Interview Q&A / System Design
System Design interview questions
Caching, scaling, databases and classic design problems for beginners.
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- How do health checks work in a load balancer?
The load balancer probes each server regularly, for example an HTTP request to /health. After several failures it stops sending traffic to that server, and after several successes it adds it back.
BeginnerQuick answer1 min read - What does '99.9% availability' mean, and how much downtime is that?
99.9% availability, called three nines, allows about 8.76 hours of downtime a year, roughly 43 minutes a month. Each extra nine cuts the allowed downtime by ten times and costs much more to achieve.
BeginnerQuick answer1 min read - What is a single point of failure and how do you remove it?
A single point of failure is one component whose failure takes the whole system down. You remove it with redundancy: run several copies, add failover, and spread them across machines and zones.
BeginnerQuick answer1 min read - What is the difference between availability and reliability?
Availability is whether the system is up and responding when asked. Reliability is whether it keeps giving correct results over time without failing. A system can be available yet wrong, so it is available but not reliable.
BeginnerQuick answer1 min read