04 Mar 2021, 14:06

The Refinery, an Analogy for Distributed Systems

Back when I was in Engineering school, my first-year internship happened in a refinery. In retrospect, this turned out to be extremely relevant for my current job in tech.

The subject of my internship was the optimization of an existing process. The unit had been planned on paper by an engineer on another continent, installed according to specs, and it turned out to be … not working so well.

Don’t believe Tech is unique

A refinery is a distributed system. There are specs and basically internal contracts on each sub-unit regarding the quantity it should process per day, what the requirements for inputs and the desired output characteristics are. Instead of queries, the inputs and outputs are, you know, oil and gas.

There are continuous and batch processes. Just like in tech, the interface between these is the subject of a lot of literature and ops knowledge.

In tech, services have availability and latency SLOs. In a refinery, there are input and output SLOs (plus specs like purity, sulfur content, water content, etc.).

In tech, there are error budgets. In a refinery, you have emission budgets as a limiting factor. You may only send x amount of NOx or SOx or CO2 into the air over the course of the day. You may only go over the target value for n hours per month, otherwise the company pays a fine. The water that leaves the grounds may only be so-and-so polluted and have at most y degrees of temperature, otherwise there is another fine. And so on.

And just like in tech, contractors do the darndest things, although in tech, you rarely get a truckload of methanol dumped into your waste water stream.

Safety and Postmortem culture

In a tech company, we might say that prod is on fire when there is an incident, but in a refinery, things might literally be on fire or explode.

I quickly learned to be distrustful of recently renovated spaces, because that means it may have been blown up recently. My intern office was recently renovated; at some point, they showed me the photos of the incident that had all but destroyed the building the previous year.

The oil and gas industry has had a strong safety culture for years. The postmortem culture that SRE is so proud of had been a thing in the industry for years. Thus, when there was a large fire in a refinery in Texas City (the refinery was run by a competitor!), we got hold of a copy of the postmortem analysis, so that we could learn from it and avoid making the same mistakes. There was a meeting where we went through it together, discussed and acknowledged various lessons.

I am not sure how blameless these postmortems are. The problem is that in a real incident, people can get hurt or killed. At some point, it involves responsibility in a legal sense. The director of a unit may be responsible for incidents resulting in damages to others, including in a criminal court. Fortunately (?) for tech, incidents usually do not have this kind of real-world impact.

Dev/Ops Divide

Maybe even more than in tech, there is a divide between “dev” (as in, process engineers and planners) and “ops” (actual operator crews who are literally opening and closing giant valves with their two hands).

As a budding chemical engineer, I interned in process engineering. I quickly found out that many process engineers don’t actually bother finding out how the units they planned perform day to day. On the other hand, the most helpful thing I could do was to sit with the ops in the control room, drink a lot of coffee, and listen to their mental models of the unit. They could perfectly explain it in terms of “when I open this valve too much, the temperature over here jumps up and I have to open this other valve to compensate”. But they cannot tell you why. The trick of the good engineer is to take what you know about physics, thermodynamics, etc. and translate this into an understanding of what actually happens.

Often, what actually happens is not what the planning says. In all cases, there are variables that you did not take into account (outside temperature anyone?).

In addition, some real-world boundary conditions are hard to express in correlation matrices and linear programming models. For example: “If you open this valve too much for too long, the bit in the middle will clog with corrosive white slime deposits”. What kind of coefficient is that?

Your Monitoring is lying

Just like in tech, your monitoring is lying to you! You could have a miscalibrated sensor, but what is more likely is that the sensor is placed in a way that it does not show you the true picture.

In our case, there was a laser measurement in a gas conduit where the laser sometimes passed through the stream of flowing gas and sometimes it didn’t. The wildly fluctuating output has no basis in the actual gas composition. In another part of the unit, a sensor was placed in such a way that it was disturbing the flow, so it was actually modifying the behavior of the system.

My two most important variables (the composition of the feed stock and the composition of ammonia) were not even available as measurements. In a refinery power plant, you burn whatever is left over at that moment. For gas, that may be pure hydrogen, pure butane or anything in between. And don’t get me started on the disgusting sludge they call heavy gas-oil.

As for the concentration of ammonia in the ammonia feed (yes, really), I asked for a manual measurement so I could have at least one data point. They told me it was impossible. I asked why. The answer was “when we open the storage drum, it catches fire. The contents are pyrophoric.” Yay.


The recommendation at the end of my internship was to switch off the unit and to tear it down. My manager agreed but it turned out to be impossible because other units had come to depend on this process to consume their input or output feeds.

So for all I know, the piece of crap unit I tried to optimize in the early 2000s is still there, making the surrounding air just a tiny bit dirtier.


Do not believe that tech is different from other, existing disciplines of engineering. There are other industries that have worked on distributed systems. Many software engineers I know are interested in the aerospace industry for a similar reason, which usually means binging on plane crash documentaries. But throughout many different industries, you can find solutions to many of the same issues that you have with your distributed cloud microservice mesh, or whatever.