Two candidates were interviewing for a Sr. Engineer role @ Uber and were given these requirements...for their system design problem... Requirements: – The system must have low latency – It must handle sudden traffic spikes during peak events – It must enforce strict per-user rate limits Same question. Very different interviews. Hence, Very Different Results. Candidate 1: – For low latency, I will add Redis as a cache – For spikes, I will put Kafka in front of the system – For rate limiting, I will use a Redis-based token bucket On paper, it sounds fine. But this is exactly where many Senior and Staff engineers fail. They treat requirements like keywords. They hear “low latency” and respond “Redis”. They hear “spikes” and respond “Kafka”. They hear “rate limiting” and respond “token bucket”. There is no thinking in between. Candidate 2: “Before I walk through a design, I want to clarify a few things.” – When you say low latency, what are we targeting for p95 and p99? – What is the read-to-write ratio and approximate QPS at peak? – Are we rate-limiting per user, per IP, per API key, or a combination? – How strict is the rate limit? Do we drop requests or degrade gracefully? Only after that, they start. First, they sketch a simple, single region baseline. Then they check it against the numbers. Next, they layer in complexity only where it is needed. When they reach rate limiting, they do not just say “Redis token bucket”. They think out loud. – Local in process rate limiting is simple, but not safe for distributed systems. – A central Redis based limiter works, but what about race conditions and clock skew? – If we add per user, per IP, and per API key limits, how does that affect storage and cost? – What happens when Redis is down, do we fail open or fail closed? Candidate 1 threw tools at the problem. Candidate 2 modeled the problem, quantified it, then selected tools. A few weeks later, Candidate 2 got the offer. Candidate 1 kept wondering what went wrong. In 2025, with AI becoming mainstream, you cannot stand out by naming components. Any decent AI can say “use Redis, Kafka, and a token bucket”. Senior and Staff level signal is not tool recall. It is structured reasoning. If your system design answers sound like a list of technologies, you are competing with a search engine. If your system design answers sound like careful thinking under constraints, you are competing at a Senior and Staff level. ------------------ Checkout my Java+Microservices+System design ebook here matamgi.com/java-interview-e…