Rehabilitating "you can't manage what you can't measure"

There’s a management aphorism that goes “you can’t manage what you can’t measure”. It is … controversial. W. Edwards Deming, for example, famously derided it. But I think there are two ways to interpret this quote, and they have very different takeaways.

One way to read this is to treat the word measure as a synonym for quantify. When John Allspaw rails against aggregate metrics like mean time to resolve (MTTR), he is siding with Deming in criticizing the idea of relying solely on aggregate, quantitative metrics for gaining insight into your system.

But there’s another way to interpret this aphorism, and it depends on an alternate interpretation of the word measure. I think that observing any kind of signal is a type of measurement. For example, if you’re having a conversation with someone, and you notice something in their tone of voice or their facial expression, then you’re engaged in the process of measurement. It’s not quantitative, but it represents information you’ve collected that you didn’t have before.

By generalizing the concept of measurement, I would recast this aphorism as: what you aren’t aware of, you can’t take into account.

This may sound like a banal observation, but the subtext here is “… and there’s a lot you aren’t taking into account.” A lot of things that are happening in your organization, your system, are largely invisible. And those things, that work, is keeping things up and running.

The concept that there’s invisible work happening that’s creating your availability is at the heart of the learning from incidents in software movement. And it isn’t obvious, even though we all experience it directly.

This invisible work is valuable in the sense that it’s contributing to keeping your system healthy. But the fact that it’s invisible is dangerous because it can’t be taken into account when decisions are made that change the system. For example, I’ve seen technological changes that have made it more difficult for the incident management team to diagnose what’s happening in the system. The teams who introduced those changes were not aware of how the folks on the incident management team were doing diagnostic work.

In particular, one of the dangers of an action-item-oriented approach to incident reviews is that you may end up introducing a change to the system that disrupts this invisible work.

Take the time to learn about the work that’s happening that nobody else sees. Because if you don’t see it, you may end up breaking it.

An old lesson about a fish

Back when I was in college [1], I was required to take several English courses. I still remember an English professor handing out an excerpt from the book ABC of Reading by Ezra Pound [2]:

No man is equipped for modern thinking until he has understood the anecdote of Agassiz and the fish:

A post-graduate student equipped with honours and diplomas went to Agassiz to receive the final and finishing touches. The great man offered him a small fish and told him to describe it.

Post-Graduate Student: ‘That’s only a sunfish.’

Agassiz: ‘I know that. Write a description of it.’

After a few minutes the student returned with the description of the Ichthus Heliodiplodokus, or whatever term is used to conceal the common sunfish from vulgar knowledge, family of Heliichtherinkus, etc., as found in textbooks of the subject.

Agassiz again told the student to describe the fish.

The student produced a four-page essay. Agassiz then told him to look at the fish. At the end of three weeks the fish was in an advanced state of decomposition, but the student knew something about it.

I remember my eighteen-year-old self hating this anecdote. It sounded like Agassiz just wasted the graduate student’s time, leaving him with nothing but a rotting fish for his troubles. As an eventual engineering major, I had no interest in the work of analyzing texts that was required in English courses. I thought such analysis was a waste of time.

It would take about two decades for the lesson of this anecdote to sink into my brain. The lesson I eventually took away from it is that there is real value in devoting significant effort to close study of an object. If you want to really understand something, a casual examination just won’t do.

To me, this is the primary message of the learning from incidents in software movement. Doing an incident investigation, like studying the fish, will take time. Completing an investigation may take weeks, even months. Keep in mind, though, that you aren’t really studying an incident at all: you’re studying your system through the lens of an incident. And, even though the organization will have long since moved on, once you’re done, you’ll know something about your system.

[1] Technically it was CEGEP, but nobody outside of Quebec knows what that is.

[2] Pound is likely retelling an anecdote originally told by either Nathaniel Shaler or Samuel Hubbard Scudder, both of whom were students of Agassiz.

There is no escape from the adaptive universe

If I had to pick just one idea from the field of resilience engineering that has influenced me the most, it would be David Woods’s notion of the adaptive universe. In his 2018 paper titled The theory of graceful extensibility: basic rules that govern adaptive systems, Woods describes the two assumptions [1] of the adaptive universe:

  1. Resources are always finite.
  2. Change is ongoing.

That’s it! Just two simple assertions, but so much flows from them.

At first glance, the assumptions sound banal. Nobody believes in infinite resources! Nobody believes that things will stop changing! Yet, when we design our systems, it’s remarkable how often we don’t take these into account.

The future is always going to involve changes to our system that we could not foresee at design time, and those changes are always going to be made in a context where we are limited in resources (e.g., time, headcount) and hence will have to make tradeoffs. Instead, we tell ourselves a story about how next time, we’re going to build it right. But, we aren’t, because the next time we’ll also be resource constrained, and so we’ll have to make some decisions for reasons of expediency. And the next time, the system will also change in ways we could never have predicted, invalidating our design assumptions.

