Heartbeat mechanism in HLD - System Design Exercise

Design: Heartbeat Mechanism System

Design covers heartbeat sending, receiving, monitoring, and alerting. Out of scope are client implementation details and dashboard UI design specifics.

Functional Requirements

FR1: Detect if a client or server is alive by sending periodic heartbeat signals

FR2: Support up to 10,000 concurrent clients sending heartbeats

FR3: Trigger alerts if heartbeat is missed for more than 30 seconds

FR4: Provide a dashboard to show live status of all clients

FR5: Ensure minimal network overhead for heartbeat messages

Non-Functional Requirements

NFR1: Heartbeat interval must be configurable but default to 10 seconds

NFR2: System must handle network delays and temporary outages gracefully

NFR3: Latency for detecting a missed heartbeat should be under 5 seconds after timeout

NFR4: System availability target is 99.9% uptime

NFR5: Heartbeat messages should be lightweight (under 1 KB)

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Heartbeat sender (client or server component)

Heartbeat receiver service

State store or cache to track last heartbeat timestamps

Alerting and notification system

Dashboard or monitoring UI

Load balancer or API gateway

Design Patterns

Polling vs push-based heartbeat

Timeout and retry mechanisms

Circuit breaker pattern for unhealthy clients

Event-driven architecture for alerting

Caching for fast heartbeat status lookup

Reference Architecture

  +------------+       Heartbeat       +----------------+
  |  Clients   | --------------------> | Heartbeat      |
  | (10,000)  |                       | Receiver       |
  +------------+                       +----------------+
                                         |       |
                                         |       | Updates last
                                         |       | heartbeat time
                                         v       v
                                   +---------------------+
                                   | State Store (Redis)  |
                                   +---------------------+
                                         |       |
                                         |       | Triggers alerts
                                         |       | if timeout
                                         v       v
                                   +---------------------+
                                   | Alerting Service    |
                                   +---------------------+
                                         |
                                         v
                                   +---------------------+
                                   | Monitoring Dashboard|
                                   +---------------------+

Components

Clients

Any client platform

Send periodic heartbeat messages to indicate they are alive

Heartbeat Receiver

Stateless REST API or TCP server

Receive heartbeat messages and update client status

State Store

Redis or in-memory key-value store

Store last heartbeat timestamp per client for quick lookup

Alerting Service

Event-driven microservice

Detect missed heartbeats and send alerts/notifications

Monitoring Dashboard

Web UI with real-time updates

Display live status of all clients and alerts

Load Balancer

Nginx or cloud LB

Distribute heartbeat requests across receiver instances

Request Flow

1. Client sends heartbeat message every 10 seconds to Heartbeat Receiver.

2. Heartbeat Receiver validates and updates the last heartbeat timestamp in State Store.

3. Alerting Service periodically scans State Store for clients missing heartbeat beyond 30 seconds.

4. If a missed heartbeat is detected, Alerting Service triggers notifications.

5. Monitoring Dashboard queries State Store to show live client statuses and alerts.

Database Schema

Entities: - Client: client_id (PK), metadata - HeartbeatRecord: client_id (FK), last_heartbeat_timestamp Relationships: - One-to-one between Client and HeartbeatRecord - HeartbeatRecord updated on each heartbeat received

Scaling Discussion

Bottlenecks

Heartbeat Receiver can become overwhelmed with 10,000+ concurrent heartbeat messages

State Store may face high read/write load for heartbeat timestamps

Alerting Service scanning large datasets may cause latency

Network bandwidth could be stressed by frequent heartbeat messages

Solutions

Use multiple Heartbeat Receiver instances behind a load balancer to distribute load

Use a highly performant in-memory store like Redis with sharding for State Store

Implement incremental or event-driven alerting instead of full scans

Optimize heartbeat message size and interval; consider adaptive heartbeat frequency

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing architecture and data flow, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain why heartbeat is needed and how it helps detect failures

Discuss trade-offs in heartbeat interval and message size

Describe components and their responsibilities clearly

Highlight how system handles scale and failure scenarios

Mention monitoring and alerting importance for operational health