Push flows: MRP and DRP

It's sometimes said that Kanban works on a pull-flow basis and MRP on a push-flow basis. In reality, things aren't quite so simple, because an MRP-type method can very well generate a flow of products on the shop floor whose characteristics approach those of a just-in-time (JIT) pull flow, if sales uncertainty is low, lead times are short, and launches are made as late as possible. Conversely, a Kanban system applied with an excessive number of tickets, on the pretext that the machines must not stop, is more akin to a push flow than a pull flow.

The distinction between push and pull flow is not so much a question of the production scheduling method as of how it is applied:

• if production is started at any point in the process, following the order defined by a work schedule, as soon as resources are available (materials, machines, manpower) and without taking into account the short-term needs of the downstream process (or the end consumer), the flow is said to be pushed;

• if the decision to produce depends on the short-term needs of the downstream process (or the real needs of the consumer), the flow is said to be pulled.

Push flow reflects the viewpoint of a "producer", who seeks to make the most of his resources by keeping to his delivery schedule. Pull flow reflects the viewpoint of a supplier seeking to deliver the precise service the customer requires.

In the article "Enjeux de la logistique" in this treatise [3] , Pascal Eymerie discusses the idea of complexity in logistics systems, and in particular the independence of industrial management decisions in the case of shared resources. The complexity of a logistics system is increased by various factors: variety of products manufactured, inertia of capacity and flow decisions, uncertainty of commercial demand, existence of production hazards (machine breakdowns, quality problems, etc.).

But how do you actually manage a complex system? This article traces the evolution of MRP (Manufacturing Resource Planning) and DRP (Distribution Resource Planning) push-flow production and distribution planning systems.

Before the advent of MRP and DRP methods, production and distribution management often boiled down to inventory level management. MRP and DRP methods are particularly well suited to industries which manufacture and assemble a large number of components to order, or in small batches, using batch production methods. This is where production flows are the most difficult to manage. These methods are particularly well suited to the mechanical and assembly industries. Extensions have been...