DSA TrainerSystem Design · Unit 10
Availability & Failover
It is 2 a.m. and the one machine running your database has a hardware failure. Every request that touches data now errors. Your two app servers are perfectly healthy, and it does not matter at all: the whole product is down until someone wakes up, provisions a new machine, and restores from a backup. Hours, if you are lucky.
Nothing in that story is exotic. Machines fail constantly: disks die, power supplies burn out, someone unplugs the wrong cable. Availability is the discipline of designing so that the failure of one thing does not take the product down with it. The two moves are redundancy (have a second one ready) and failover (detect the death and switch to the second one, fast, ideally with no human involved).
Interviewers love this topic because it is pure judgment: any component you point at, they can ask "and what happens when that dies?" This unit gives you the habit of asking that question yourself, before they do.
The rest of the System Design course is premium
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