Backend Loop Questions That Actually Test System Design
Five backend coding questions where the surface is a function but the real signal is your system-design instincts. None of them want the cleverest algorithm; all of them want the right data model and the right failure mode.
By @arjunrivera
March 24, 2026
·
Updated August 15, 2026
439 views
13
Rate
Implement a deduplication helper for incoming webhook deliveries. Each delivery has a signed id; the same id may arrive multiple times because the sender retries. The signal: do you ask about the time window, do you reach for a TTL, and do you flag the failure mode when storage is unavailable.
What you'd see
What you'd see in Redis after dedup.mark_seen('evt_abc'): key wh:dedup:evt_abc with a 3-day TTL. dedup.has_seen('evt_abc') inside the window returns True and the retry is rejected. If Redis is unreachable, has_seen falls open (returns False, event is processed) and on-call sees the alarm fire on the cache-miss spike.
4 more questions and all solutions are locked.
Purchase this item to access all questions, code snippets, and solutions.