Design: Design a Notification Service

Endpoints

Add the operations your service exposes. Method, path, and status codes make your API much easier to review.

Start with a template

Applies to all endpoints

Policies that aren't specific to a single endpoint — auth, rate limits, versioning, and other notes.

Authentication

Rate limiting

Versioning

Notes (idempotency, redirect choice, error shape, anything else)

Diagram

Draw the components and how traffic flows between them. Notes below the canvas are optional — the Walkthrough panel is the primary place to narrate flows.

Draw the architecture so the components and connections tell the core story; flow narration in Notes is optional but recommended.

Components

Click palette â†’ add

Drag edge dot â†’ connect

Double-click node/edge â†’ rename

Shift+drag or box-select â†’ multi

Click any component on the left to add it here.

Drag the edge of a node to connect it to another.

Double-click any arrow to label it.

Notes (optional) — flow narration, trade-offs, chosen alternatives0 elements on canvas

Request walkthrough

Trace each core requirement as an ordered sequence of hops through your diagram. Use component names from your canvas for the From / To columns.

1

Accept notification requests from upstream services via API or event bus.

FromToAction / payload

1.

2.

2

Deliver notifications via push (APNs/FCM), SMS (Twilio), and email (SES/SendGrid).

FromToAction / payload

1.

2.

3

Respect user notification preferences (channel opt-in/out, quiet hours, frequency caps).

FromToAction / payload

1.

2.

4

De-duplicate: the same notification to the same user should not be sent twice.

FromToAction / payload

1.

2.

5

Support priority levels (critical: OTP/security, high: transactional, low: marketing).

FromToAction / payload

1.

2.

Storage schema

For each entity, declare how it's stored. Sharding key is the interesting one — pick the access pattern it optimises for.

NotificationRequest

An inbound request: recipient, type, channel hint, priority, template, payload.

In-memory / derived

Storage type

Primary key

Sharding / partition key

Critical fields

Notes (indexes, TTL, access pattern)

UserPreferences

Per-user channel opt-ins, quiet hours, frequency caps, device tokens.

In-memory / derived

Storage type

Primary key

Sharding / partition key

Critical fields

Notes (indexes, TTL, access pattern)

Template

A parameterized message template (subject, body, deep link) per notification type × channel.

In-memory / derived

Storage type

Primary key

Sharding / partition key

Critical fields

Notes (indexes, TTL, access pattern)

DeliveryRecord

Tracks each notification: status (queued → sent → delivered/failed/bounced), timestamps, retry count.

In-memory / derived

Storage type

Primary key

Sharding / partition key

Critical fields

Notes (indexes, TTL, access pattern)

Component choices

Pick one per row and give a one-line reason. These are the concrete technology decisions your diagram implies.

Load Balancer

How traffic is distributed to your app servers.

Queue / Stream

Async work buffer for writes/fan-out.

Database

Primary durable store for entities.

Worker / Dispatcher Pool

The async worker tier that drains the queue and does the slow work (delivery, fan-out, embedding).

Your diagram

No components drawn yet — edit the diagram before answering.

Iterate on your design — don't start over.

Each scenario below probes a specific weakness in a typical HLD. Reference components from your diagram by name, describe what breaks and at what load, then name the minimum change that fixes it. Strong answers identify the precise failure mode — not just "scale it up".

1

APNs (Apple Push Notification Service) is down for 10 minutes. What happens to the millions of queued push notifications?

Probes: failure mode analysis

Your answer

Design a Notification Service

API & core entities

Endpoints

Applies to all endpoints

High-level design

Deep dives