Enterprise OAuth2/OIDC Authorization Platform for Hundreds of Banks
Zero-Trust Architecture for Hundreds of Banks and 10M+ Daily Users
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OAuth2 Zero-Trust Platform
Zero-trust authentication security operations center
The Challenge
Building Zero-Trust Authentication Platform for Hundreds of Banks with 100% Legacy Compatibility
this major German banking IT provider runs core banking and digital channels for hundreds of banks with tens of millions of accounts, 10M+ daily users and double-digit billions of account bookings per year – plus securities and custody bookings. The challenge was to design and implement a completely new enterprise-grade authentication and authorization platform supporting Zero-Trust architecture while maintaining 100% compatibility with existing non-standard OAuth2 processes, enabling gradual modernization without disruption.
No existing authentication framework - greenfield project requiring architectural decisions from ground up
Must support hundreds of independent banks with approximately 10M+ daily active users across Germany
Existing OAuth2 service has extensive custom, non-standard processes requiring 100% backward compatibility
Zero-Trust security requirements for highly regulated banking environment (BaFin compliance)
No persistent data storage allowed - session and access rights management only in memory/cache
Extreme reliability requirements - banking systems cannot afford downtime (99.9% SLA)
Must provide path to gradually remove custom processes and modernize to standards-compliant OAuth2
Performance requirements: sub-100ms latency for 12,000+ peak tokens/second
Modern DevOps standards needed - team required uplift to state-of-the-art practices
The Solution
Modern OAuth2/OIDC Platform with Zero-Trust Architecture & GraalVM Native Performance
As Lead Developer and Architect, I designed and implemented the complete OAuth2/OIDC authorization framework using Spring Authorization Server (SAS) as the foundation. The architecture strictly separates inner and outer authentication flows with a modern, modular approach while providing 100% backward compatibility with legacy processes. Progressive optimization achieved 71% latency reduction (300-500ms → 68-92ms) and 247% throughput improvement through JVM tuning and GraalVM Native compilation.
Core Framework
Spring Authorization Server (SAS) as base, extended with custom modules for enterprise requirements and legacy compatibility layer
Session Management
Custom Redis-based distributed session replication with Kafka event sourcing - read-once/write-once pattern reducing Redis workload by 80% while improving consistency
Authentication Flows
Strict module separation: Outer OAuth2 flow via SAS + Inner authentication via Central Authentication Service + Legacy compatibility layer
Token Generation
External token service implementation with modern JDK 21 features, JSR-107 caches, and custom security enhancements
Zero-Trust
Complete Zero-Trust architecture with mTLS, certificate-based authentication, and least-privilege access
Smart Migration Routing
Custom Envoy filters with intelligent destination prediction based on token characteristics, banking source, and metrics - enabling zero-downtime migration
Critical Challenges
Key technical hurdles and how they were overcome
100% Backward Compatibility Migration
Problem
Old OAuth2 service had extensive custom, non-standard processes. New framework must provide 100% compatibility while modernizing to standards-compliant OAuth2/OIDC for future maintainability
Solution
Built comprehensive compatibility layer allowing gradual removal of custom processes over time, ensuring zero disruption during migration while providing clear path to OAuth2 standards
Migrated hundreds of banks without a single authentication failure
Impact
Seamless migration with ability to modernize incrementally to standard OAuth2 flows
Redis Session Replication at Massive Scale
Problem
Redis replication with user data caused excessive memory usage (45GB cluster-wide) and traffic across the hundreds-of-banks infrastructure, impacting performance and costs
Solution
Implemented custom session & persistence stack with read-once/write-once pattern, combining Redis caching with Kafka event sourcing for controlled load/save with extremely high consistent state
Impact
Achieved 80% lower Redis workload (45GB → 9GB) while maintaining high consistency and security
Zero-Downtime Migration Strategy
Problem
New and old services use completely different storage and handling. Sticky sessions don't work across incompatible stacks. Migration must be seamless for millions of active users across hundreds of banks
