November 5, 2025

We get a tour of a method by which AWS engineers could have prevented the expensive outage they had. (And you can prevent your outages too!) We also talk a little about extracting knowledge from experts and how to do it right.

You have arrived at the mid-week hump. Have a $container of $beverage and enjoy some reading before you speed along with the rest of your life.

New articles

AWS DynamoDB Outage Analysis

AWS had a very expensive outage recently. They published a summary based on root cause analysis – a common technique in our field. This article shows you an analysis method that is better, can be performed before the outage happens, takes a couple of hours to do, and would have uncovered all the problems that contributed to the outage, and some other ones to boot. This is the magic of systems theory.

Full article (18–60 minute read): AWS DynamoDB Outage Analysis

Flashcard of the week

During cognitive task analysis (CTA), i.e. when we try to figure out how the mind of an expert works, we may want them to construct a timeline of something that happened, so we can ask them things in context. (Forcing experts to answer questions out of context yields nothing useful.)

The critical decision method (CDM) is one of those ways we structure a CTA interview. The first step of the CDM (once we have selected an expert and they have decided on an event to discuss) is to construct a timeline of the event.

What prompts should be used to help people construct a timeline?

We saw the first step was constructing this timeline. For additional hints, the second step is to identify key decision points in said timeline.

The third step is to figure out, for each key decision point, what knowledge the expert drew on, what they observed in their environment that helped them decide, what priorities they followed, what they predicted would happen as a consequence of their decision, etc. The fourth step is to dig into counterfactuals, like what would the expert have done if some information was not available, or what would a beginner have done differently, or what would the expert have done if they had more time, or which part of the expert’s training was not meaningful for this task, etc.

This flashcard is about the first step, constructing the timeline. The answer to what prompts can be used is

None!

During construction of the timeline, we don’t want to interfere with the expert’s memory of the situation. If we offer advice or prompts or ask what-ifs at this time, we risk planting false memories and increase the occurrence of confabulations.

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