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Design a smart lock system

PremiumIoT systemEasy

  • BLE
  • Digital keys
  • Offline access

Quick answer

This ioT system interview question asks you to design a smart lock system. Aim for a design that collects data from many devices and acts on it reliably, then explain device connections, ingestion, storage and trade-offs. Interviewers want to see how you handle unreliable networks, device scale and data volume.

The problem

Design a smart lock system: a battery-powered door lock that opens from the owner's phone over BLE, from anywhere through a Wi-Fi bridge, and with keypad codes for guests. The hard parts are opening the door with no internet, revoking a guest key on a lock that is offline, and lasting more than a year on four AA cells.

The lock has a motorised deadbolt, a keypad, a door position sensor and a BLE radio. A small bridge plugged into a nearby wall socket links the lock to home Wi-Fi. Owners give family and guests digital keys that can be limited to certain days and hours.

What must a smart lock system do?

  • Phone unlock: A phone with a valid key unlocks the door over BLE within 1 second, with or without internet.
  • Remote unlock: The owner unlocks from anywhere through the bridge within 3 seconds.
4 more requirementsUnlock to see all

How well must it do it?

  • Security: A recorded BLE exchange can never be replayed to open the door later.
  • Never locked out: Low batteries, a dead bridge or a cloud outage never stop the owner from getting in; the inside thumb-turn always works by hand.
2 more requirementsUnlock to see all

What scale should you design for?

Use these numbers in your estimates.

The scale numbers unlock with this questionUnlock

Key concepts

  • BLE
  • Digital keys
  • Offline access
  • Challenge response
  • Wi fi bridge
  • Battery budget

How you’ll practise

6 short steps · save and come back any time
  1. Requirements
  2. Estimate
  3. Device and connectivity
  4. Ingestion and processing
  5. Device management and updates
  6. Wrap-up

Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.

What do interviewers look for?

In any IoT system design round.

  • Clarifies device count, message rate and payload size
  • Handles devices that go offline and reconnect
  • Plans ingestion that absorbs bursts
  • Separates hot real-time data from long-term storage
  • Covers device identity, updates and security
  • Explains what runs on the device and what runs in the cloud

Questions people ask about this interview question

Which design type is this question?
It is an ioT system question. You design a smart lock system and explain your choices the way you would in an interview.
Is this question asked at entry level?
It is marked Easy, so it suits engineers who are new to ioT system interviews. It is also a good warm-up before harder questions.
What should I cover first?
Start with the devices, what they send, how often and what must happen with it. Agree on scope with the interviewer before you design anything.
Can I practise it for free?
It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.

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Updated · Written by the SevyDevy team

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