What is the difference between backup and replication, and why is replication not a backup?
What they are really testing: Whether you grasp that replication propagates mistakes in real time, while a backup is a point-in-time copy that protects against logical errors. A dangerous misconception if confused.
A real interview question
What is the difference between backup and replication, and why is replication not a backup?
What most people say
drag me
“Replication makes copies, so it already serves as a backup.”
This is the dangerous misconception. Replication copies your data in real time, including a bad delete or ransomware encryption, so every replica is corrupted too. Only a point-in-time backup lets you recover from logical errors.
The follow-ups they ask next
You have three replicas and someone runs a bad DELETE. What happens?
The delete replicates to all three instantly, so every replica loses the data too. Replication has no earlier version to recover from. Only a point-in-time backup taken before the delete can restore it.
Why should backups be immutable or air-gapped?
So ransomware or a malicious/compromised account cannot delete or encrypt the backups themselves. If your backups can be tampered with from the same access as production, they may not survive the very incident you need them for.
What the interviewer is listening for
- Knows replication propagates mistakes in real time
- Backup = point-in-time recovery from logical errors
- Wants both + immutable backups
What sinks the answer
- "Replication is a backup"
- No notion of recovering an earlier state
- Unaware of ransomware/immutability
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“Replication [keeps real-time copies for availability/durability/failover, but it faithfully copies mistakes, a bad delete/corruption/ransomware hits every replica instantly], so it is not a backup. A backup is [a point-in-time copy you restore from, recovering from logical errors]. You need [both: replication for hardware failure, backups (retained + immutable/air-gapped) for human error and ransomware].”
Keep going with storage
Foundation
What is the difference between durability and availability for storage?
Junior
Explain the difference between object, block, and file storage, and give one good use case for each.
Junior
How does object storage achieve such high durability?
Junior
What consistency guarantees does object storage give, and why does it matter?
Junior
For block storage, what is the difference between IOPS and throughput, and how does it affect volume choice?
Mid
Design durable, highly available storage for user-uploaded images. Which service do you choose and why?
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