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Deterministic Simulation Testing in Go

I came across the idea of deterministic simulation testing when I watched the conversation between the TigerBeetle CEO and ThePrimeagen. During the conversation he mentioned that they built a tool called the VOPR, which would deterministically inject faults into their application and test it.

The goal of DST is to uncover novel bugs before they occur in production. This is achieved by first removing all sources of randomness in our application. Some sources of randomness include time, I/O devices, and OS thread scheduling. When sources of randomness is removed we can be sure that the application code is executed in the exact same manner, and in the exact same order, every single time.

The reason why we want the app to run deterministically is so that we can trace the exact code path the app took to encounter a bug. This is powerful because more often than not we find bugs in production that we simply cannot reproduce.

The simulation aspect of DST is mainly focused on simulating the kind of faults we find in the real world: dropped packets, duplicated packets, high latencies, something completely unexpected like bit rot, etc. By simulating these faults we can observe how our app behaves before it happens in the real world.

Goals

Fast
Require minimal/no changes in source code

Features & Progress

Simulated clock
- Get current time
- Register callback that is called after at least the given amount of time has passed
- Register callback that is called at regular intervals
- Sleep
- Cancel timers
Simulated I/O
- Filesystem
- Network
Simulated faults
Simulated nodes
- Scheduler
- Network interface
Simulator. Tie it all together.

Latest Update

Maybe I'm going about it wrong

Jan 28, 2026

I just had a thought today, more like a train of thoughts.

One of the goals, a very important one, of this project is to limit or not require the user to change their codebase to use the simulator.

I need to simulate time, I/O, scheduling, etc. and to solve each one I’ve had to introduce some kind of interface that ultimately requires the user to change their codebase. This obviously doesn’t line up with my goals here.

Then I had the thought: why not just write an interpreter for Go? This way I still have control over scheduling, time, etc and the user won’t need to change their codebase.

I’ve never written an interpreter before. I have written parsers though so I’m hoping that experience will help here. Maybe I should think some more? I don’t know.

Go’s standard library already has packages for parsing Go programs so maybe I should start there.

Previous Updates

Initial Thoughts

Jul 12, 2025

The VOPR is very cool. What I think it does is provide interfaces to time, I/O, etc. as a dependency and expects applications to be written that way.

Traditionally you simply expect interfaces to these as part of the standard library and not really as something that can be swapped out.

import "time"

func MyFunc() {
    time.Now()
}

If instead we treated them instead as dependencies then a simulator can swap out the implementation.

func MyFunc(clock Clock) {
    clock.Now()
}

If we can introduce interfaces for time, I/O, etc. then we can simulate their behaviour and more importantly inject some faults.

func FileReader(io IO) {
    io.Read(file) // simulator can inject a fault here
}

The first thing that sticks out to me is that I may need to refactor my code so that the simulator can do its thing. I’d like to avoid that entirely if possible.

Clocks

Aug 22, 2025

I wanted to tackle time simulation first as I began working on this project. I felt this would be the most difficult part of the project and also the most interesting.

Currently we simply import the time package and use its public functions.

import "time"

func main() {
    time.Now()
}

What I tried to do with my clock interface is to keep it very similar to the time package’s interface. This way, I believe, users of this project won’t need to change their codebase too much to benefit from a clock that can be simulated.

// A Clock returns the current time and can create timers
// and tickers. An pplication should use the same clock
// throughout its execution.
type Clock interface {
        Now() time.Time
        NewTimer(d time.Duration) (Timer, <-chan time.Time)
        NewTicker(d time.Duration) (Ticker, <-chan time.Time)
        Sleep(d time.Duration)
}

// A Timer represents an event in the future. This
// interface is kept deliberately similar to the design
// of time.Timer.
type Timer interface {
        Reset(d time.Duration) bool

        Stop() bool
}

The way the clock works is simple: it spawns a goroutine that creates and updates timers, sleeps, and ticks i.e. moves the clock time forward. This goroutine waits to receive from specific channels forever.

This design has the nice property that creating timers and sleeping has the same interface as the time package and even blocks the current goroutine while sleeping.

Scheduler

Oct 24, 2025

It has become clear to me that I need to implement a scheduler as well. We cannot predict in what order goroutines will execute and how the instructions will interleave. This is a source of non-determinism and it needs to be eliminated.

To illustrate assume two goroutines G1 and G2. Both G1 and G2 contain just two instructions.

Goroutine   Instruction

G1          I1
G1          I2

G2          I1
G2          I2

Just in this very simple case there are SIX different ways these two goroutines could interleave.

    1           2           3

G1, I1  |   G1, I1  |   G1, I1
G1, I2  |   G2, I1  |   G2, I1
G2, I1  |   G1, I2  |   G2, I2
G2, I2  |   G2, I2  |   G1, I2


    4           5           6

G2, I1  |   G2, I1  |   G2, I1
G2, I2  |   G1, I1  |   G1, I1
G1, I1  |   G2, I2  |   G1, I2
G1, I2  |   G1, I2  |   G2, I2

If you’ve ever built concurrent applications you know how this can end.

I’ve spent some time and started implementing a basic scheduler. The scheduler has a concept of jobs. It keeps two lists, one for the jobs it is currently running and another for jobs that need to be run in the future. The queued jobs are executed once the current list of jobs is completed.

Jobs can be blocked for a number of reasons: I/O, sleep, etc. The scheduler expects these events to be communicated with it through channels. It runs jobs in a separate goroutine and listens for these events. If it hears nothing and the job completes then we assume the job is completed. Otherwise the job is marked as blocked and queued.

I realise this is too simplistic and probably will bite me in the back.

What About Channels?

Jan 27, 2026

It’s been a hot minute. The project has been sitting on ice, largely because I’ve been busy with my day job.

Over the last few months I went back and forth on the scheduler. At one point I even thought I don’t actually need a scheduler and scrapped whatever I implemented.

No, the scheduler is definitely needed. I definitely need some way to guarantee Goroutines will execute in some definite order.

There are several difficulties that I’ve been trying to wrestle with in my mind with regard to the scheduler. For any two Goroutines it doesn’t really matter in what order they run if they don’t affect each other i.e. no shared state. This holds true only for the simplest of programs.

In the general case multiple Goroutines will most likely affect each other. I was going down a spiral when I was considering how I’m supposed to order reads and writes if two Goroutines were reading from and writing to the same file. I came to the conclusion that it’s not my problem: that’s such a strange scenario and the user is actually supposed to use locks to synchronise such a setup, assuming they want the operations to be ordered.

That’s when I remembered Go has channels. And channels are used to synchronise Goroutines. If the jobs submitted to my scheduler sends to channels and/or waits to receive from channels then the sheduler will be blocked; there is no way for the scheduler to get this information.

Ouch.

One way out of this hole is to introduce a custom Channel type.

type Channel interface {
    Send(val any)
    Recv() (val any, ok bool)
    TrySend(any) bool
    TryRecv() (val any, closed bool, ok bool)
}

We can in theory wrap channels using this interface. The downside however is that we will lose out on some nice language features: <-, ->, and select.

I don’t like this.

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