Scrum and Kanban: Like Chocolate and Peanut Butter
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This post is primarily geared toward folks who have been doing Scrum for at least 6-9 months and are curious about what options are out there for further improving their process. One reason for looking beyond Scrum may be that you are bumping your head on the iteration boundaries. A common place to look is Extreme Programming (XP). There are many great practices in XP that complement Scrum. Even if you personally don’t want to do pair programming, XP has much more to offer than just pair programming, take a look!
Whether you’ve looked at XP or not, another great place to take a look is Kanban. Kanban is a process into itself and can be adopted whether you are doing Scrum or not, but I will be looking at Kanban as a source of good ideas to apply to an existing Scrum process.
One of the reasons that I am assuming 6-9 months of Scrum as the starting point is that Kanban requires a certain level of experience with breaking work down into small user stories and doing One Piece Flow which I believe is easier to attain via Scrum than starting with Kanban from scratch.
One of the biggest and most noticeable differences between Scrum and Kanban is that Kanban doesn’t have iterations. At first this seems outlandish, but consider that most of the impact of having iterations is at the beginning and end of the iteration. In the midst of an iteration, the iteration boundaries are immaterial; everything you are doing is story-oriented. In that respect, Scrum and Kanban are the same.
Our Old Friend: The Decoupling Principle
Now imagine that you are in the midst of a Scrum iteration… and the end of the iteration never comes. Whenever the team finishes a story, they move on to the next story, just as in Scrum. The difference here is that the “todo hopper” is being constantly filled by the product owner.
One question at this point is “but what about the per-iteration activities such as retrospectives?” And that is one of the first things about Kanban that can be applied to Scrum, with no need to remove the iterations themselves. There is an underlying principle in Kanban which is a well known and widely used principle in programming: the decoupling principle. When doing Kanban, you still need to do the equivalent of planning, assignment, estimation, retrospectives, delivery, etc. In Kanban, all of these activities are decoupled from each other whereas in Scrum they are all coupled to the iteration boundary.
How can this be applied to Scrum? Consider retrospectives. If you are just starting with Scrum, you probably have an iteration length of 1 month (or four weeks). From that it follows that you will have a retrospective once per month. If you eventually end up with an iteration length of 1 week, then it follows that you will have a retrospective every week. But this actually seems like the wrong way to set the cadence of retrospectives. Wouldn’t it be better to have the cadence of retrospectives meet the need for them? If it eventually makes sense to do a retrospective every week, doesn’t it make sense to get the benefit of them on a weekly basis when you are just starting Scrum?
Go deeper: “Applying the Decoupling Principle to Scrum”
Do One Thing and Do it Well
Kanban has something called “work in progress limits.” Limiting work in progress is a concept that comes from Lean. The basic idea is that the less you have in progress the simpler everything becomes and the more likely you are to actually get work finished. If you believe in Scrum then by extension you pretty much also believe in work in progress limits. An iteration is in effect a work in progress limit. If you have an iteration length of two weeks and a velocity of 60 story points then you are saying that you will take on no more than 60 points of work per two week period. Kanban takes this further and says that in general the smaller you can make your work in progress limits, the better.
One of the most straightforward ways to create a WIP limit is to do a headcount of the # of developers on the team. If you have three developers, that gives a WIP limit of 3. That means that you never have more than 3 stories in progress at any given time. If you have been doing Scrum for a while, are good at breaking work down into small stories, and are doing One Piece Flow well, then what you are doing now is very similar to having WIP limits.
Take it to The Limit
Once you are comfortable with having a WIP limit, the next step is to remove iterations all together. Hopefully, this doesn’t seem quite so outlandish any more. After all, you are just exchanging one constraint for another. There is a hidden danger here though, which is one of the reasons that I strongly recommend building up the discipline of One Piece Flow first.
Let’s say you have an iteration length of two weeks. That also puts a two-week limit on your stories. In Kanban, if you only have a WIP limit on the number of stories you can take on, you could end up doing things like working on a task for months and months without finishing it. Luckily, there is a simple cure for this problem: impose either a maximum elapsed time rule, a per-story story point limit, or both.
One more thing to consider here is that if you had an iteration length of 4 weeks, there's a good chance that most of the stories were no more than a week from start to finish, and probably on the order of days. You may want to consider having a per-story elapsed time limit of a week, possibly 2 weeks to start.
Kanban in Action
Now that we’ve taken a look at the various aspects of Kanban, let’s see what this would look like in practice. First of all, we’ve started with Scrum so we still have a backlog, a Scrum Master, a Product Owner, user stories, and the like. As soon as a story is completed, the team takes another story from the “todo” list and starts working on it. That then triggers the product owner to add another story from the backlog to the todo list. Stories move from backlog, to todo, to coded, to tested, to done and then on to "shipped" on a regular basis. If you have been doing Scrum well, this is essentially what you have been doing anyway, but now you no longer have the awkward iteration boundaries. Instead, you have a continual flow of stories with no need to stop artificially.
Yes You Kanban
In summary, a good approach to take when adopting some or all of Kanban is:
- Do Scrum for 6-9 months
- Get good at breaking work into small user stories
- Get good at doing One Piece Flow
- Apply the decoupling principle to your Scrum implementation
- Impose work in progress limits
- Drop iterations and impose a story point limit
Limited Work in Progress Society (Kanban resources)
Kanban vs. Scrum (Friendly comparison)
One day in Kanban Land (Kanban Cartoon!)
Scrumban (Fully mixed!)
If you are in the process of moving to Kanban from Scrum or adopting some of Kanban in your Scrum implementation, let us know how it is going! What worked well for you and what are some of your lessons learned?
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