Design Smart Mirror - System Design | WinJob

Problem statement: ambient smart mirror platform

Design a bathroom/kitchen smart mirror that overlays calendar, weather, fitness, and news on semi-transparent glass while respecting privacy in a shared home.

Numbers to state early

Metric A: 8M installed mirrors
Metric B: p95 widget paint < 180ms
Metric C: local-first UI

Mechanism

The mirror runs Android Things on an edge SoC with a compositor that dims the LCD behind two-way glass; cloud only supplies widget payloads and OAuth tokens—never raw camera frames by default.

Failure and edge cases

Steam fogging the glass, mis-detected face switching to a child's calendar, and ISP outage during morning rush must not blank the mirror—cached layouts and offline clocks remain.

Open with privacy-by-default (camera shutter + on-device face embeddings) before listing microservices; cite 8M installed mirrors when challenged on fleet scale.

Design pressure (understanding)

Shared bathrooms need profile handoff under 400ms without storing video; proximity IR wakes the panel only when someone stands within 60cm.

Java

javaOne Dark Pro

1 public final class MirrorSession {
2   private final String householdId;
3   private final String profileId;
4   private final long layoutVersion;
5  
6   public MirrorSession(String householdId, String profileId, long layoutVersion) {
7     this.householdId = householdId;
8     this.profileId = profileId;
9     this.layoutVersion = layoutVersion;
10   }
11 }

Python

pythonOne Dark Pro

1 @dataclass(frozen=True)
2 class MirrorSession:
3     household_id: str
4     profile_id: str
5     layout_version: int

TypeScript

typescriptOne Dark Pro

1 export interface MirrorSession {
2   householdId: string;
3   profileId: string;
4   layoutVersion: number;
5 }

Why interviewers care

Smart Mirror interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem statement: ambient smart mirror platform that proves you understand real outages, not happy-path diagrams.

Key Highlights

•8M installed mirrors
•Resident → Cloud
•Design a bathroom/kitchen smart mirror that overlays calendar, weather, fitness, and news on semi-transparent glass whil.

1	public final class MirrorSession {
2	private final String householdId;
3	private final String profileId;
4	private final long layoutVersion;
5
6	public MirrorSession(String householdId, String profileId, long layoutVersion) {
7	this.householdId = householdId;
8	this.profileId = profileId;
9	this.layoutVersion = layoutVersion;
10	}
11	}

1	@dataclass(frozen=True)
2	class MirrorSession:
3	household_id: str
4	profile_id: str
5	layout_version: int

1	export interface MirrorSession {
2	householdId: string;
3	profileId: string;
4	layoutVersion: number;
5	}