Core Code
Core Code Modern Dev Handbook
SYSTEM-DESIGN Contents
Constraints & Core Metrics
What Is System Design (Production-First) Functional vs Non-Functional Requirements Constraints, Assumptions, and Design Inputs Latency vs Throughput Tail Latency (p95/p99) and Why It Dominates UX SLI, SLO, SLA: The Reliability Contract Capacity Estimation Basics Bottlenecks and Queues (Little's Law Intuition)
Scaling & Traffic Management
Vertical vs Horizontal Scaling Load Balancing Fundamentals Caching Strategies Overview Cache Invalidation and Consistency Rate Limiting at Scale Backpressure: Preventing Overload Timeouts and Retries (Safe Defaults) Graceful Degradation and Traffic Shedding
Data & Consistency
Database Scaling Basics Replication: Read Replicas and Failover Consistency Models (Strong vs Eventual) Sharding Basics Partitioning and Hotspot Avoidance Idempotency and Deduplication Eventual Consistency and UX Schema Evolution and Backward Compatibility
Reliability Patterns
Timeouts: Bounding Damage Retry Strategies (Jitter, Caps, Budgets) Circuit Breaker Bulkhead Pattern Fallbacks and Graceful Degradation Load Shedding and Admission Control Backups and Restore Strategy Disaster Recovery (RPO/RTO)
Observability & Control
Golden Signals: Latency, Traffic, Errors, Saturation Metrics: SLIs and SLO Targets Logging at Scale (Structure and Cost) Correlation IDs and Context Propagation Distributed Tracing Overview Alerting and Runbooks Capacity Planning and Saturation Postmortems and Continuous Improvement
Case Studies
Case Study: URL Shortener Case Study: File Upload and Serving Case Study: Notification System Case Study: Chat System Overview Case Study: Rate Limiter Service Case Study: Social Feed (Fan-out Trade-offs) Case Study: Search Autocomplete Case Study: Analytics Ingestion Pipeline
Distributed Systems Foundations
CAP Theorem Consensus Basics Leader Election Distributed Locking Clock Skew Split Brain Quorum Reads and Writes Eventual vs Strong Consistency (Practical)
Service Communication Patterns
Synchronous vs Asynchronous Request-Response Pattern Message Queues Pub/Sub Pattern Event-Driven Architecture Saga Pattern CQRS Overview Outbox Pattern API Gateway Pattern Backend For Frontend (BFF)
Data at Scale Advanced
Index Design Deep Dive Query Planning Multi-Region Databases Read-After-Write Guarantees Global Databases Data Locality Large Table Strategies Background Jobs Design
Multi-Region Architecture
Multi-Region Basics Active-Active Active-Passive Geo DNS Cross-Region Replication Global Latency Optimization Regional Failover
Cost-Aware System Design
Cost vs Performance Storage Cost Trade-offs Cache Cost Analysis Overprovisioning vs Risk Cloud Pricing Mental Model Right-Sizing Instances
Security Architecture
Zero Trust Service-to-Service Auth Secrets Management Encryption at Rest Encryption in Transit DDoS Protection Multi-Tenant Data Isolation
System Design Interview
Design Twitter Design YouTube Design Uber Design Payment System Design E-commerce Platform Design Distributed Cache

Active-Active

Design highly available active-active systems.
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Active-Active: Two Regions Serving Traffic

Active-active means multiple regions serve production traffic at the same time. The main benefit is regional resilience and local user latency. The main cost is correctness complexity: when both regions accept traffic, you must define how state stays consistent.

Active-Active Variants

  • Active-active reads, single-writer writes: easiest form; reads are local, writes route to a home region.
  • Geo-partitioned active-active: each region owns a subset of data (tenant/user home region).
  • Multi-writer active-active: any region can write any data; requires conflict resolution.

Core Challenge: Write Coordination

If two regions can write the same record, conflicts are inevitable. If your domain cannot tolerate conflict semantics (like last-write-wins), avoid multi-writer for that dataset. A production rule: if you cannot explain conflicts to users, do not accept multi-writer writes for that flow.

Operational Challenge: Partial Partition

Active-active failures are often partial. One region may be healthy, but cross-region replication or routing is degraded. Without clear ownership and timeouts, traffic can bounce between regions, creating tail-latency spikes and inconsistent reads.

Make Locality the Default

Active-active succeeds when most requests are served fully within one region. The more cross-region calls you have on the hot path, the less benefit you get and the more p99 suffers.

Production Checklist

  • Define ownership: single-writer, geo-owned, or multi-writer per dataset.
  • Define read-after-write behavior per endpoint.
  • Instrument replication lag and conflict rates (if applicable).
  • Practice regional isolation: can one region run without the other?

Production-First Takeaway

Active-active is not “turn on multi-region.” It is a strict locality and ownership discipline. Start with active-active reads + single-writer writes, then evolve only if the product truly requires multi-writer behavior.

Active-Passive →