Jairo Junior

Posted on Apr 26

Know coroutines doesn't mean you know concurrency, read this to know how to understand it.

#kotlin #programming #rust #ai

Hello, I’m Jairo 👋

The dev.to writer you love the most.

(Ok… maybe not yet 😅 but let me believe that for a second.)
First of all, thank you for taking the time to read this.

I hope you learn something new — or at least enjoy the journey.

🚀 Why I built a Thread Pool

This week I was looking for projects to improve my software engineering skills — not just frameworks or tools, but the fundamentals.

Lately, I’ve been learning Rust as my “low-level playground” language, building things closer to the system level. My goal is to understand what happens under the hood.

So I started exploring ideas like:

building a Redis-like database from scratch
understanding memory management
working with threads and concurrency

And that’s when I decided:

“Why not build my own thread pool?”

🧠 Why a Thread Pool?

When learning a new language, we usually focus on things like:

strings
collections
classes / structs

But one thing that’s often ignored:

👉 how concurrency actually works

A thread pool solves a very real problem:

❌ One thread per task → high memory usage

✅ Fixed workers → controlled concurrency

Instead of creating a new thread for every job, we:

create a limited number of workers
reuse them
keep them alive
let them consume tasks from a queue

⚙️ How I built it

The idea is simple:

main → pushes jobs → shared queue → workers → results queue

Workers:

stay alive forever
sleep when there’s no work
wake up when a new job arrives
execute the job
store the result

🧩 Core Architecture

Queue (Arc + Mutex + Condvar)

↓

Workers (threads)

↓

Result Queue

🧪 Real code example

Here’s the core of the worker loop:

loop {
    let mut process = shared_queue.get_process();
    let response = process.run();
    result_queue.push(Result::new(response, process.get_id()));
}

Workers:

continuously fetch jobs
execute them
store results

Shared Queue (the heart of everything)

pub struct Queue {
    processes: Arc<(Mutex<VecDeque<Process>>, Condvar)>,
}

Why this?

Arc → shared across threads
Mutex → safe mutation
VecDeque → efficient queue
Condvar → sleep/wake mechanism

Adding a job

pub fn add(&self, process: Process) {
    let (processes, condvar) = &*self.processes;

    processes.lock().unwrap().push_back(process);
    condvar.notify_one();
}

Getting a job (blocking wait)

pub fn get_process(&self) -> Process {
    let (lock, condvar) = &*self.processes;
    let mut processes = lock.lock().unwrap();

    while processes.is_empty() {
        processes = condvar.wait(processes).unwrap();
    }

    processes.pop_front().unwrap()
}

👉 This is where the magic happens:

if no jobs → thread sleeps
when job arrives → thread wakes up

🧠 Arc + Mutex (the big lesson)

This was the hardest part for me.

Arc

Allows multiple threads to own the same data

Mutex

Ensures only one thread mutates at a time

Together:

Arc<Mutex<VecDeque<Process>>>

This gives you:

shared ownership
safe mutation
no race conditions

Important detail

let queue_clone = queue.clone();

This does NOT clone the data.

It creates a new pointer to the same memory.

Mutex lock

let guard = mutex.lock().unwrap();

Means:

“I want exclusive access to this data”

If another thread is using it:

👉 you wait.

😴 Condvar (sleeping workers)

Without this, your workers would:

loop forever → burning CPU

With Condvar:

no job → sleep

new job → wake up

This makes your system:

efficient
responsive
production-like

💡 What I learned

At some point I realized:

I wasn’t just learning Rust…

I was rebuilding what a dispatcher does internally.

Things became much clearer:

how thread pools work
how coroutines are implemented under the hood
how shared memory works
why synchronization matters

🤖 Using AI as a teacher (not a code generator)

One important thing about this project: I didn’t use ChatGPT to generate the solution for me.

Instead, I used it as a teacher.

Whenever I got stuck, I asked things like:

“Why does this break?”
“What is the responsibility of this component?”
“Am I modeling this correctly?”

And then I implemented everything myself.

That made a huge difference.

Because instead of just copying code, I was forced to:

understand the concepts
reason about concurrency
fix my own mistakes

And honestly, that’s where the real learning happens.

📦 The project

You can check the full implementation here:

👉 https://github.com/jairo-dev-junior/thread-pool

🎯 Final thoughts

This project was one of those “click moments”.

Rust forces you to think about:

ownership
memory
concurrency
synchronization

No magic. Just control.

And yeah… I still need to implement a clean shutdown 😅

Thanks for reading until here 🙏

See you in the next article 🚀

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