[mrkurt]

This is the distribute-your-Rails-app-without-making-any-code-changes version of that story. It works great for apps that are 51% or more read heavy. You drop our library in, add a region, and off you go. The library takes care of eventual consistency issues.

HTTP requests that write to the DB are basically the same speed as "Heroku, but in one place". If you're building infrastructure for all the full stack devs you can target, this is a good way to do it.

Distributing write heavy work loads is an application architecture problem. You can do it with something like CockroachDB, but you have to model your data specifically to solve that problem. We have maybe 5 customers who've made that leap.

In our experience, people get a huge boost from read replicas without needing to change their app (or learn to model data for geo-distribution).

[anon3949494]

It's also trivial to serve read requests from a caching layer or via a CDN. At any sufficient scale, you're probably going to need a CDN anyway, whether your database is replicated or not. You don't want every read to hit your database.

[jrockway]

I don't think this is that trivial. I've never seen it done correctly. It typically manifests itself as not being able to read your own writes, and I see this all the time (often from companies that have blog posts about how smart their caching algorithm is). For example, you add something to a list, then you're redirected to the list, and it's not there. Then you press refresh and it is.

I guess that's acceptable because people don't really look for the feedback; why do users add the same thing to the list twice, why does everyone hit the refresh button after adding an item to the list, etc. It's because the bug happens after the user is committed to using your service (contract, cost of switching too high; so you don't see adding the cache layer correspond to "churn"), and that it's annoying but not annoying enough to file a support ticket (so you don't see adding the cache layer correspond to increased support burden).

All I can say is, be careful. I wouldn't annoy my users to save a small amount of money. That the industry as a whole is oblivious to quality doesn't mean that it's okay for you to be oblivious about quality.

(Corollary: relaxing the transactional isolation level on your database to increase performance is very hard to reason about it. Do some tests and your eyes will pop out of your head.)

[anon3949494]

> It typically manifests itself as not being able to read your own writes

Multi-region databases with read replicas face the same issue

[mrkurt]

Database read request are not the same as readonly HTTP requests. I am much happier having all requests hit my app process than I am trying to do the CDN dance.

Right now your choices are: run a database in on region and:

1. Use the weird HTTP header based cache API with a boring CDN

2. Write a second, JS based app with Workers or Deno Deploy that can do more sophisticated data caching

3. Just put your database close to users. You can use us for this, or you can use something like Cloud Flare Workers and their databases.

My hot take is: if something like Fly.io had existed in 1998, most developers wouldn't bother with a CDN.

Weirdly, most Heroku developers already don't bother with a CDN. It's an extra layer that's not always worth it.