Because we are forever trapped in the adaptive universe.

[1] If you watch Woods’s online resilience engineering short course, which precedes this paper, he mentions a third property: surprise is fundamental. But I think this property is a consequence of the first two assumptions rather than requiring an additional assumption, and I suspect that’s why he doesn’t mention it as an assumption in his 2018 paper.

There is no escape from Ashby's Law

[V]ariety can destroy variety

W. Ross Ashby

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.

Hamlet (1.5.167-8)

In his book An Introduction to Cybernetics, published in 1956, the English psychiatrist W. Ross Ashby proposed the Law of Requisite Variety. His original formulation isn’t easy to extract into a blog post, but the Principia Cybernetica website has a pretty good definition:

The larger the variety of actions available to a control system, the larger the variety of perturbations it is able to compensate.

Like many concepts in systems thinking, the Law of Requisite Variety is quite abstract, which makes it hard to get a handle on. Here’s a concrete example I find useful for thinking about it.

Imagine you’re trying to balance a broomstick on your hand:

This is an inherently unstable system, and so you have to keep moving your hand around to keep the broomstick balanced, but you can do it. You’re acting as a control system to keep the broomstick up.

If you constrain the broomstick to have only one degree of freedom, you have what’s called the inverted pendulum problem, which is a classic control systems problem. Here’s a diagram:

From the Wikipedia Inverted pendulum article

The goal is to move the cart in order to keep the pendulum balanced. If you have sensor information that measures the tilt angle, θ, you can use that data to build a control system to push on the cart in order to keep the pendulum from falling over. Information about the tilt angle is part of the model that the control system has about the physical system it’s trying to control.

Now, imagine that the pendulum isn’t constrained to only one degree of freedom, but it now has two degrees of freedom: this is the situation when you’re balancing a broom on your hand. There are now two tilt angles to worry about: it can fall towards/away from your body, or it can fall left/right.

You can’t use the original inverted pendulum control system to solve this problem, because that only models one of the tilt angles. Imagine you can only move your hand forward and back, but not left or right. Because of this, the control system won’t be able to correct for the other angle: the pendulum will fall over.

The problem is that the new system can vary in ways that the control system wasn’t designed to handle: it can get into states that aren’t modeled by the original system.

This is what the Law of Requisite Variety is about: if you want to build a control system, the control system needs to be able to model every possible state that the system being controlled can get into: the state space of the control system has to be at least as large as the state space of the physical system. If it isn’t, then the physical system can get into states that the control system won’t be able to deal with.

Bringing this into the software world: when we build infrastructure software, we’re invariably building control systems. These control systems can only handle situations that it is designed for. We invariably run into trouble when the systems we build get into states that the designer never imagined happening. A fun example of this case is some pathological traffic pattern.

The fundamental problem with building software control systems is that we humans aren’t capable of imagining all possible states that the systems being controlled can get into. In particular, we can’t imagine the changes that people are going to make in the future that will create new states that we simply could not ever imagine needing to handle. And so, our control systems will invariably be inadequate, because they won’t be able to handle these situations. The variety of the world exceeds the variety our control systems are designed to handle.

Fortunately, we humans are capable of conceiving of a much wider variety of system states than the systems we build. That’s why, when our software-based control systems fail and the humans get paged in, the humans are eventually able to make sense of what state the system has gotten itself into and put things right.

Even we humans are not exempt from Ashby’s Law. But we can revise of our (mental) models of the system in ways that our software-based control systems cannot, and that’s why we can deal effectively with incidents. Because of how we can update our models, we can adapt where software cannot.

The downsides of expertise

I’m a strong advocate of the value of expertise to a software organization. I’d even go so far as to say that expertise is a panacea.

Despite the value of expertise, there are two significant obstacles to organizations to leverage expertise as effectively as possible.

Expertise is expensive to acquire

Developing expertise is expensive for an organization to acquire. Becoming an expert requires experience, which takes time and effort. An organization can hire for some forms of expertise, but no organization can hire someone who is already an expert in the org’s socio-technical system. And a lot of the value for an organization is having expertise in the behaviors of the local system.

You can transfer expertise from one person to another, but that also takes time and effort, and you need to put mechanisms in place to support this. Apprenticeship and coaching are two traditional methods of expertise transfer, but also aren’t typically present in software organizations. I’m an advocate of learning from incidents as a medium for skill transfer, but that requires its own expertise for doing incident investigation in a way that supports skill transfer.

Alas, you can’t transfer expertise from a person to a tool, as John Allspaw notes, so we can’t take a shortcut by acquiring sophisticated tooling. AI researchers tried building such expert systems in the 1980s, but these efforts failed.

