security scan kanban

There's something about the typical kanban board I see that feels wrong. Let me try to explain. It's to do with what kanban means; what a kanban is. A true kanban is a physical thing used to regulate supply and demand. For example, consider putting your hand-luggage through an airport security scanner. You put your hand-luggage into empty trays. The trays go through the scanner. At the other side you take your hand-luggage out of the trays. The emptied trays go back to the start (often on a separate conveyor belt underneath) for someone else to put their hand-luggage into. Often you're ready to put your hand-luggage through the scanner but you have to wait because there are no empty trays. The number of trays represents the capacity of the system. Each tray is a kanban.

Now let's take a look at a typical kanban board such as the one on the left. It has two columns; one column for the activity of Wibbling which has a work-in-progress (WIP) limit of 5, and one column for the activity of Fubaring which has a work-in-progress limit of 4. There are 3 blue stories being Wibbled and 4 blue stories being Fubar'd.

How is this board supposed to be used? I count the number of blue stories currently being Wibbled - 1,2,3. I look up to the top of the Wibbling column and see its WIP limit is 5. I know 3 is less than 5 so I know there is some spare Wibbling capacity. If I want to know how much I can do the math, 5-3==2. And repeat... I count the number number of blue stories currently being Fubar'd - 1,2,3,4. I look up to the top of the Fubaring column and see its WIP limit is 4. I know 4 is the same as 4 so there is currently no spare Fubaring capacity.

It seems odd to have two columns that look exactly the same when one has spare capacity and the other does not!

My problem is that 5 and 4 are abstractions!
5 is not the same as 5 trays.
5 is just 5.
But 5 trays is [Tray Tray Tray Tray Tray].

What's important about the 5 isn't the 5.
What's important about the 5 is that it represents 5 trays.
What's important about the 5 is the relationship between the 5 and the number of blue stories it limits. The Wibbling blue story WIP limit is abstracted to a 5 but the blue stories are not. I have 1,2,3 blue stories in the Wibbling column but I'm not representing them with a 3, I'm representing them with 1,2,3 actual blue story cards.

Here's the alternative 'scan kanban' I'm thinking of.
I remove the 5 WIP limit from the top of the Wibbling column. Instead I put in 1,2,3,4,5 orange  Tray s.
I remove the 4 WIP limit from the top of the Fubaring column. Instead I put in 1,2,3,4 red  Tray s.

I add the rule that every  Story  must always be in a tray.

Now I can instantly see the Wibbling column has some spare capacity because there are some  Tray s with no  Story  in them. I can count the spare capacity if I want -  1 ,  2 . No math needed. Easy. I can instantly see the Fubaring column has no spare capacity because all the  Tray s contain a  Story .

Now I have a kanban board that actually has kanban on it!
Each tray is a kanban.
Now I can experience pull.

Suppose the workers to the right of the Fubaring column use green  Tray s. They send an empty  Tray  to the Fubaring column to signal they're ready to pull a  Story .

The workers in the Fubaring column move a done  Story  from their  Tray  into the empty  Tray .

The workers to the right of the Fubaring column pull the  Story  in its  Tray  back into their column.

The workers in the Fubaring column now have an empty  Tray .

The workers in the Fubaring column can send their empty  Tray  to the Wibbling column to signal they're ready to pull a  Story .

Thus each  Story  flows left to right while each empty  Tray   Tray   Tray  moves right to left.

Once we have a system under control we can try to improve. For example, we can see what happens when we reduce the WIP limit by 1 in the Wibbling column. We simply remove a  Tray .





Caveat Emptor: I've never seen this style of kanban in actual use. It's just an idea so far. As always, if anyone has any feedback it would be much appreciated.

Update! Here's a follow up blog entry.