
A New Copy!
Today we tackle Chapter 7 of Daniel S. Vacanti’s Actionable Agile Metrics for Predictability: An Introduction (buy a copy today). The chapter is titled, Conservation of Flow Part I. The flow of work into and out of a process is incredibly important for establishing predictability. Chapter 7 explains the concept of conservation of flow mentioned (almost in passing) in Chapter 6.
We begin with a reminder of Little’s Law Assumption One: the average arrival and departure rates have to be equal. Vacanti uses an example that is near and dear to my heart, an airport. If planes arrive faster than they depart, at some point all of the gates will be filled followed by the tarmacs, and if the scenario continues for too long the runways would be compromised. The functionality of the airport would initially be stressed, then compromised, and in the end it would fail. This analogy holds for software development teams and organizations. Conservation of flow explains why committing to every request is a losing strategy for satisfying the need to deliver business value.
Vacanti suggests that one way of regulating arrival rate is to add an arrival column. The arrival column contains only the work that has met the conditions of ready and the team has pulled to work on. The arrival column should have a WIP limit. Over any period of time, the work that enters the arrival column should equal the work that departs the process (arrivals = departures). Remember the arrival column is not the backlog.
The arrival column establishes a crisp definition for arrivals. Equally as important is having a crisp (Vacanti uses the terms clear and unambiguous) definition of when a piece of work leave the system or process. This is often known as a definition of done.
In the text, Vacanti provides several excellent charts that show how variability in arrival or departure rates affect predictability (for example, see figures 7.6 and 7.7). For example, as arrival rates increase in relation to departures, WIP increase as does approximate cycle time. Graphically, predictability occurs when the slope of the arrival rate and departure (completion) rate is the same. This shows a steady amount of WIP and consistent approximate cycle time. This scenario will generate a “pretty” cumulative flow diagram (CFD). Vacanti admonishes the reader that a pretty CFD is not a guarantee of a healthy system, but certainly a pretty decent start.
One of the benefits of re-reading a book like Actionable Agile Metrics for Predictability: An Introduction is that you immediately find nuances ideas that you have forgotten. This week I found myself discussing conservation of flow and drawing the relationship between WIP, throughput and approximate cycle time when discussing why starting more than can reasonably be finished in a two-week increment will lead to erratic results.
Previous Installments
Week 2: Flow, Flow Metrics, and Predictability
Week 3: The Basics of Flow Metrics
Week 4: An Introduction to Little’s Law
Week 6: Workflow Metrics and CFDs
Support the author (and the blog), buy a copy of Actionable Agile Metrics for Predictability: An Introduction by Daniel S. Vacanti
Get your copy and begin reading (or re-reading)!
December 10, 2017 at 4:10 am
Vacanti’s description of the average arrival rate constantly exceeding the average departure rate at an airport is a great visualization of an unsustainable work environment. Yet, in software, the incoming feature list flies under the radar and piles up as WIP. The CFD (Cumulative Flow Diagram) is a great tool to help people see this effect.
It is so important to have a crisp definition of when the clock starts for an work-item. In scrum, it is when a user story is accepted by the team during sprint planning.
We need to avoid the situation where the business or the engineering manager thinks the clock has started for a feature, but the team does not. Or worse, the team is not even aware of the ticking feature.
December 10, 2017 at 1:43 pm
Agreed! Everyone n the process needs to understand when the clock begins and when the clock is turned off. Almost like every feature or story should come with a chess timer!.
December 10, 2017 at 10:11 pm
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December 23, 2017 at 11:57 pm
[…] 4: An Introduction to Little’s Law Week 5: Introduction to CFDs Week 6: Workflow Metrics and CFDs Week 7: Flow Metrics and CFSs Week 8: Conservation of Flow, Part I Week 9: Conservation of Flow, Part […]
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January 9, 2018 at 11:57 pm
[…] working on a story (or any other piece of work) the base assumption is that the work will complete (conservation of flow – Actionable Agile Metrics). While once a team starts working on a story actively it will typically complete the story, […]
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