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Comment by CSMastermind | Hacker News Reader

One of the things I think I did right in my career is working on payment and billing systems early on.

It does teach you how to build at least semi-reliable software because people tend to get really mad when you screw with their money. It also is a good introduction to regulation and compliance.

Idempotency was is one of the concepts you get taught on day one in that field (as kind of demonstrated by that being the example used by the author in the article).

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Interestingly, I didn’t have to be taught about idempotency early on in my career; I’m pretty sure I didn’t even know what that word meant. I’m a naturally curious person and I was always curious about the failure cases for the software I worked on—e.g., “What happens if this request is successful but times out before returning a response to the client?” I honestly assumed every programmer asked these types of questions, but I learned the hard way over the years that this isn’t the case.

The only reason I knew about idempotency is because growing up I was a little shit and I learned that you could break a LOT of electronics by getting it to do a second thing midway before finishing doing the first thing.

If it had a screen and buttons, I would try to break it.

So I started striving for highly reliable systems not because there are professional bad actors out there or spammers or to achieve high performance… but because there’s another little shit out there.

> getting it to do a second thing midway before finishing doing the first thing

I'm not sure this is the kind of failure idempotency is design to tackle.

Idempotency tackles the issue of handling multiple requests for doing the same thing, isn't it?

> Idempotency tackles the issue of handling multiple requests for doing the same thing, isn't it?

yeah... there should be no extra side effect if you call something more than once on it with the same parameters. ie foo(x) is the same as foo(foo(x)) is the same as foo(foo(foo(x)))

or in networking GET x is the same as GET x followed by a GET x etc... the GET request doesn't (*shouldn't) change anything.

It gave me an intuitive understanding of state machines and idempotency is one solution to transitions.

If you start with State A and a call changes it to State B, what does running the call again do? A->B? But you’re already at B. Shit’s going to break. Redesign your system.

In this example, it seems the API should return some 4xx error.

If there's a process with two steps, moving from Step A to B requires the process to be at Step A and it's already at step B, it should return an error. At first glance, this doesn't seem the kind of problem that idempotency is supposed to prevent...

Instead of idempotency you can return a 4xx error too. You use idempotency because it’s logistically less complicated for the client.

Hopefully many do, but it is much easier to talk about when you learn the terminology.

Yeah, I need to send this to one of my vendors. We send them an order number, and they send us back an order key from their system. However, we can only look up an order by that key, and they will happily accept and process duplicate order numbers. Therefore, if for whatever reason we don't receive a key during the order submission process, we have no way to look up the order afterwards by order number to see if it went through and what the key is. They don't seem to understand why that is bad.

Another good one is instant messaging. Both latency and reliability (exactly once delivery) are important.

Doing so using microservices will teach you a lot more (but may not be immediately applicable as it's not so favored).

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