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Open source research and development in distributed systems

SimKube: Part 1 - Why do we need a Simulator?

Published on: Aug. 28, 2023 @ 11:00
Written with ❤️ by drmorr

Alright, it's time for my first multi-part series on this blog! I'm gonna talk about my first "big" project that I've been working on at Applied Computing. I think this is going to be a three (or maybe four?) part series. I'll link to the other posts from here as I write them. Here's the outline:

Excited? I am! So buckle up, it's gonna be a ride.

Understanding distributed systems sucks #

If you've spent any time at all working with large distributed systems, you'll know that trying to understand their behaviour is really hard. Distributed systems are, well, distributed — which means that by definition, they are broken up into a bunch of tiny1 components which operate mostly in isolation but periodically have to communicate something about their state to other components in the system. A lot of the research in distributed systems has been around "what happens if something goes wrong with the network"2, which admittedly is a very hard problem. There has been comparatively less research around "what happens when everything is going normally?", which I might argue is an even harder problem.

To put it another way, distributed systems have emergent behaviour. From the Wikipedia article,

emergence occurs when a complex entity has properties or behaviors that its parts do not have on their own, and emerge only when they interact in a wider whole.

This is almost the textbook definition of a distributed system: we have a bunch of small components that create a complex entity, and the complex entity only works because of the interactions of the small components. The trouble is that emergent behaviour is extremely hard to reason about. You can understand each of the small components perfectly in isolation and still not understand the system. You can understand the system at a small scale perfectly, and still not understand the system at a large scale. And distributed systems like Kubernetes, which are built up of dozens or hundreds of "small components" are extremely susceptible to this. In fact, I would say that 75% of the incidents I've witnessed at big companies have been caused because of emergent behaviour that we didn't understand.

Actually it's even worse than that. Because we don't understand the behaviour, when something breaks (and it will break), we don't understand how to fix it. So we make a guess, and apply a band-aid, and change some parameter to fix the immediate problem. But then two months later, something else breaks, and we apply another band-aid, and then another, and then three years later your system is coated in band-aids that themselves interact in unpredictable ways until you can't even fix your existing problem without creating a new one.

This is obviously not a sustainable long-term situation, but what can we do about it? If you come from a mathematics or theoretical CS background, you might try to prove the behaviour of your system. If you can formalize your system and apply logical reasoning, maybe you can show that "undesired" behaviour can't happen. This is not going to be easy, though: formal verification is nearly intractable even for non-distributed systems3. Even ignoring that small difficulty, to have a stable and reliable system we need to somehow be able to reason about all the failure modes we'd like to avoid, and almost by definition we don't know all the failure cases of an emergent system.

Another option, which is more tractable, is testing. Writing good tests is a really important part of software development, and you will find all kinds of arguments on the Internet about the "best" way to write tests. But one of the truisms about testing is that "the bigger and more integrated the system under test, the harder the tests are to write". Tests of small, isolated components are easy; tests of a few components working together are harder; end-to-end tests of the entire system are very hard to write and very hard to maintain4. And you still have the problem that you can only test for the failure cases that you can think of!

A third method that you can use, which initially sounds really promising, is simulation. We don't know how the system will behave with this change, so let's just simulate it and see what happens! Then we can have a lot of confidence in our change! Easy peasy, this blog series is over.

Except… it's not really that simple. The main problem with simulating complex systems is that you have to ignore some of the complexity in order to create the simulation. And when you start ignoring complexity, the results of your simulation are going to diverge from reality, which means you can't always trust your simulation5. So the name of the game here is trying to pick the critical parts of the system that drive the majority of the behaviour that you're interested and throw away the rest. And then hope that the resulting simulation provides some interesting insights6.

Simulating distributed systems sucks #

Back when I was a baby developer, I wrote a simulator as a part of Clusterman, which was an autoscaler that we built at Yelp for Apache Mesos. I'm maybe biased, but it worked pretty well, and it actually did help our infra teams uncover bugs before shipping code out to production. The main problem we ran into back then is that "maintaining a simulator" requires duplicating a bunch of effort whenever things change. First you have to make the change to your production system, and then you have to also make changes to your simulator so that it continues to behave "the same". Eventually for Clusterman, the way things worked in the real world diverged enough from the simulator, and we didn't have enough headcount to maintain two parallel code paths, that most of the simulation code became useless and was removed.

