The data structures that matter in system design are not always the ones you spend the most time while preparing for coding interviews.

Arrays, linked lists, and binary search trees are still useful foundations, but system design interviews usually focus on structures that show up in real systems: Bloom filters, LSM trees, inverted indexes, consistent hashing, and sketches.

They help large systems make practical trade-offs around memory, latency, disk access, accuracy, and operational complexity. You use them when data does not fit in memory, disk reads are too slow, keys need to move across machines, or exact answers are too expensive to compute at scale.

In this chapter, we will cover the data structures that come up most often in system design interviews. For each one, we will look at how it works, where it is used, and how to explain it clearly in an interview.