast/Famine scenario. There was a lot of waste on this line and it impacted daily

 production in a serious way. As the team lead, I set out to observe, interview the operators, and

collect data on this line. I quantified the cost to Amazon that was a result of the Feast/Famine

scenario — costs in terms of actual dollars resulting from missed orders, upgraded orders,

overtime of operators, product damage, and safety issues.

The distribution you see above is best approximated by the Poisson Distribution, which means that

the Mean and the Standard Deviation are approximately the same. What does this mean? The

 picture above graphically shows the Feast scenario — product in totes arrive at the constraint ALL

at the same time.

Solution? I led a team of software and industrial engineers that re-engineered this line and we

implemented a DBR solution, where the pack-rate at the Constraint would dictate what theupstream pick-rate should be. In other words, we made sure that the pick-rate would never be

higher than the pack-rate. This solution worked and Amazon saved a lot of money and customers

 benefit.

Drum-Buffer-Rope is an intuitive solution to a seemingly complex problem. DBR is seen in many

areas including Agile Development, Manufacturing, and Medicine (Emergency Room Visits). It’s

an effective business tool to manage the constraints that every business faces.

Oh, by the way, camping was great!