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Financial Systems Design: Metered Billing & Rating Engine
Welcome to the comprehensive Systems Design guide for 19-usage-billing-engine. This documentation series analyzes how an enterprise financial metering, rating, and invoicing platform operates under extreme scale.
System Design Chapter Index
| Chapter | Topic | Focus & Architectural Scope |
|---|---|---|
| 14.1 | Requirements & Capacity Estimation | Functional/Non-functional requirements and mathematical load modeling. |
| 14.2 | Architecture Styles & Ingestion Pipes | Modular monolith, event-driven streaming, and service decomposition. |
| 14.3 | Networking & Communication Protocols | Reverse proxies, HTTP/1.1 vs HTTP/2/3, REST vs gRPC vs SSE. |
| 14.4 | Load Balancing & Tenant Partitioning | Layer 7 load balancing, tenant hashing, and horizontal worker autoscaling. |
| 14.5 | Databases & Immutable Financial Ledgers | PostgreSQL range partitioning, double-entry schemas, and B-tree indexing. |
| 14.6 | Caching & Idempotency Key Stores | Write-Through Redis caches, Bloom filters, and cache stampede prevention. |
| 14.7 | Messaging, Queues & Stream Processing | Kafka / Redis Streams event log, tumbling aggregation windows, and DLQs. |
| 14.8 | Consistency, Concurrency & Locking | ACID transactions, SELECT ... FOR UPDATE, and debit-credit conservation. |
| 14.9 | Reliability, Resilience & Audits | Circuit breakers, retry backoff, and daily differential reconciliation jobs. |
| 14.10 | API Design & Rate Limiting | Idempotency headers, batch ingestion, and token bucket rate limiters. |
| 14.11 | Security, Multi-Tenancy & SOX Compliance | Row-level tenant isolation, SHA-256 API keys, and automated PII scrubbing. |
| 14.12 | Search, Anomaly Detection & AI | Holt-Winters spend forecasting and LLM dispute explanation copilots. |
| 14.13 | Real-Time Telemetry & Drawdowns | Redis atomic counters and sub-second prepaid credit wallet drawdowns. |
| 14.14 | Observability & Operations | Structured audit logs, Prometheus metrics, and deployment pipelines. |
| 14.15 | Performance Engineering & Precision | Fixed-point integer cents vs IEEE 754 floats and benchmark analysis. |
| 14.16 | Cost Modeling & Infrastructure Scaling | Financial cost breakdown at current scale vs 100x ($340 vs $3,200/mo). |
| 14.17 | Classic System Design Scenarios | Designing AWS CloudWatch Billing, Stripe Invoicing, and Rate Limiters. |
| ADR | Architecture Decision Records (ADR) | Formal log of all major architectural decisions and trade-offs. |