That depends on the use case. HN is not a good example. I am referring to business applications where users submit data. Ofc in these cases we are looking at 00s not millions of users. The answer is good enough.

> If your writes are fast, doing them serially does not cause anyone to wait.

Why impose such a limitation on your system when you don't have to by using some other database actually designed for multi user systems (Postgres, MySQL, etc)?

Because development and maintenance faster and easier to reason about. Increasing the chances you really get to 86 million daily active users.

So in this solution, you run the backend on a single node that reads/writes from an SQLite file, and that is the entire system?

Thats basically how the web started. You can serve a ridiculous number of users from a single physical machine. It isn't until you get into the hundreds-of-millions of users ballpark where you need to actually create architecture. The "cloud" lets you rent a small part of a physical machine, so it actually feels like you need more machines than you do. But a modern server? Easily 16-32+ cores, 128+gb of ram, and hundreds of tb of space. All for less than 2k per month (amortized). Yeah, you need an actual (small) team of people to manage that; but that will get you so far that it is utterly ridiculous.

Assuming you can accept 99% uptime (that's ~3 days a year being down), and if you were on a single cloud in 2025; that's basically last year.

I agree...there is scale and then there is scale. And then there is scale like Facebook.

We need not assume internet FB level scale for typical biz apps where one instance may support a few hundred users max. Or even few thousand. Over engineering under such assumptions is likely cost ineffective and may even increase surface area of risk. $0.02

It goes much further than that.. a single moderately sized VPS web server can handle millions of hard-to-cache requests per day, all hitting the db.

Most will want to use a managed db, but for a real basic setup you can just run postgres or mysql on the same box. And running your own db on a separate VPS is not hard either.

>How often does the typical user write to the DB

Turns out a lot when you have things like "last accessed" timestamps on your models.

Really depends on the app

I also don't think that calculation is valid. Your users aren't going to be purely uniformly accessing the app over the course of a day. Invariably you'll have queuing delays above a significantly smaller user count (but maybe the delays are acceptable)