Requirements

Log workouts
View history & basic stats.
Sync across devices.
DAU: Around 10 million users, as fitness apps can attract a large user base.
Concurrency: At peak times, you might expect around 5-10% of DAU to be active simultaneously. This would translate to approximately 500,000 to 1 million concurrent users.

User Activity Data: Typically, activity logs and workout data are retained for at least 1-2 years. This allows users to track their progress over time and review historical data.
Personal Information: User profiles and personal data might be retained for as long as the user maintains their account, but it's good practice to allow users to delete their data upon account deletion.
Aggregate Data: For analytics purposes, aggregated data can often be retained indefinitely, as it doesn't identify individual users.
Type of Activity: Running, cycling, swimming, weightlifting, etc.
Duration: Total time spent on the workout.
Distance: For activities like running or cycling, the distance covered.
Calories Burned: Estimated calories burned during the workout.
Heart Rate: Average and peak heart rate during the workout (if applicable).
Repetitions and Sets: For strength training, the number of sets and reps performed.
Workout Intensity: User-defined or calculated intensity level.
Location: GPS data for outdoor activities, if applicable.
Date and Time: When the workout took place.