[BrentOzar]

> SQL Server (the db you mention) is no slouch when paired with additional CPUs (for vertical linear scaling) and allows for transactionally safe replication to distributed clusters of SQL Servers

Additional CPUs are $7,000 USD per core, and replication is labor intensive. Transactional replication has a nasty habit of breaking as the source tables are changed, and Availability Groups have a ton of bugs (as evidenced by any recent Cumulative Update.)

Saying that SQL Server scales is like saying your wallet scales to hold any amount of money. Sure, it might, but it’s up to you to put the coin in, and it’s gonna for a lot of coin - compared to scaling out app servers, who have generally near no license cost, and code is synchronized at deploy time.

As to recommending you a place to read, I hate to say this, but you could start with my blog. Pretty much every week, I’ve got real life examples turned into abstract tutorials on there from companies who hit scaling walls and had to hire me for help. (Past client examples: Stack Overflow, Google.)

[taffer]

> Additional CPUs are $7,000 USD per core, and replication is labor intensive

Probably still cheaper than trying to impement scalable transactions in higher layers.

[BrentOzar]

> Probably still cheaper than trying to impement scalable transactions in higher layers.

Transactions, yes, I totally agree. That's what databases are for: reliably storing and retrieving data. It's where you start doing domain logic that things get tougher, like (and I wish I was joking) calling cross-continent web services from the database and building HTML inside SQL.

[jmnicolas]

> PostgreSQL has this and it's absolutely free.

Literally the second sentence ...