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

Design Payment System

Design secure and reliable payment processing.
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Problem Statement

Design a payment system that safely creates charges, handles retries, prevents duplicates, and records immutable ledgers. The system must prioritize correctness and auditability over raw throughput.

Functional Requirements

  • Create payment intents / charges
  • Confirm and capture payments
  • Handle refunds and chargebacks
  • Maintain a ledger for auditing

Non-Functional Requirements

  • Strong correctness (no double charge, no lost charge)
  • Idempotent APIs for client retries
  • Auditability and immutable history
  • Secure handling of sensitive data

Idempotency Is Mandatory

Clients will retry on timeouts. Every write endpoint must accept an idempotency key and return the same result for the same key within a defined window. This prevents duplicate side effects even under replay.

Ledger as the Source of Truth

Use an append-only ledger for financial events. Do not “update balances” directly as the primary record; compute balances from ledger entries or maintain derived balances with strict reconciliation.

Workflow and State Machine

Payments are multi-step. Model states explicitly (created, authorized, captured, failed, refunded). Enforce valid transitions and make each transition idempotent.

External Provider Integration

Payment providers can be slow or return ambiguous outcomes. Handle “unknown” states by recording pending status and reconciling via webhooks and periodic polling. Never assume a timeout means failure.

Security Constraints

Use tokenization and avoid storing raw card data. Encrypt at rest and in transit. Strictly control access to keys and secrets. Redact logs aggressively.

Production-First Takeaway

Payment systems are correctness-first. Design idempotency into every write, maintain an immutable ledger, model states explicitly, and reconcile external ambiguity with webhooks and controlled retry logic.

Design E-commerce Platform →