Walk into any system design interview involving file storage, and S3 will almost certainly come up.

The reason is simple: S3 solves the hard problems of storing unstructured data at scale, durability, availability, and cost optimization, so you can focus on building your application.

It looks deceptively simple: you PUT an object, you GET an object, and it “just scales.” That simplicity is exactly why S3 shows up everywhere—from storing profile photos and video segments to backups, data lakes, logs, and machine learning datasets.

But the real value of S3 isn’t the API. It’s the set of guarantees and trade-offs behind it: durability through replication, availability through distributed control planes, performance shaped by key design and request patterns, and cost shaped by lifecycle policies and storage classes.

If you only know “S3 stores files,” you’ll miss the design decisions that make systems reliable and efficient at scale.

This chapter will give you the depth to discuss S3 confidently in interviews.

1. When to Choose S3