Concentrated expertise is dangerous

Organizations tend to foster local experts: a small number of individuals who have a lot of expertise with aspects of the local system. These people are enormously valuable to organizations (they’re often very helpful during incidents), but they represent single points of failure. If these individuals happen to be out of the office during a critical incident, or if they leave the company, it can be very costly to the organization. My former colleague Nora Jones calls this the islands of knowledge problem.

What’s worse, high concentration of expertise can become a positive feedback loop. If there’s a local expert, then other individuals may use the expert as a crutch, relying on the expert to solve the harder problems and never putting in the effort to develop their own expertise.

To avoid this problem, we need to develop the expertise in more people within the organization, which, is as mentioned earlier, is expensive.

I continue to believe that it’s worth it.

Conveying confusion without confusing the reader

Confusion is a hallmark of a complex incident. In the moment, we know something is wrong, but we struggle to make sense of the different signals that we’re seeing. We don’t understand the underlying failure mode.

After the incident is over and the engineers have had a chance to dig into what happened, these confusing signals make sense in retrospect. We find out that about the bug or inadvertent config change or unexpected data corruption that led to the symptoms we saw during the incident.

When writing up the narrative, the incident investigator must choose whether to inform the reader in advance about the details of the failure mode, or to withhold this info until the point in time in the narrative when the engineers involved understood what was happening.

I prefer the first approach: giving the reader information about the failure mode details in the narrative before the actors involved in the incident have that information. This enables the reader to make sense of the strange, anomalous signals in a way that the engineers in the moment were not able to.

I do this because, as a reader, I don’t enjoy the feeling of being confused: I’m not looking for a mystery when I read a writeup. If I’m reading about a series of confusing signals that engineers are looking at (e.g., traffic spikes, RPC errors), and I can’t make sense of them either, I tend to get bored. It’s just a mess of confusion.

On the other hand, if I know why these signals are happening, but the characters in the story don’t know, then that is more effective in creating tension in my mind. I want to read on to resolve the tension, to figure out how the engineers ended up diagnosing the problem.

When informing the reader about the failure mode in advance, the challenge is to avoid infecting the reader with hindsight bias. If the reader thinks, “the problem was obviously X. How could they not see it?”, then I’ve failed in the writeup. What I try to do is put the reader into the head of the people involved as much as possible: to try to convey the confusion they were experiencing in the moment, and the source of that confusion.

By enabling the reader to identify with the people involved, you can communicate to the reader how confusing the situation was to the people involved, without directly inflicting that same confusion upon them.

The Gamma Knife model of incidents

Safety researchers love using metaphors as a framework to describe how accidents happen, which they call accident models.

One of the earliest models, dating back to 1931, is Herbert W. Heinrich’s domino model of accident causation:

Image source: https://risk-engineering.org/concept/Heinrich-dominos

About sixty years later, in 1990, James Reason proposed the Swiss cheese model of accident causation:

By Davidmack – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31679759

About seven years later, in 1997, Jens Rasmussen proposed the dynamic safety model. This model doesn’t have an evocative a name as “domino” or “Swiss cheese”. I like to call it the “boundary” model, because everyone talks about it in terms of drifting towards a safety boundary:

This diagram originally appears in Rasmussen’s paper Risk management in a dynamic society: a modelling problem. I re-created the diagram from that paper.

I haven’t encountered a good metaphor that captures the role of multiple contributing factors in incidents. I’m going to propose one and call it the Gamma knife model of incidents.

Gamma knife is a system that surgeons use for treating brain tumors by focusing multiple beams of gamma radiation on a small volume inside of the brain.

Multiple beams of gamma radiation converge on the target. From the Radiosurgery wikipedia page.

Each individual beam is of low enough intensity that it doesn’t affect brain tissue. It is only when multiple beams intersect at one point that the combined intensity of the radiation has an impact.

Every day inside of your system, there are things that are happening (or not happening(!)) that could potentially enable an incident. You can think of each of these as a low-level beam of gamma radiation going off in a random direction. Somebody pushes a change to production, zap! Somebody makes a configuration change with a typo, zap! Somebody goes on vacation, zap! There’s an on-call shift change, zap! A particular service hasn’t been deployed in weeks, zap!

Most of these zaps are harmless, they have no observable impact on the health of the overall system. Sometimes, though, many of these zaps will happen to go off at the same time and all point to the same location. When that happens, boom, you have an incident on your hands.

Alas, there’s no way to get rid of all of those little beams of radiation that go off. You can eliminate some of them, but in the process, you’ll invariably create new ones. There are some you can’t avoid, and there are many that you don’t even see, unless you know how to look for them. One of the reasons I am interested in otherwise harmless operational surprises is that they can reveal the existence of previously unknown beams.