Lessons from Moving a Live Production Database describes the complex process of migrating a massive dataset while keeping the service available to users.

The case study covers specific strategies for achieving zero downtime during a high-risk infrastructure change.

But is zero downtime always worth the engineering effort?

Imagine that you have two options:
1. Spend several weeks preparing and testing a zero-downtime migration.
2. Schedule 20 minutes of downtime during a low-traffic period.
Considering that 99.9% availability allows approximately 43 minutes of downtime per month, which option would you choose?
What factors would change your decision: revenue loss, SLA penalties, customer expectations, rollback complexity, or the size of the engineering team?
https://www.tines.com/blog/zero-downtime-database-migrations-lessons-from-moving-a-live-production/