Solution
Developed custom Envoy filters with intelligent destination prediction based on token length, banking source, and custom metrics - routing users to correct stack automatically
Live migration of tens of millions of users between incompatible authentication systems with only minimal session loss overall
Impact
Practically zero lost sessions during migration (<0.001%), soft rollout without any issues
Extreme Performance Requirements
Problem
Authentication latency directly impacts customer experience across millions of daily banking users. Legacy: 300-500ms was too slow for modern digital banking
Solution
Multi-phase optimization: (1) JVM tuning achieved 180-220ms, (2) GraalVM Native compilation reached 68-92ms - 71% total latency reduction with 247% throughput increase
Impact
Customer-facing operations now feel instant (sub-100ms), enabling superior user experience
Business Impact
Measurable value delivered to the business
New Business Opportunities
Custom constraint-based SSO and authorization now configurable instead of code-intensive, enabling new products and complex authorization scenarios previously impossible or requiring extensive development effort
Cost Savings
€4.92M vs. Auth0 Enterprise (€5M vendor cost avoided) + €200k infrastructure reduction through GraalVM Native optimization
Infrastructure Efficiency
From €280k to €80k/year hardware costs while achieving 247% better performance through Native compilation
Customer Experience
Latency reduced from 300-500ms to 68-92ms - banking operations now feel instant to 10M+ daily users
Developer Experience Revolution
Testcontainers integration testing, Spotless code quality, JDK 25, semantic versioning with automated releases & release notes, Docker Compose + local K8s overlay + Istio emulation - complete cloud-independent local development with all services mocked and runnable
Unlimited Growth Potential
Platform architecture eliminates traditional scaling limitations - handles peak loads effortlessly and supports exponential growth without performance degradation. Successfully tested with 30M users with zero architectural changes required.
Innovations
Groundbreaking solutions that set new standards
Hybrid Session Architecture (Redis + Kafka)
Novel combination of Redis caching with Kafka event sourcing for session management, implementing read-once/write-once pattern for optimal performance
Industry first: 80% reduction in cache workload while simultaneously improving consistency and security
Impact: Became reference architecture for all future services at this major German banking IT provider
Smart Envoy Routing for Live Migration
Custom Envoy filters with intelligent destination prediction based on token characteristics, banking source, and custom metrics - routing millions of users between incompatible stacks
Zero-downtime migration of millions of users between completely different authentication architectures
Impact: <0.001% session loss during migration - unprecedented success for banking scale with only minimal session loss overall
GraalVM Native in Mission-Critical Banking
First major German bank to deploy GraalVM Native for mission-critical authentication (0.25s startup, 520 req/s, 68-92ms latency)
93% startup time reduction enabling elastic scaling and 71% infrastructure cost savings in banking production
Impact: New internal standard at this major German banking IT provider: all services now migrating to GraalVM Native for cost/performance optimization
Compatibility-First Modernization Architecture
Architectural pattern allowing 100% backward compatibility with non-standard processes while providing incremental path to standards-compliant OAuth2/OIDC
Solved the 'legacy migration paradox' - achieving full compatibility AND modernization simultaneously
Impact: Enabled migration of hundreds of banks without disruption, with clear roadmap to gradually remove all custom processes
"The OAuth2/OIDC platform has become the foundation of our banking authentication infrastructure. The Zero-Trust architecture and modern DevOps practices raised our entire team's capabilities. The ability to maintain full legacy compatibility while modernizing was crucial for our cooperative banking network at this scale."
Technologies Used
core
persistence
infrastructure
devops
security
testing
performance
Need Enterprise OAuth2/OIDC Expertise at This Scale?
If your organization requires a custom, high-security authentication platform that can't be solved with off-the-shelf SaaS solutions, especially with legacy compatibility requirements at massive scale (millions of users, strict compliance), let's talk.
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