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System Design: Payments where every balance must add up. SQL or NoSQL? Best choice: SQL. Transactions keep related records in step.
AI Engineering: Answer questions across 10,000 documents. Long context or RAG? Best choice: RAG. Retrieves only the passages that matter.
AI Engineering: The same three steps, every time. Workflows or Agents? Best choice: Workflows. Fixed steps: cheaper, faster and easy to test.

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Server concurrency: handling many requests at once

Learn through videos, clear notes, and interactive explanations, at your own pace.

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Part 1 · Fundamentals

01System design fundamentals

01Introduction to system design
02Request lifecycle: what happens when you open a website
03Server concurrency: handling many requests at once
04Latency and throughput: measuring speed and capacity
05Functional and non-functional requirements
06Capacity estimation: users, storage and traffic
07Core system properties: availability, durability, consistency
08Design tradeoffs: making and explaining decisions

02Networking, APIs and load balancing

03Authentication, authorization and security

Module 01 · Lesson 3 of 14Free

Server concurrency: handling many requests at once

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On this page

By the engineers

Harpreet Singh@harpreet_dev
Concurrency finally clicked
I’d read about thread pools and event loops a dozen times. The animated walkthrough made the trade-off obvious in five minutes.
Grace MensahSenior engineer, e-commerce
Better design reviews
The key numbers sheet lives in my bookmarks now. I use it every time we size a new service.
Omar Haddadr/cscareerquestions
Ready for interviews
I walked into my system design round with a framework instead of buzzwords, and it showed.
Ananya IyerBackend engineer, fintech
Depth without the padding
Every lesson gets to the point. I can finish one on my commute and actually remember it.
Rohan Kulkarni@rohan_ships
Applied it the same week
After the caching lesson I fixed a stampede problem we’d been living with for months.
Sneha Joshir/ExperiencedDevs
Now my team’s default
I point new hires here first. The diagrams explain things better than our internal docs.
Minh Tran@minhtran_io
Worth every evening
I study after work, so short lessons matter. Each one leaves me with something I can explain to someone else.
Arjun MehtaPlatform engineer, SaaS
Trade-offs, not recipes
Most courses tell you what to use. This one explains when it stops working, which is what I actually needed.

Harpreet Singh@harpreet_dev
Concurrency finally clicked
I’d read about thread pools and event loops a dozen times. The animated walkthrough made the trade-off obvious in five minutes.
Grace MensahSenior engineer, e-commerce
Better design reviews
The key numbers sheet lives in my bookmarks now. I use it every time we size a new service.
Omar Haddadr/cscareerquestions
Ready for interviews
I walked into my system design round with a framework instead of buzzwords, and it showed.
Ananya IyerBackend engineer, fintech
Depth without the padding
Every lesson gets to the point. I can finish one on my commute and actually remember it.
Rohan Kulkarni@rohan_ships
Applied it the same week
After the caching lesson I fixed a stampede problem we’d been living with for months.
Sneha Joshir/ExperiencedDevs
Now my team’s default
I point new hires here first. The diagrams explain things better than our internal docs.
Minh Tran@minhtran_io
Worth every evening
I study after work, so short lessons matter. Each one leaves me with something I can explain to someone else.
Arjun MehtaPlatform engineer, SaaS
Trade-offs, not recipes
Most courses tell you what to use. This one explains when it stops working, which is what I actually needed.

Tolu AdeyemiSoftware engineer, payments
Finally, a shared vocabulary
Three years in and I still learned something. Now my team and I argue about the same diagram instead of past each other.
Priya Thapa@priyathapa_
Built for my phone
I do most of my learning on the metro. The lessons load fast and read well on a small screen, which is rare.
Aditya Menonr/learnprogramming
Gaps I didn’t know I had
I thought I understood load balancing. The quiz at the end proved otherwise, and the explanation fixed it.
Amara Okeke@amaraokeke
The visuals do the work
The diagrams are the best part. I sketch them from memory now before every design discussion.
Karthik Reddyr/developersIndia
Cleared my SDE-2 loop
Two weeks of evenings with this and I had a clear way to break down any design question they threw at me.
Linh NguyenSoftware engineer, travel tech
Clear without dumbing down
It respects that I’m an engineer. No filler, no hype, just the reasoning behind each choice.
Vikram Choudharyr/devops
Ops finally makes sense
Coming from infrastructure, I never saw the full request path. These lessons connected the pieces for me.
Meera KrishnanSenior engineer, insurance
What I recommend now
When juniors ask where to start with system design, I send them here. It’s the resource I wish I’d had.

