Just-In-Time, Work-In-Progress and Kanban
Posted by Pete McBreen 05 May 2022 at 04:50
There are several interesting parallels between Scrum style development and Just-In-Time manufacturing systems, the most useful is the idea of Work-In-Progress (WIP).
- In manufacturing, having a lot of WIP means that all workstations can be kept busy, at the cost of a lot of investment in the partially finished work and a really slow response to changes. With enough WIP any supply chain disruptions have minimal impact, since there are always enough in somewhere in the manufacturing system.
- Scrum has the same sort of problem if there are too many epics/stories/tasks in progress, but the WIP is less obvious other than the problem that there is no working software being delivered from the process. It solves the problem of the product owner being slow to document new epics/stories, which is the manufacturing equivalent of supply chain disruption.
Overall low WIP is preferable for a responsive process, but the downside of too low a WIP level is that it will reveal any process issues that exist. In the manufacturing world, the metaphor that was commonly used was that of water flowing in a river. If you slow the volume of water flowing, then the rocks in river become visible, which should be used as a signal to increase the volume of the WIP until the rocks in the stream could be removed (meaning that the process issues could be fixed).
A specific manufacturing example would be the setup time to start processing a new part. With a large setup time, it makes sense to have a large batch size (and hence a larger WIP) so that the setup time can be amortized over the larger batch. The obvious fix is to do the necessary work on fixtures to reduce the setup time, so that smaller batches are feasible, so that the WIP can be reduced.
In the software world, a branch and associated Pull Request (PR) can be viewed as a batch of WIP. A small PR that touches only a few files is fast to review and merge, with a low probability of any merge conflicts. A large PR that represents multiple days of work modifying many files takes much longer to review and merge, with a high probability of a merge conflict that will further slow the process. The normal choice is to choose to do smaller PRs, but that does not work if the review process (AKA the setup time in the manufacturing process) takes a long time.
So the eventual outcome of a slow review process is that developers will choose to have more WIP, since they will choose to make larger changes, since the review takes a long time. They will also work on multiple stories/tasks concurrently so that they can stay busy rather than being stuck idle waiting for their PR to be merged.
Kanban is a mechanism for controlling the amount of WIP.
- In manufacturing, the available space to store parts on the manufacturing floor is reduced to limit the WIP. Parts can only be produced if there are appropriate places to put the finished components.
- Software does not have the equivalent concept of intermediate storage of WIP, so the equivalent control is to limit the stories/tasks that can be in progress. This is not quite as physical a control as reducing the size and number of the finished part storage, but with appropriate enforcement it can be an effective strategy.