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the bottleneck from being starved had emerged. This buffer is not made of stock—it is a time buffer. The idea was to release the materials for the bottleneck exactly a time-buffer length before the bottleneck is supposed to begin work on the job, giving all the required resources enough time to let the parts reach the bottleneck before the scheduled time. This concept of the buffer as time—supporting the timely arrival of the parts rather than parts sitting in front of the bottleneck—was a key in understanding the paradigm shift that lies in the change from great sophistication to simplicity. It consists of understanding that buffers are necessary to deal with uncertainty, and that in order to protect a schedule, which is built of time-based instructions, we need to use time as protection. The time buffer meant that even when Murphy messes things up, the expectation is that the parts will reach the bottleneck on time in the vast majority of cases. Of course, specifying long enough time to cover for Murphy meant that in most cases the parts would arrive too early to the bottleneck and simply sit there. So, it looks like a buffer of inventory, but actually the real protector against starvation of the constraint is the time provided for parts to go through the route to the bottleneck.

The term “bottleneck” was the key term in the OPT® days, and even when the DBR methodology was developed together with the famous book The Goal (Goldratt and Cox, 1984), the terminology was based on bottlenecks. It is always important and enlightening to have a historical perspective of the development of such major managerial approaches as TOC. At that time, in 1984, the much more generic term constraint was not yet coined.

OPT® registered trademark of Scheduling Technologies Group Limited, Hounslow, U.K.

The important insight, partially acknowledged in the OPT®2 days, but becoming clearer later, is:

As complex as the production shop floor may be, the performance of the shop as a whole is impacted by a single work center, which determines both the response time and the maximum potential output of the floor.

Is there really only one capacity constraint (called CCR—capacity constrained resource), or could there be two? Well, technically, it is possible to have two, but assuming we speak about interactive resources (one feeds the other) being driven to their limits, then the performance of the shop is doomed to be unstable and even erratic because of the statistical fluctuation that inevitably occurs between dependent resources.

This chapter is not focused on DBR, but on S-DBR and the transition of the understanding that paved the way from DBR to S-DBR. We just stated one transition from OPT® to DBR and the main way is still ahead of us. Before we proceed, let’s fully understand three different aspects of the TOC approach. Each is material in understanding the development from DBR to S-DBR and the internal logic of S-DBR.

Three Views on Operations Planning and Execution

The basic TOC philosophy was first expressed by the Five Focusing Steps (5FS), which already explain the logic of the TOC production planning and its related BM control. The second viewpoint recognizes the difference of defining the rules behind planning in a world with a significant amount of uncertainty (planning with uncertainty) versus planning to optimize in a deterministic world. At the time of the execution whatever is dictated in planning lays out the objectives and the resulting actions. But then, there is a need to define the rules for the decision-making required to deal with the impact of “Murphy” in executing the plan. It is fascinating to realize that defining the rules for planning and execution lead to the S-DBR planning rules and the role of BM in leading the execution decision-making.

The third viewpoint looks at the achievements of Henry Ford and Taiichi Ohno and their focus on flow as the central objective of operations. It seems that even that viewpoint fully supports the TOC methodology for production planning and execution. The three different