kanban push-me-pull-you

I've been thinking some more about kanban and visualization. A while ago I blogged about my security-scan kanban idea of introducing proper physical kanban onto the board. Instead of writing a work-in-progress limit of 5 at the top of a column you introduce 5 physical "empty-tray" kanbans. Different columns use different coloured kanban. For example, here's a very simple kanban board with a Wibbling limit of 4 yellow trays and a Fubaring limit of 5 red trays. The stories are in blue and must always be in a tray.

I mentioned how this allows genuine pulling. For example, the Fubarers can signal they're ready to pull a story from the Wibblers by moving an empty red kanban tray into the Wibbling column.

I said that the Wibblers could simply move a done blue story from its yellow kanban into an empty red kanban. Something about that bothered me and I think I now know what it is. Once again it's about visualization. What bothers me is that there is no representation of whether a blue story is done until it's moved into an empty kanban. There is no visual display of whether a blue story actually is "in-progress" and being worked on, or whether its "in-queue" and waiting for an empty red kanban. That feels wrong.

More recently, I blogged about about the common pattern of splitting each column; one for ongoing, one for done, like this. Something about the done column didn't feel quite right either.

So here's what I'm thinking. As well as moving empty-kanbans upstream to signal a pull, you can also move full-kanbans downstream to signal a push. For example, when a Wibbler finishes a blue story they don't leave it in their Wibbling column, waiting for a red kanban pull signal, they move it, still in its yellow kanban, into the Fubaring column, like this:

Ideally, the departing full-yellow-kanban will be just-in-time to meet an arriving empty-red-kanban between the columns...

...and the blue-story will flow from the full-yellow-kanban into the empty-red-kanban:

On the other hand, if the Wibblers are working much faster than the Fubarers then the Fubarer's column will fill up with full-kanban push-requests:

As the Fubarer's column fills up with full-kanban push-requests from upstream it's likely it will also be filling up with empty-kanbans pull-requests from downstream (in green say):

Push-me-pull-you is very visual:

• There's one representation of flow ; thin columns with kanban of one colour only.
• There's a different representation of lack of flow ; fat columns filling up (at the bottleneck) with two or three different coloured kanbans.

Push-me-pull-you points to problems:

• If the yellow Wibblers work faster than the downstream red Fubarers, the Wibbler's full yellow kanban get stuck downstream in the Fubarer's column, inviting the Wibblers to help the Fubarers.
• If the red Fubarers work faster than the upstream yellow Wibblers, the Fubarer's empty red kanban get stuck upstream in the Wibbler's column, inviting the Fubarers to help the Wibblers.

Push-me-pull-you has helped me understand:

• pull does not preclude push; you can have push and pull.
• wip-limits are not limited to pull systems; wip-limits would help in push systems too.