Design real-time per-status device counts
Company: Verkada
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
## Scenario
You have **1,000,000 devices**. Every **minute**, each device sends a message containing:
- `device_id`
- `timestamp`
- `status` (one of **10 possible statuses**)
## Task
Design a system that can **compute in real time** the **number of devices in each status** for the **previous minute** (i.e., for each completed minute window, output 10 counts).
## Clarifications to address
In your design, state assumptions and handle:
- Late or out-of-order messages
- Duplicate messages (e.g., retries)
- Missing heartbeats
- How “previous minute” is defined (processing time vs event time)
## Output / Consumption
Assume a dashboard or API needs to read the counts for each minute (and optionally recent history like last 1–24 hours).
Quick Answer: This question evaluates a candidate's ability to design scalable, fault-tolerant real-time aggregation systems that produce per-minute status counts, including considerations for event-time versus processing-time semantics, out-of-order and late arrivals, deduplication, and missing heartbeats.