Design real-time per-status device counts

Q: Design real-time per-status device counts

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.

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Question

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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).

Design real-time per-status device counts

Quick Overview

Scenario

Task

Clarifications to address

Output / Consumption

Solution

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