System Design Interview: Sharded Counters Explained

Learn how to explain sharded counters in a system design interview — what the interviewer is really testing, how to walk through the solution, and where most candidates go wrong.

April 12, 202612 min read

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System Design

distributed-systems

scalability

databases

interview-prep

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System Design Interview: Sharded Counters Explained

Learn how to explain sharded counters in a system design interview — what the interviewer is really testing, how to walk through the solution, and where most candidates go wrong.

April 12, 202612 min read

This is premium content

Upgrade to Pro to read the full article and access 200+ premium resources, coding problems, and system design lessons.

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The Question That Trips Up Senior Engineers#

Here's a scenario I've seen play out dozens of times in system design interviews. The interviewer says something like: "Design a system that can handle millions of concurrent likes on a viral tweet." The candidate nods confidently, draws a box labeled "Counter Service," adds a database underneath it, and says "we'll just increment a counter in the DB."

The interviewer writes something down. It's not good notes.

Here's the thing most people miss — this question isn't really about counting. It's about write contention, hot partitions, and whether you understand what happens to a system when millions of requests all hammer the same resource simultaneously. The interviewer is checking if you can reason about why simple solutions bre

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