Design
Fitness Tracker App
Design a fitness tracking app that logs workouts, steps, and goals, and computes progress analytics.
This is a Low-Level Design (LLD) problem. New to LLD vs HLD? Start here.
01 - Why interviewers ask this
It tests write-time vs. read-time aggregation trade-offs — interviewers check whether you pre-aggregate trends instead of summing raw sensor events on every dashboard load.
02 - Where this system exists in the real world
You interact with this design every day.
Fitbit / Apple Health / Strava tracking apps
Corporate wellness challenge platforms
Physical therapy progress tracking tools
03 - What you'll master
Solve this once. Know it forever.
Aggregate high-frequency sensor data efficiently
Design a rule engine for achievements/badges
Balance write-time vs. read-time computation
04 - What you'll design
6 functional · 2 non-functional requirements.
Log workouts (type, duration, calories, distance)
5 more requirements inside
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05 - Companies that ask this
You may face this exact question in your next interview.
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