Below is a screenshot of Vizceral, a tool that was built by a former teammate of mine at Netflix. It provides a visualization of the interactions between the various microservices.

Vizceral uses moving dots to depict how requests are currently flowing through the Netflix microservice architecture. Vizceral is able to do its thing because of the platform tooling, which provides support for generating a visualization like this by exporting a standard set of inter-process communication (IPC) metrics.

What you don’t see depicted here are the interactions between those microservices and the telemetry platform that ingest these metrics. There’s also logging and tracing data, and those get shipped off-box via different channels, but none of those channels show up in this diagram.

In fact, this visualization doesn’t represent interactions with any of the platform services. You won’t see bubbles that represent the compute platform or the CI/CD platform represented in a diagram like this, even though those platform services all interact with these application services in important ways.

I call the first category of interactions, the ones between the application services, as first-class, and the second category, the ones where the interactions involve platform services, as second-class. It’s those second-class interactions that I want to say more about.

These second-class interactions tend to have a large blast radius, because successful platforms by their nature have a large blast radius. There’s a reason why there’s so much havoc out in the world when AWS’s us-east-1 region has a problem: because so many services out there are using us-east-1 as a platform. Similarly, if you have a successful platform within your organization, then by definition it’s going to see a lot of use, which means that if it experiences a problem, it can do a lot of damage.

These platforms are generally more reliable than the applications that run atop them, because they have to be: platforms naturally have higher reliability requirements than the applications that run atop them. They have these requirements because they have a large blast radius. A flaky platform is a platform that contributes to multiple high-severity outages, and systems that contribute to multiple high-severity outages are the systems were reliability work gets prioritized.

And a reliable system is a system whose details you aren’t aware of, because you don’t need to be. If my car is very reliable, then I’m not going to build an accurate mental model of how my car works, because I don’t need to: it just works. In her book Human-Machine Reconfigurations: Plans and Situated Actions, the anthropologist Lucy Suchman used the term representation to describe the activity of explicitly constructing a mental model of how a piece of technology works, and she noted that this type of cognitive work only happens when we run into trouble. As Suchman puts it:

[R]epresentation occurs when otherwise transparent activity becomes in some way problematic

Hence the irony: these second-class interactions tend not to be represented in our system models when we talk about reliability, because they are generally not problematic.

And so we are lulled into a false sense of security. We don’t think about how the plumbing works, because the plumbing just works. Until the plumbing breaks. And then we’re in big trouble.