The Payments Engineer Playbook

The Payments Engineer Playbook

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3 Articles To Start Building Payments Software

Alvaro Duran's avatar
Alvaro Duran
May 04, 2025
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Every company operating online has a software system built to handle their money.

But only a tiny few run without problems.

Engineers who get hired to maintain these systems are a dime a dozen. They come in, do what they’re told, spend some time learning the ropes, and eventually abandon, leaving a wake of false assumptions, technical debts, and operating nightmares in their wake.

You don’t want to be one of these generalists. You want to do a good job.

But the task of building payment systems is daunting:

  • Money is on the line

  • You must act quickly

  • Mistakes are hard to fix in retrospect

So, if you’re asking yourself “how can I stay on top of the payment systems I’m responsible for?”

Then you’ve come to the right place.

Here are 3 articles on the basics of building ledgers

The must read for every engineer who wants to get good at building ledger technology quickly:

The Intuition of Money

The Intuition of Money

Alvaro Duran
·
Jan 8
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The API of digital cash, what makes digital cigarettes look exactly like bank notes, consists of the following intuitions: Atomicity, Conservation, Fungibility and Storability (and the hidden one, Non-reproducibility).

Ledgers are simply a way to translate physical cash into digital money. To the extent that they embody the properties of money, that translation becomes seamless.


A Tale of Two Ledgers

A Tale of Two Ledgers

Alvaro Duran
·
Jan 15
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Most ledgers can only server one of two use cases. The most powerful can serve the two.

Double-entry accounting has existed for thousands of years. And yet, turning physical ledgers into digital ones requires a deeper understanding of what constitutes a source of truth, and the limitations imposed by the CAP theorem.


Engineers Do Not Get To Make Startup Mistakes When They Build Ledgers

Engineers Do Not Get To Make Startup Mistakes When They Build Ledgers

Alvaro Duran
·
September 11, 2024
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I used to work for a startup that, on every transaction, simply lost track of a couple of cents. As if they fell from our pockets every time we pulled out our wallets.

At this startup, a stock trading platform, the engineering team had followed the mantra of “make it work, make it right, make it fast”; we refused to build a double-entry accounting system.

We could’ve taken the time to build it right. We could’ve done things better. But we didn’t.

Stories like this don’t get aired very often because they’re embarrassing. But I believe it happens all the time.

Not losing track of money is the bare minimum. And yet, every engineer in the industry has a story of a time when they screwed that up big time. That shouldn’t be happening anymore.


And if you want to dive deeper into the topic, check out the ledgers tag.

Here are 3 articles on building payment systems

These systems are responsible for accepting and submitting payments from and to external entities. Building them is tricky, not because they are conceptually difficult, but because they look easy on the surface. These articles will help you navigate the hidden obstacles and the accidental sources of technical debt:

Stripe Made The Obvious Choice When Building Its Payments API. It Took Two Years To Fix It.

Stripe Made The Obvious Choice When Building Its Payments API. It Took Two Years To Fix It.

Alvaro Duran
·
July 31, 2024
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You’ll find a lot of content out there on how to use Stripe’s API.

However, if you want to build payment systems, there isn’t much content out there. Stripe, obviously, isn’t going to help you build one. And so, there’s little about why Stripe’s API was built that way.

The truth is that Stripe's redesign is less simple, but serves a more valuable purpose. Even if it isn’t the simplest route.

Because the simplest route is a mistake.

Stripe’s API is the best way to programmatically develop and interact with a Payment Service Provider out there. 10 years ago, it was as simple as 7 lines of code to get your payments up to speed with them. But they’ve abandoned that, and for good reason: it was wrong.


Cringey, But True: How Uber Tests Payments In Production

Cringey, But True: How Uber Tests Payments In Production

Alvaro Duran
·
August 7, 2024
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There’s pretty much one way to produce high quality software. Use it, and fix all the bugs you can find in it. In time, the easy bugs are gone.

Unlike the human body, old software is so healthy. If that’s the only way to produce high quality software, the only illnesses you’re going to find are the exotic ones.

Payment systems are an extreme version of this

Uber tests its payments system in production. On the summer of 2024, I argued that you should too, and it took Hacker News by storm.


Payments Patterns: Elements of Reusable Money Software

Payments Patterns: Elements of Reusable Money Software

Alvaro Duran
·
Jan 2
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A payment is a promise made by an authorized party about a transfer.

Payment methods look diverse, but that’s because you have the wrong framework to approach them. A good taxonomy on all payment methods, existing and none, can make or break the architecture of your payments system.


And if you want to dive deeper into the topic, check out the payment systems tag.

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Sashi Kumar Nagulakonda's avatar
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Engineers Do Not Get To Make Startup Mistakes When They Build Ledgers
Practical accounting for fintech engineers, and also how not to repeat my silly mistakes.
Sep 11, 2024 • 
Alvaro Duran
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The Payments Engineer Playbook
The Payments Engineer Playbook
Engineers Do Not Get To Make Startup Mistakes When They Build Ledgers
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Cringey, But True: How Uber Tests Payments In Production
Well-run payment systems are developed by engineers who understand what is the best use of their time: to catch unknown unknowns, and to do it fast.
Aug 7, 2024 • 
Alvaro Duran
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The Payments Engineer Playbook
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Cringey, But True: How Uber Tests Payments In Production
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Why Payments Engineers Should Avoid State Machines
Event-driven money software is replayable and pull-based. Why are state machines still prevalent in payments?
Sep 25, 2024 • 
Alvaro Duran
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Why Payments Engineers Should Avoid State Machines
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