Tolu AdeyemiSoftware engineer, payments
Finally, a shared vocabulary
Three years in and I still learned something. Now my team and I argue about the same diagram instead of past each other.
Priya Thapa@priyathapa_
Built for my phone
I do most of my learning on the metro. The lessons load fast and read well on a small screen, which is rare.
Aditya Menonr/learnprogramming
Gaps I didn’t know I had
I thought I understood load balancing. The quiz at the end proved otherwise, and the explanation fixed it.
Amara Okeke@amaraokeke
The visuals do the work
The diagrams are the best part. I sketch them from memory now before every design discussion.
Karthik Reddyr/developersIndia
Cleared my SDE-2 loop
Two weeks of evenings with this and I had a clear way to break down any design question they threw at me.
Linh NguyenSoftware engineer, travel tech
Clear without dumbing down
It respects that I’m an engineer. No filler, no hype, just the reasoning behind each choice.
Vikram Choudharyr/devops
Ops finally makes sense
Coming from infrastructure, I never saw the full request path. These lessons connected the pieces for me.
Meera KrishnanSenior engineer, insurance
What I recommend now
When juniors ask where to start with system design, I send them here. It’s the resource I wish I’d had.

Start free.
Go deeper when you’re ready.

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Annual
$56.55 first year
Billed annually. Cancel any time.
- All courses and paid lessons
- New courses while subscribed
- Premium papers and reading
~~Usual price $87~~ 35% off
Lifetime
$37.70 paid once
Pay once. Keep exploring.
- All courses and paid lessons
- Future courses included
- Premium papers and reading
~~Usual price $58~~ 35% off

A few things you
might be wondering.

Who is LUMO for?

Developers and engineers with some programming knowledge who want a deeper understanding of systems and AI. The courses progress from foundations to more complex concepts, so you can strengthen the basics and keep going.

What does a lesson include?

The planned learning experience combines self-paced, pre-recorded videos, written notes, visual explanations, interactive diagrams, and exercises or questions. The free course preview lets you try written explanations and knowledge checks.

How do the free lessons work?

Selected lessons sit inside the regular course curriculum and are marked Free. You can start with those and upgrade to unlock the paid lessons. The current prototype includes three working System Design lesson previews.

Are the Labs free?

Yes. Labs are LUMO’s own free interactive tools. External resources are separate links to useful documentation, research, and other tools.

What does lifetime access include?

A one-time purchase includes all courses, paid lessons, premium reading, and future courses. Pricing and final purchase terms will be available before checkout opens.

Is there mentoring or a live schedule?

Learning is self-paced. There are no live classes, mentors, or personalised feedback sessions. Learn through the material, interactive explanations, exercises, and questions in your own time.

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Your next step
starts with
understanding.

Choose a course, open a free lesson, and explore an idea you’ve wanted to understand.

Start learning

Become the engineer who understands why.

Know the concepts. Make the decisions.

Choose what you want to understand next.

Keep exploring beyond the lesson.

Server concurrency: handling many requests at once

Change a variable. See what changes.

Read beyond the explanation.

A direction for your curiosity.

A considered reading shelf.

By the engineers

Concurrency finally clicked

Better design reviews

Ready for interviews

Depth without the padding

Applied it the same week

Now my team’s default

Worth every evening

Trade-offs, not recipes

Finally, a shared vocabulary

Built for my phone

Gaps I didn’t know I had

The visuals do the work

Cleared my SDE-2 loop

Clear without dumbing down

Ops finally makes sense

What I recommend now

Start free. Go deeper when you’re ready.

A few things you might be wondering.

Your next step starts with understanding.

Become the engineer
who understands why.

Know the concepts.
Make the decisions.

Choose what you want
to understand next.

Keep exploring
beyond the lesson.

Change a variable.
See what changes.

Read beyond
the explanation.

A direction for
your curiosity.

A considered
reading shelf.

Start free.
Go deeper when you’re ready.

A few things you
might be wondering.

Your next step
starts with
understanding.