The 3am page that defines your career
At 3am, an availability zone goes dark. If your system is well-designed, nothing happens, traffic shifts, replicas take over, and you sleep through it. If it isn't, you wake up to a dead service, a CEO asking "how long until we're back," and a second question that's even worse: "how much data did we lose?" Those two questions, how long and how much, are the entire subject of this article.
Most engineers conflate high availability and disaster recovery. They're not the same thing, they cost wildly different amounts, and treating them as one is how teams either over-spend on resilience they'll never use or discover, during an actual outage, that their "backups" can't be restored. This is a senior topic because the hard part isn't the technology. It's deciding how much resilience is worth paying for.
Note: Who this is for: Engineers who can already deploy a service and now own its uptime. You should know what an availability zone and a database replica are. We use AWS terms, but Multi-AZ, cross-region replication, and the RTO/RPO framework map directly onto Azure and GCP.
HA vs DR: survive now, recover later
