HLDsystem_design~12 mins

Read replicas in HLD - Architecture Diagram

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System Overview - Read replicas

This system uses read replicas to improve database read scalability and availability. It handles many read requests by distributing them across multiple replicas, while writes go to the primary database.

Key requirements include fast read response times, data consistency with some replication delay, and fault tolerance for database failures.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  +-------------------+
  |                   |
  v                   v
Primary DB          Read Replica 1
  |                   |
  |                   v
  |               Read Replica 2
  |
  v
Cache

Components

User

client

Sends requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to appropriate services and manages read/write separation

Primary DB

database_primary

Handles all write operations and replicates data to read replicas

Read Replica 1

database_replica

Handles read requests to reduce load on primary database

Read Replica 2

database_replica

Additional read replica for scalability and availability

Cache

cache

Stores frequently accessed data to speed up read responses

Request Flow - 9 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Cache

Cache → API Gateway

API Gateway → Read Replica 1 or 2

Read Replica 1 or 2 → API Gateway

API Gateway → User

API Gateway → Primary DB

Primary DB → Read Replica 1 and Read Replica 2

Failure Scenario

Component Fails:Primary DB

Impact:Writes fail and data cannot be updated; reads from replicas may be stale

Mitigation:Promote a read replica to primary; use replication lag monitoring to ensure data freshness

Architecture Quiz - 3 Questions

Test your understanding

Which component handles write operations in this architecture?

APrimary DB

BRead Replica 1

CCache

DLoad Balancer

Design Principle

This architecture uses read replicas to separate read and write workloads, improving read scalability and availability. Caches reduce database load further. Replication delay introduces eventual consistency, which is acceptable for many read-heavy applications.