Sunday, September 09, 2007

The Kanban Juice War

I posed what I believed to be an innocuous problem for the kanbandev group at Yahoo and inadvertently started a bit of a row. The problem was as follows:

In a hotel restaurant there is a table with a juice dispenser. Next to the juice dispenser is a stack of trays with glasses. Sometimes the glasses run out, and a queue of customers build up. What is the best way to prevent glass outages?

I had a solution, but I wasn't happy with it. It involved using a photocell to detect when the glasses were in danger of running out. One complication was that the serving personnel could not easily see if the glasses were running out. The table with the juice dispenser is in an alcove. The, usually very busy, serving personnel move along paths that make it difficult to see the trays. If there is a queue of people waiting for juice, they obscure the trays, and any simple kanban mechanisms. Of course, if the arrival rate of thirsty restaurant guests is great enough, a queue may form even if there is plenty of juice and glasses.

I quickly got several suggestions on how to implement various kanban mechanisms to replenish the glasses. All of the suggestions were good, but only a few will work in the environment at the restaurant. (Or so I believe. I've been there only once, and it was awhile ago.) This was what I had expected, but one member of the group told me that my problem was a distribution problem, not a process scheduling problem, and therefore it would be more appropriate to ask the question in another group.

This sparked some heated debate. For once, I didn't wade into the thick of it. There were some big guns on both sides of the debate. One thing that became clear to me, is that there was a difference of opinion about some of the fundamentals, like what a pull system is, when to apply kanban or DBR mechanisms, and stuff like that.

I like diagrams. Mailing lists are not diagram friendly, so I thought I'd blog about my point of view. It allows me to describe the system under discussion with a diagram like this:

(Click the image to get a full size view.)

As you can see, the system has two legs, a glass leg, and a juice leg. (Some of it is conjecture. For example, I do not know if the restaurant dilutes concentrated juice, but it seems a reasonable supposition.)

The basic conflict, as I understand it, is described in the following evaporating cloud:

(Click the image to get a full size view.)

I asked about the reason for kanban not being applicable, and got the answer that the system is not a pull system because the operator of the system - the restaurant management - is not able to control the arrival rate of the work - the customers who want a drink of juice.

This seemed strange to me on two counts:
  • Pull systems control the flow of resources in a production process by replacing only what has been consumed. This has nothing to do with controlling the arrival rate of customers. (Controlling the arrival rate of work orders is important, but it is not something that defines a pull system.)
  • There was an assumption that the system was not a pull system, but suitable for a TOC distribution system approach. TOC distribution systems are pull systems.
When cracking a cloud, it is important to be able to prove assertions like the two I made above. I checked several references, but these will be sufficient to prove my point:

For definitions of the term pull system, see

For explanations of the TOC distribution system approach, see
  • Viable Vision, Ch 7. Distribution: From Push to Pull, Gerald I. Kendall;
So claiming that the problem isn't a kanban problem because the system under discussion isn't a pull system, does not hold. The system is a pull system, because the act of using glasses should trigger replenishment of the trays. The problem is that the current mechanism for doing so, visual inspection of the trays, is too unreliable.

Maybe there is some other reason to consider this a distribution problem? Well, distribution problems are primarily about sizing, composing, and placing buffers. The TOC distribution system is based on two ideas:
  1. Pool the inventory where there is the greatest predictability
  2. Use a pull system to replenish exactly what has been consumed at short intervals
Would such a solution be of help? No. The primary problem here is not where to put buffers. Increasing the glass buffer would not help, unless the buffer is big enough to last through breakfast. Even then, the restaurant might run out of glasses by dinner time. It is the mechanism for replenishing the buffer of glasses that is faulty. The second step does tell us to use a pull system, but it does (of course) not specify the exact mechanism to use.

On the other hand, proving that this is not primarily a distribution problem, does not prove that it is a problem that can be solved with kanban (or DBR). What does kanban provide? A visual aid that tells when to replenish a buffer in a pull system. Sounds rather abstract. How about translating it into something specific:

A visual aid that tells when to place a new tray of glasses next to the juice machine.

Much better. This is exactly what a kanban solution is. It also satisfies step 2 in the TOC distribution mechanism. Could it be that kanban systems can extend throughout the entire supply chain? Yes they can.

The kanban idea originated by studying how wares are replenished on the shelves of supermarkets. This is a classic kanban application, but it also ties into distribution problems. Supermarkets don't make anything, they are retail outlets, so they are fed by distributors.

A kanban system is just a signaling mechanism for replenishing resources used by a process stage in a pull system. Whether that pull system is a manufacturing system, or a distribution system does not really matter. That is not to say a manufacturing system and a distribution system are necessarily alike in all other ways. They are not.

Thus, the Kanban Juice War was caused by misunderstandings:
  • It was not understood that the juice dispenser system under discussion is a pull system
  • It was not understood that the TOC distribution system is a pull system
  • It was not understood that it is irrelevant whether the system under discussion is a production system or a distribution system, because kanban is applicable to all pull systems
The evaporating cloud was broken in two places:
Focusing on buffering won't work, because the problem is the faulty triggering mechanism. Choosing between kanban and distribution solutions is also wrong, because kanban solutions may be a part of distribution solutions that are based on pull systems.

When someone misunderstands, the responsibility may lie with the sender of the message, as well as with the receiver. As I am the original sender, I may be guilty of garbling the problem description. Hard for me to say. The message seemed clear to me, but that proves nothing. If you understand the problem description it does not prove anything either. I have restated the problem, and I am using illustrations. This may make a lot of difference. On the other hand, if you don't get the the problem explanation in this post, then I have probably goofed in my original posting to the mailing list too.

The whole thing may seem like a storm in a glass of juice, but I find the Juice War interesting. If we "experts" can't agree on how to define a very simple problem like this, how are we going to sell TOC and Lean solutions to the people who need them so desperately? Why do TOC experts themselves use the Evaporating Cloud technique to resolve conflicts so rarely? Is it because the technique does not work? Or, is it because it does work very well? (I suspect the latter, probably because I find clouds very useful.)

2 comments:

Anonymous said...

Hi

You may have already been offered this as a suggested solution. One possibility is to have the trays holding the glasses at the bottom of the stack to be of a different type or colour. When the staff see the different glasses are being used (e.g. when tables are cleared) this would trigger the loading of addition trays of glasses. Staff would first restack the type of glass used to trigger the replemment then add the other variety of glasses on top. The assumption would be that the addition of the new glasses would be sufficient to ensure that “special” glasses were removed from the system before customer used up all of the “normal” glasses.

Henrik said...

Interesting suggestion, but it has three problems:

1. The serving personnel would have to sort the glasses before putting them on the trays. This would create too much setup work.

2. The serving personnel move in paths that make it hard for them to see the trays.

3. Even if the servers do look, if there is a short queue of 2-3 people, the trays on the table will be blocked from view.

One of the suggestions I got was to stick a large paper with colored bands behind the trays. That would achieve the same effect as the colored glasses. Unfortunately, though a very good suggestion, it won't work under the given circumstances.

Of course, one might change those circumstances...