Secure Payments Platform
Financial Services Company
Context
A mid-sized financial services company operated a payments platform built in 2015. The system processed $2B in annual transactions but relied on deprecated cryptographic libraries and had no formal security audit trail. Regulatory pressure and a near-miss security incident prompted a complete rebuild.
Constraints
- —Zero downtime during migration — transactions could not be interrupted
- —Full backwards compatibility with 47 existing API integrations
- —SOC 2 Type II compliance required within 6 months of launch
- —Engineering team of 3, no additional hires approved
- —Legacy database schema could not be modified until all integrations migrated
Engineering Decisions
Rust for the core transaction engine
Memory safety guarantees without garbage collection pauses. Critical for latency-sensitive payment processing. The compile-time guarantees reduced the surface area for runtime security vulnerabilities.
Event sourcing for transaction history
Immutable audit trail required for compliance. Event sourcing provided natural support for debugging, replay, and temporal queries without additional infrastructure.
Hardware Security Modules for key management
PCI DSS requirement. Chose cloud-managed HSM to avoid operational burden. Implemented key ceremony protocols for root key generation.
Security Considerations
- •All cryptographic operations isolated to HSM-backed service
- •Zero-knowledge proofs for certain transaction verification steps
- •Defense in depth: network segmentation, mTLS between services, secret rotation
- •Penetration testing by third-party before each release
- •Security review gate in CI/CD pipeline
Performance Considerations
- •P99 latency target: 50ms for transaction validation
- •Achieved 23ms P99 through connection pooling and prepared statement caching
- •Redis used for idempotency keys and rate limiting, not as primary data store
- •Horizontal scaling tested to 10x current load
UX Trade-offs
- —API versioning over breaking changes — increased maintenance burden but preserved integrator trust
- —Detailed error codes over generic failures — enabled better debugging at cost of information disclosure risk (mitigated with error code documentation)
- —Webhook retry logic with exponential backoff — balanced reliability with integration simplicity
Failures & Corrections
Failure
Initial HSM integration caused 200ms latency spikes
Correction
Implemented key caching for symmetric operations. Asymmetric operations remained on-demand. Reduced latency impact to <5ms.
Failure
Event sourcing queries became slow at scale during testing
Correction
Introduced CQRS pattern with materialized views for read-heavy operations. Write path remained event-sourced.
Final Architecture
Microservices architecture with 4 core services: Gateway (Go), Transaction Engine (Rust), Notification Service (Node.js), and Reporting Service (Python). All services communicate via gRPC with mTLS. PostgreSQL for primary storage, Redis for caching, Kafka for event streaming.
Outcome
Successfully migrated 47 integrations over 3 months with zero transaction failures. Achieved SOC 2 Type II certification on first attempt. P99 latency improved from 180ms to 23ms. Security audit found zero critical issues.
Why It Matters
This project demonstrated that security and performance are not trade-offs. Thoughtful architecture decisions enabled both. The event-sourcing approach created a foundation for future compliance requirements (audit trails, temporal queries) without architectural changes.
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