patterns

MONOPOLY — Design Patterns Deep Dive

When to Use

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Table of Contents

1. CQRS
2. Event Sourcing
3. Saga Pattern
4. Circuit Breaker
5. Bulkhead
6. Strangler Fig
7. Sidecar / Service Mesh
8. Outbox Pattern
9. Consistent Hashing
10. Backpressure
11. Leader Election
12. Two-Phase Commit

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1. CQRS (Command Query Responsibility Segregation)

What it is: Separate the read model (Query) from the write model (Command) into distinct services, databases, or code paths.

When to use:

• Read load is 10×+ write load (most web apps)
• Read queries are complex aggregations over write data
• Need to optimize read and write paths independently
• Domain model is complex (DDD contexts)

Implementation:

Write Path:  Client → Command API → Write DB (normalized, PostgreSQL)
Read Path:   Client → Query API  → Read DB (denormalized, Redis / Elasticsearch)
Sync:        Write DB → CDC (Debezium) → Message Queue → Read DB updater

Trade-offs:

• ✅ Independent scaling of read and write
• ✅ Optimized schemas for each operation type
• ❌ Eventual consistency between write and read models
• ❌ Increased complexity; two models to maintain

Real-world users: Amazon (order service), LinkedIn (feed)

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2. Event Sourcing

What it is: Store state as a sequence of immut