Doing Work in Small Batches and Limiting Work in Progress
Recently Software Development has been borrowing a lot of ideas from Japanese Lean Manufacturing. In a way this mirrors the progression from the Detroit way of building cars pioneered by Henry Ford to the modern way pioneered by Toyota.
The original Ford production line approach was that a car was assembled on an assembly line by going through a series of specialized stations that were optimized to do one things really well and efficiently, so the car would go from perhaps the attach doors stations to the add Window glass station. Each station was optimized to do its job really well and the priority was to keep the assembly line moving no matter what. If the assembly line ever stopped then the hundreds of people working on it would be idled and this was considered a disaster. If problems occurred then they were noted and fixed post-assembly line. This process was then improved by optimizing each station to be quicker and more automated. Generally this produced a very good system of producing a very high volume of identical cars.
From the outside a Toyota assembly line looks nearly identical. But it operates very differently. Toyota didn’t have the same large market as Detroit and didn’t need the volume. They needed to produce more specialized cars and needed to switch their assembly lines quickly from one run to another. Their emphasis wasn’t on making very specialized machines to do one task quickly; it was on producing general machines that could do a number of different things and could be switched quickly from doing one job to another. Toyota then does small runs on their assembly line rather than one massive long run. The big benefit to doing things in small batches was that quality problems are found much sooner. Hence they can stop the assembly line on the first car and fix a problem, rather than producing thousands of cars with the problem and then possibly not fixing the problem because it’s too expensive at that point. This also re-enforces the practice of fixing problems right away rather than letting them pile up and that it’s more cost effective to have the production line idle while the problem is fixed than having to fix it later.
Another example from the book “Lean Thinking” by James Womack and Daniel Jones recounts a story of one of the authors stuffing envelopes with their two young children. The children wanted to take the approach of folding all the letters, then putting all the folded letters in the envelopes then sealing all the envelopes then putting stamps on all the envelopes. This seems like an efficient and intuitive way to approach the problem. However the author insisted they do it the less intuitive way of one complete envelope at a time, i.e. folding one letter, stuffing it in an envelope, sealing it and putting a stamp on and then going on to the next one. Which way is faster? It turns out that researchers have timed groups of people performing this task and it turns out the one at a time approach is faster! It has a couple of other advantage as well. One is that if there is a problem, you find out right away; if the envelope is the wrong size, you find out on the first letter, not after you have folded all the letters, this could save you then having to refold all the letters to fit the envelopes. Similarly if you take a break you know how far along you are, if you’ve completed 40 out of 100 letters then you are 40% done and you can estimate your remaining time easily. You can’t do this when you do each step completely. This process is called “single piece flow” and is an example of doing a task as lots of small batches rather than one large batch. This is a general principle you want to apply in everything you do, resisting the old Ford type assembly line type mentality. You also receive the first finished product off the assembly line quicker, so you can validate it with your customer before too many more are produced to ensure you are producing the right thing.
Kanban vs Scrum
To some degree batch size is at the heart of the difference between the Kanban and Scrum methods of developing software. Scrum tends to batch things into sprints (usually two or three weeks long) which is much smaller than traditional waterfall development, but Kanban wants to take it further. In Kanban you only work on one or two small things at a time and finish one before starting the next things. You operate on lots of small batches, but you also have to limit how many batches (or tasks) you can have in progress at a time. This is called limiting work in progress (WIP). In Kanban you don’t worry about batching things up into sprints, just keeping the jobs small and limiting WIP. Kanban requires a lot of discipline and often if a development team hasn’t successfully progressed from waterfall to scrum, then adopting Kanban is a disaster.
The goal here is to produce small high quality increments that are immediately validated by customers. This way if you are going in the wrong direction, you won’t get too far before discovering it and not too much time will have been wasted (only a couple of small batches).
How do you get customer feedback, one small increment at a time? Obviously you can’t have dozens of customers download, install and play with a new version of your product every time you add a small increment and then interview them about it. But there are techniques you can use to do just this. The first is to produce your software via “continuous deployment”. This means that every build of your software is automatically deployed to customers whether that means pushing it to a web server or loading it on an automatic software updater service. You need to be able to control which customers get it and you need to be able to roll it back if something goes wrong. To be confident with this sort of procedure you need really good automated testing in place to ensure you haven’t accidentally broken something. Then to find out whether your increments (or batches) provide customer value, you need really good instrumentation in your product to report back on customer usage, whether they use the feature, how long it takes to perform tasks and any other useful statistics.
Software products are large and complicated systems. Managing that complexity is a major challenge. Breaking large tasks down into many small tasks is a powerful tool to reducing complexity. Combining many small but high quality parts results in emergent complexity, but again it is much easier to manage by adding one small thing at a time rather than suddenly throwing several large items into the mix at once.
Most of the ideas in this blog posting come from my reading the “Lean Startup” by Eric Ries over the holidays. Quite a good book with many useful ideas on product development and innovation. Reducing Batch size and limiting WIP appeal to me to help make Sage more agile and innovative as we move forward.