The second problem I ran into with our simulator is "where does the data come from?" In other words, we want to simulate a system running at scale, but how do we know what "at scale" looks like? You could randomly generate 100,000 pods and schedule them at random on 2,000 nodes, but it's fairly well-known that random data isn't often a good representation of reality. The other thing you can do is collect a "trace" of real data and then use that (or a perturbation of it) in your simulation, but how do you know you're collecting the right trace? Separately, how do you store that data and ensure that it's useful in the future?

The last problem I encountered during this time was that understanding the results of a simulation is extremely challenging. We as an industry don't (in my opinion) have a good story around doing data analysis on distributed systems7. In Clusterman, for example, I wrote a bunch of bespoke code to generate some (very pretty, imo) graphs, but if we wanted to change the analysis or look at something different, I had to go write a bunch more code, which proved to not be sustainable.

After that experience, I took a pretty hard-line "simulation is impossible" stance for a few years, but I've recently been revisiting that position. Maybe this is because I've matured8 as a developer and realized that saying something is "impossible" isn't helpful, but also maybe this is because the existence of tools like Kubernetes-in-Docker (aka kind) make it possible to actually run a full-fledged distributed cluster locally with all the "real" control-plane components present. This solves the first (and, imo, the hardest) issue that I ran into with the Clusterman simulator—kind ensures that we don't have to worry about drift between "real" Kubernetes and "simulated" Kubernetes.

From there, we pretty much can see a path to the solution, which coincidentally lines up with the outline for the rest of this blog series. First we need to build some small components to scale the number of simulated nodes in a cluster up and down, and we need to mock out the actual workloads on those nodes (we'll use Virtual Kubelet for this). Next, we need to describe what a trace looks like, and build some tools to collect and store them; at the core, this is just a data collection problem, and we roughly know how to do that. Lastly, we need to answer the data analysis question—this is the one part that I don't have a good solution for right now, but I don't think that this is impossible, either, and I have a few ideas for how to proceed.

Everything sucks #

Well, my goodness, that blog post was negative! Hopefully it wasn't too negative. My goal here was to try to outline the problem space and explain why this is a hard (but not too hard) and important problem to solve. I've talked with quite a few people who really want some way to simulate their Kubernetes clusters; and more to the point, this is also something that I want. As I start getting into my main goal of "doing scheduling in Kubernetes better", I'm going to need some way to compare the performance between the default scheduler and whatever it is that I'm going to build, and simulation is one way to do that. So that's where we'll be going for the rest of this series! Hope you enjoy.

Thanks for reading,

~drmorr

The main article is long enough already, but I do want to call out that there is some related work in this area. I'll just include a few links below with a brief description, and also the caveat that I may be missing something — If I am missing a project here, I'd love to know about it!

  1. And here I use the word "tiny" very liberally ↩

  2. See also: CAP theorem ↩

  3. There has been a bunch of progress on this in recent years; there's been some efforts to formally verify the Linux kernel, and Airbus has been using formal methods to verify their avionics software for over twenty years! But there's still a long way to go before we can easily do formal verification for arbitrary software. ↩

  4. And to top it off, if you're running an end-to-end test at scale, it's gonna cost you a pretty penny, so you're probably not going to do it very often. ↩

  5. In physics, this is the three-body problem. If you have two objects and you want to understand how they interact, you can run a computer simulation (or solve for a closed-form solution). If you have three or more objects and you want to understand how they interact, you can't. Or, well, you can, but the way to understand it is to set them up and press "go", and then watch what happens—you can't have a computer simulate it for you. Three-body systems are chaotic, which means that very small changes in their initial conditions lead to wildly divergent outcomes.

    Perhaps amusingly, I think about this problem in the context of video and role-playing games too. There's a big tension in game design between making things realistic and making things fun. Games try to simulate some version of reality with dice rolls and stats and complex rules, but there's almost always a situation where the rules break down. You get to a point where the only way to simulate reality is to "try it in real life and see what happens" but this becomes problematic when you're trying to stab other creatures with a sword for fun. ↩

  6. All models are wrong, but some are useful. ↩

  7. There're a lot of reasons for this: metrics collection is hard, visualizing data is hard, and statistics is hard. Things have gotten a bit better in recent years with Prometheus and Grafana, but these are monitoring tools, not data analysis tools, which seem like the same problem at first glance until you realize that they're subtly different in a hundred different ways. ↩

  8. Dear peanut gallery: shut up. ↩