Redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe.

Think of a Spare Tire:

1. Your car has 4 wheels to drive.
2. If one wheel pops, you are stuck on the road.
3. The Spare Tire in the trunk is your Redundancy.
4. You don't use it for driving normally, but if the main one fails, it saves your life.

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Types of Redundancy

1. Active Redundancy (N+1): multiple identical components are running. If one fails, the others just keep going (e.g., 5 servers instead of 4).
2. Passive Redundancy (Standby): A second component is sitting there, turned off. It only turns on if the main one dies.

Layers of Redundancy

• Hardware: Dual power supplies, RAID hard drives.
• Service: Multiple instances of the same microservice.
• Geographic: Servers in both Virginia (USA) and Tokyo (Japan) so even a hurricane can't take your site down.

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No Single Point of Failure (SPOF)

Your goal in any design is to eliminate the SPOF. Interview Scenario: "You have a Load Balancer, 3 Web Servers, and 1 Database." The Catch: The Database is the SPOF! If it dies, the whole system dies. The Fix: Add a redundant Database (Replica).

Cost vs Reliability

Redundancy is expensive. You are paying for hardware that you aren't using. Pro Answer: Only add redundancy where the cost of a failure (Downtime/Loss of user trust) is higher than the cost of the extra hardware. 🔁