Read-heavy vs write-heavy systems in HLD - Design Approaches Compared

Design: Read-Heavy vs Write-Heavy System Design

Design focuses on architectural differences and optimizations for read-heavy and write-heavy systems. Does not cover UI or client-side implementation. Does not include detailed security or compliance requirements.

Functional Requirements

FR1: Support a system that can handle either predominantly read operations or predominantly write operations.

FR2: Ensure data consistency and availability for both read-heavy and write-heavy workloads.

FR3: Optimize for low latency in read operations for read-heavy systems.

FR4: Optimize for high throughput and durability in write operations for write-heavy systems.

FR5: Provide scalability to handle up to 100,000 requests per second.

FR6: Support eventual consistency or strong consistency based on workload needs.

Non-Functional Requirements

NFR1: Read-heavy system should have p99 read latency under 100ms.

NFR2: Write-heavy system should sustain 100,000 writes per second with durability guarantees.

NFR3: Availability target of 99.9% uptime for both systems.

NFR4: Data storage must be reliable and scalable.

NFR5: System should handle spikes in traffic gracefully.

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Load balancers

Caching layers (e.g., Redis, Memcached)

Databases optimized for reads (e.g., read replicas, columnar stores)

Databases optimized for writes (e.g., log-structured merge trees, write-ahead logs)

Message queues for write buffering

Replication and consistency mechanisms

Design Patterns

Cache-Aside pattern

CQRS (Command Query Responsibility Segregation)

Event Sourcing

Read Replicas and Sharding

Write-Ahead Logging

Batching and Bulk Writes

Reference Architecture

                    +-------------------+                    
                    |    Clients        |                    
                    +---------+---------+                    
                              |                              
               +--------------+--------------+               
               |         Load Balancer        |               
               +--------------+--------------+               
                              |                              
          +-------------------+-------------------+          
          |                                       |          
+---------v---------+                   +---------v---------+
|   Read-Heavy DB   |                   |   Write-Heavy DB  |
| (with Read Cache) |                   | (with Write Queue)|
+---------+---------+                   +---------+---------+
          |                                       |          
          |                                       |          
   +------+-----+                         +-------+------+   
   | Read Cache |                         | Write Queue  |   
   +------------+                         +--------------+   

Components

Load Balancer

Nginx, HAProxy, or Cloud LB

Distributes incoming client requests evenly to backend servers.

Read-Heavy Database

Relational DB with read replicas (e.g., PostgreSQL with replicas)

Handles mostly read queries with replicas to scale reads.

Read Cache

Redis or Memcached

Caches frequent read queries to reduce database load and latency.

Write-Heavy Database

NoSQL DB optimized for writes (e.g., Cassandra, MongoDB)

Handles high volume of writes with efficient storage and durability.

Write Queue

Kafka or RabbitMQ

Buffers incoming writes to smooth spikes and enable asynchronous processing.

Request Flow

1. Client sends request to Load Balancer.

2. For read-heavy system: Load Balancer routes read requests to Read Cache or Read-Heavy DB.

3. Read Cache is checked first; if cache miss, query goes to Read-Heavy DB.

4. For write-heavy system: Load Balancer routes write requests to Write Queue.

5. Write Queue buffers writes and asynchronously persists them to Write-Heavy DB.

6. Write-Heavy DB ensures durability and replication for fault tolerance.

7. Clients receive responses after read or write operations complete.

Database Schema

Entities depend on application domain but generally include: - Data Entity: Main data stored. - Read Replica: Copies of primary data for scaling reads. - Write Log: Append-only log for write-heavy systems. Relationships: - One-to-many between primary data and read replicas. - Eventual consistency between write log and main data store.

Scaling Discussion

Bottlenecks

Read DB becomes overloaded with too many read requests.

Cache misses cause high latency and DB load.

Write DB cannot sustain high write throughput.

Write Queue becomes a bottleneck if not scaled.

Network bandwidth limits between components.

Solutions

Add more read replicas and scale horizontally for reads.

Increase cache size and improve cache hit ratio with better keys.

Use partitioning/sharding to distribute writes across multiple nodes.

Scale write queue horizontally and use multiple consumers.

Optimize network usage and colocate components where possible.

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying read/write ratios. Spend 20 minutes designing architecture and explaining components. Spend 10 minutes discussing scaling and trade-offs. Reserve 5 minutes for questions.

Clarify workload characteristics (read-heavy vs write-heavy).

Explain caching benefits and cache miss handling.

Discuss consistency models and trade-offs.

Describe how to scale reads with replicas and writes with sharding.

Mention asynchronous processing for writes to improve throughput.

Highlight monitoring and failure recovery strategies.