system-design

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name: system-design description: Use this skill when designing large-scale systems, choosing between architectural patterns, evaluating trade-offs, designing for scalability and reliability, or preparing for system design interviews. Trigger on keywords: system design, architecture, scalability, microservices, monolith, distributed system, design trade-offs, high availability, load balancing, caching strategy, database selection.

System Design

Design Process (Always Follow This Order)

1. Clarify requirements — functional AND non-functional
2. Estimate scale — users, requests/sec, data volume, read/write ratio
3. Define the API — what does the system expose?
4. Design the data model — what data, how stored, how accessed?
5. High-level architecture — major components and their relationships
6. Deep dive — zoom into the hardest or most critical parts
7. Identify bottlenecks — and how to address them

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Requirements Clarification

Before designing anything, answer:

Functional: What does the system do? Scale: How many users? Requests/sec? Data volume? Growth rate? Availability: What's the acceptable downtime? (99.9% = 8.7h/yr, 99.99% = 52m/yr) Latency: What's the acceptable response time? (p99) Consistency: Strong vs eventual consistency? (CAP theorem) Read/Write ratio: Read-heavy? Write-heavy? Mixed? Geography: Single region? Global? Data sovereignty requirements?

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Scalability Patterns

Horizontal vs Vertical Scaling

	Vertical	Horizontal
Method	Bigger machine	More machines
Limit	Hardware ceiling	Theoretically unlimited
Cost	Expensive	More cost-efficient
Downtime	Required	Zero downtime possible
Complexity	Simple	Requires load balancing, distributed design

Default: design for horizontal scaling from the start.

Load Balancing

• Round Robin — equal distribution, simple, doesn't account for load
• Least Connections — routes to server with fewest active connections
• Consistent Hashing — for distributed caches, minimizes redistribution on scaling

Caching Strategy

Layer	Tool	What to Cache
CDN	Cloudflare, CloudFront	Static assets, public API responses
Application	Redis, Memcached	Session data, computed results, DB query results
Database	Query cache, materialized views	Expensive aggregations

Cache Invalidation Strategies:

• TTL (Time-to-Live) — expire after N seconds. Simple, may serve stale data.
• Write-through — update cache on every write. Strong consistency, slower writes.
• Write-behind — async cache update after write. Fast writes, eventual consistency.
• Cache-aside — app manages cache explicitly. Most flexible.

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Data Architecture

SQL vs NoSQL Decision

Use SQL When	Use NoSQL When
Data is relational	Data is unstructured or variable schema
ACID transactions required	Horizontal scaling is priority
Complex queries and joins	High write throughput needed
Data integrity is critical	Eventual consistency is acceptable

Database Sharding

Partition data across multiple databases:

• Horizontal sharding — split rows (e.g., users A-M on DB1, N-Z on DB2)
• Vertical sharding — split tables by feature (users DB, orders DB, payments DB)
• Hash-based — shard key hashed to determine partition

Read Replicas

• Primary handles writes
• Replicas handle reads
• Good for read-heavy workloads (social feeds, analytics)
• Trade-off: replication lag = eventual consistency

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Architecture Patterns

Monolith vs Microservices

	Monolith	Microservices
Complexity	Low (operational), High (code over time)	High (operational), Lower (per service)
Deployment	Simple, all-or-nothing	Complex, independent per service
Scaling	Scale everything together	Scale services independently
Team size	Small teams, early stage	Larger teams, mature product
Start with	✓ Yes	Only when monolith pain is real

Rule: Start monolith, extract services when you feel the pain. Premature microservices is a common mistake.

Event-Driven Architecture

Use when:

• Services need to be decoupled
• High throughput, async processing
• Audit trail / event sourcing needed

Tools: Kafka (high throughput), RabbitMQ (complex routing), AWS SQS (simple, managed)

CQRS (Command Query Responsibility Segregation)

Separate read and write models:

• Write model — optimized for consistency and transactions
• Read model — optimized for query performance (denormalized, pre-computed)

Good for: high read/write ratio disparity, complex domain logic

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Common System Design Components

API Gateway

• Single entry point for all clients
• Handles auth, rate limiting, SSL termination, routing
• Tools: AWS API Gateway, Kong, Nginx

Message Queue

• Decouples producers from consumers
• Enables async processing, load leveling
• Provides buffer for traffic spikes

CDN (Content Delivery Network)

• Serve static assets from edge locations close to users
• Reduce latency, reduce origin server load
• Cache invalidation is the hard part

Search

• Don't use your primary DB for full-text search
• Elasticsearch / OpenSearch for complex text search
• Typesense for simpler, faster search

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Non-Functional Requirements Cheatsheet

Requirement	Pattern
High Availability	Redundancy, failover, multi-region
Low Latency	CDN, caching, read replicas, edge computing
High Throughput	Horizontal scaling, async processing, queues
Consistency	ACID transactions, consensus protocols (Raft, Paxos)
Durability	Replication, backups, write-ahead logs
Security	Auth, encryption at rest + transit, least privilege

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Trade-Off Framework

For every design decision, explicitly state the trade-off:

Decision: [what you chose]
Why: [what problem it solves]
Trade-off: [what you give up]
Alternative: [what you considered]
When to reconsider: [conditions that would change this decision]

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Back-of-Envelope Estimation

Useful numbers:

• 1M users × 10 req/day = ~120 requests/second
• 1KB × 1M requests/day = ~1GB/day of data
• SSD read: ~0.1ms | Network: ~1ms | HDD: ~10ms
• Single server handles ~10K-50K requests/sec (depends on complexity)
• RAM: 1 server ≈ 64-256GB | Typical process ≈ 100MB-1GB