[rbjorklin]

Scaling horizontally undoubtedly introduces complexity but it also comes with some upsides:

    * DB backups are now (much) faster.
    * Smaller backups means faster restores which reduces your RTO (Recovery Time Objective)
    * If you have a well architectured application a catastrophic DB failure will now only impact a portion of your userbase instead of all of them.

There are probably more good reasons but these are the ones I could think of now.

[mbreese]

Is high availability or easier backups why people look to horizontal scaling though? I don't think that's ever been a primary reason for any story I've read. It's a great "bonus", but I can't think that it would be a compelling reason to choose horizontal vs vertical scaling.

(There are other reasons, of course...)