Control Flow!

Drum Buffer Rope (DBR) Is a process control mechanism developed from Ely Goldratt’s Theory of Constraints (TOC).  In the book The Goal, Goldratt explains how a constraint can impact the performance of a process. The TOC is based on three assumptions (see Kanban and The Theory of Constraints):

1. All systems are limited by a small number of constraints
2. There is always at least one constraint in a process
3. Flow can only be increased by increasing the flow through a constraint.

The Drum Buffer Rope concept translates the TOC into a tool that can be used for planning and control.  The constraint is the drum (or cadence) that controls the output of the process. The drum is exploited (used to its maximum effect) through the use of time buffers.  The rope synchronizes the flow of work so that input to and output from the drum is constant and shocks are avoided. Using a mountain climbing team as an example, one person will always be slower than all others; that person is the drum (constraint).  The rope that connects team members is a tool to keep the team synchronized so that the whole team ascends at a sustainable pace. The slack between members acts as a buffer so that if one member exceeds the slowest member’s climbing speed, the variance can be absorbed without impact on the team’s ascent.

Use the five-step process identified in Goldratt’s book The Goal:

1. Find the constraint
2. Exploit the constraint
3. Subordinate every other step to the constraint
4. Elevate the constraint, then
5. Repeat if the constraint has been broken

When a team identifies the constraint in their process and leverage the DBR concept to exploit that constraint in a sustainable fashion, they can achieve predictability. Apply DBR using a three-step approach:

1. Develop a detailed schedule for the drum resource. The schedule ensures the maximum sustainable utilization of the drum.  The resource can be a machine or person (yes I am aware of the sensitivity of considering people a resource).
2. Add buffers to protect the performance of that resource. Buffers are used to flatten out the variability in the flow of the process.
3. Synchronize the schedule of all other resources to the drum schedule.  The rope ensures that work-in-process is minimized across the entire process flow.

The Drum Buffer Rope puts Goldratt’s Theory of Constraints into action and promotes predictability. Processes that are predictable can be planned and can be improved!