I disagree with the opening premise:
> A single database server cannot handle such demand, so we must spread the queries and data out across many servers with database sharding
Did you max out the capacity of the best server you can buy?
Such a database can serve millions of customers (the numbers given).
You always want to scale up the other parts first, request handlers, caching, etc. The day you can no longer inspect the essential state of your system is the day your company better be included in NASDAQ and ready to pay a few hundred engineers 300k salaries.
Well, they are selling this thing so they don't want you to buy a big server (with a read replica) as that's much cheaper.
Surely the cost of running sql server on premium hardware with replication would be more than running on commodity hardware with sharding?
Most non FAANG orgs could probably serve all their customers from postgres on a laptop.
I found the article very helpful from a technical perspective, and didn't focus on the number too much, as it could easily be swapped and the decision-making process for when to shard is kinda out of scope.
But I hadn't considered this, so thanks for pushing back. Good to keep in mind their incentives.
I will say, since their product is a proxy whose interface is a single SQL connection, you should in theory be able to do dev queries through that black box, much the same as application queries? What is so scary here that it would require a hundred engineers?