DevOps measurement: Work in process limits

When faced with too much work and too few people to do it, rookie managers assign people to work on multiple tasks in the hope of increasing throughput. Unfortunately, the result is that tasks take longer to get done, and the team burns out in the process.

Instead, you should do the following:

  • Prioritize work
  • Limit how much people work on
  • Focus on completing a small number of high-priority tasks

The manufacturing sector has a long history of limiting the amount of work in process (WIP). Factories don't hold large amounts of inventory. Instead, when a customer orders a product, parts are made in-house, on-demand or are pulled from suppliers upstream as needed, and the company then assembles the product just in time. When you implement this process correctly (following a number of principles and practices), you end up with faster lead times, higher quality, lower costs, and less waste.

How to implement work in process limits

Use a storyboard. In technology, our inventory is invisible. There's no shop floor with piles of work or assembly line where we can see the progression of work. A simple way to see inventory is to write all the work the team is doing on index cards and stick them on a board. In agile methods, this is called creating a storyboard.

The following sample storyboard spans multiple functions (analysis, development, testing, operations) and shows all work for a single product.

image

(Source: "Kanban for Ops" board game, Dominica DeGrandis 2013")

A common practice with storyboards is to ink a dot onto a card for every day the card has been worked on. The team can easily see which work is blocked or taking longer than it should.

Specify limits. For each column on the board, specify the WIP limit, or how many cards can be in that column at one time. After the WIP limit is reached, no more cards can be added to the column, and the team must wait for a card to move to the next column before pulling the highest priority one from the previous column.

Only by imposing WIP limits and following this pull-based process do you actually create a Kanban board.

Determine WIP limits by team capacity. For example, if you have four pairs of developers, don't allow more than four cards in the "in development" column.

Stick to the limits. WIP limits can result in teams sitting idle, waiting for other tasks to be completed. Don't increase WIP limits at this point. Instead, work to improve your processes to address the factors that are contributing to these delays. For example, if you're waiting for an environment to test your work, you might offer to help the team that prepares environments improve or streamline their process.

Common pitfalls with work in process limits

Organizations implementing WIP limits often encounter the following pitfalls:

  • Not counting invisible work. It's important to visualize the whole value stream from idea to customer, not just the portion of the work that the team is responsible for. Without doing this, it's impossible to see the actual bottlenecks, and you'll end up addressing problems that aren't actually significant constraints to the flow of work. (This is also known as local optimums.)
  • Setting WIP limits that are much too big. Make sure your WIP limits aren't too big. If your team is splitting their time between multiple tasks or projects, that's a good sign your WIP limits are too high.
  • Relaxing WIP limits. Don't relax your WIP limits when people are idle. Instead, those people should be helping in other parts of the value stream, addressing the problems that are leading to constraints elsewhere.
  • Quitting while you're ahead. If your WIP limits are easy to achieve, reduce them. The point of WIP limits is to expose problems in the system so they can be addressed. Another thing to look for is when there are too many columns on your visual display. Instead, look for ways to simplify the delivery process and reduce hand-offs. Process improvement work is key to increasing flow.

DevOps Research & Assessment research shows that WIP limits help drive improvements in software delivery performance, particularly when they are combined with the use of visual displays and feedback loops from monitoring.

Ways to improve work in process limits

  • Make your work visible. As you do this, try to surface all of your work, making all of it visible, to several teams and stakeholders. (See visual displays for details).
  • Set WIP limits that match your team's capacity for work.
    • Account for activities like production support, meeting time and technical debt.
    • Don't allow more WIP in any given part of the process than you have people to work on tasks.
    • Don't require people to split their time between multiple tasks.
    • When a particular piece of work is completed, move the card representing that work to the next column, and pull the highest priority piece of work waiting in the queue.
  • Set up a weekly meeting for stakeholders to prioritize all work in order. Let stakeholders know that if they don't attend, their work won't get done.
  • Work to increase flow. Measure the lead time of work through the system. Record the date that work started on a card and the date work ended. From this information, you can create a running frequency histogram, which shows the number of days work takes to go through the system. This data will allow you to calculate the mean lead time, as well as variability, with the goal of having low variability: high variability means you are not scoping projects well or have significant constraints outside of your team. High variability also means your estimates and predicts about future work will not be as reliable.
  • Improve work processes. Reduce hand-offs, simplify and automate tasks, and think about how to collaborate better to get work done. After you've removed some obstacles and things feel comfortable, reduce your WIP limits to reveal the next set of obstacles. The ideal is single-piece flow, which means that work flows from idea to customer with minimal wait time or rework. This ideal may not be achievable, but it acts as a "true north" to guide the way in a process of continuous improvement.

Ways to measure work in process limits

WIP limits are something you impose rather than measure, but it's important to keep finding ways to improve. During your regular retrospectives, ask the following questions:

  • Do we know the mean lead time and variability for our entire value stream (from idea to customer)?
  • Are we finding ways to increase flow and thus reduce lead time for work?
  • Are our WIP limits surfacing obstacles that prevent us increasing flow?
  • Are we doing things about those obstacles?

What's next

  • For links to other articles and resources, see the DevOps page.
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