foundations
Durability — the guarantee you can't fudge
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Once you say 'stored', you own it forever.
Once your system says “stored”, it owns that data. Durability is the probability that data survives — usually measured in nines (11 nines = one file loss per 100 billion files per year). Availability lets you read; durability keeps the data from disappearing forever.
Interactive nines
11 nines
99.99999999900% chance any given object survives 1 year
Real-world durability targets
S3 Standard: 11 nines. Achieved via 3+ AZ replication + versioning + object-lock.
S3 Glacier: 11 nines. Same replication + cold storage tier.
EBS: 5 nines. Single AZ; snapshot to S3 for higher.
Postgres RDS Multi-AZ: ~7-8 nines with automated backups + WAL shipping.
Consumer SSD: ~3-4 nines. Single-copy = single point of failure.
The durability formula
P(loss) = 1 - (1 - p_single_replica_failure)^replicas
3 replicas of 1% annual failure prob = 1 - 0.99^3 ≈ 3% chance of losing all 3. Add cross-AZ + you go from ~3% to ~1e-9. Add cross-region backup + you hit 11 nines.
Practice what you just read
Every foundation concept has a companion quiz to close the loop.