Why sharding and partitioning are key scaling questions in system design interviews

Database sharding comes up in nearly every system design interview once a single database can no longer handle the data volume or write load. Adding read replicas only increases read capacity, leaving write throughput and storage limits unchanged, so horizontal splitting of data across machines becomes necessary. The shard key determines the quality of the design, and candidates must justify it against read and write patterns while naming trade-offs. Interviewers evaluate problem solving, communication, technical execution, and time management, not just whether the word sharding is mentioned.