Kotlin Expert in Hamburg

• Reviewing functional and technical requirements from a testing perspective
• Creating test cases and automated tests to validate requirements
• Performing manual and automated functional, end-to-end, and regression tests
• Documenting test results and tracking defects
• Using models like GPT-4, BERT, and Hugging Face Transformers for automated test case generation, analysis of test results, and improving test coverage, including bias checks and security reviews
• Techs: MS Office, Jira, Zephyr, Confluence, Tosca, stakeholder communication, Agile, Kanban, Scrum, OpenAI API, Hugging Face, PyTorch, LangChain.

traigo is VTG's digital rail logistics and fleet management platform. It processes large volumes of telemetry, mileage, geofence, sensor and wagon-movement events in near real time and provides operational services for rail logistics customers across Europe.

As part of Team Customer Selfcare, I worked on the design, implementation, optimisation and operation of large-scale backend services and event-driven processing pipelines — covering both feature development and operational ownership of business-critical production systems. I also regularly acted as first responder for production incidents, data inconsistencies and performance investigations across multiple distributed services.

• Design and implementation of event-driven backend services.
• Migration and replacement of legacy processing pipelines.
• Development of replay / rebuild mechanisms for large event datasets.
• High-throughput asynchronous event processing on SNS / SQS.
• Database and query optimisation for PostgreSQL and DynamoDB.
• Design of scalable read / write models and aggregation pipelines.
• Production troubleshooting and operational support.
• Performance tuning and infrastructure scaling.
• Design and stabilisation of integration and system tests.
• Technical concepts, architecture documentation, and cross-team collaboration.
• Support the further development of existing GitLab CI/CD pipelines

Geofence & Wagon Stay Processing

• Algorithm to detect vehicles within geofences (entry, exit, dwell time).
• Event sourcing with guaranteed chronological order within the affected time window.
• Refactored geofence event and wagon-stay processing logic for performance.
• Resolved race conditions and event-ordering problems in distributed services; server-side filtering, aggregation and optimised query pipelines.
• Repair and replay tooling for corrupted or inconsistent movement data.

Fleet Metadata & Mileage

• Modernised the service; migrated storage from DynamoDB to PostgreSQL to improve traceability and accelerate new features.
• Scalable mileage aggregation and replay mechanisms.
• Read / write models and optimised queries for high-volume mileage calculations.

Sensor & Telematics Integration

• Integrated telemetry and sensor processing pipelines.
• Snapshot and state-calculation logic for sensor systems.
• APIs and persistence models for wagon sensor data; data-quality improvements.
• Further development of a service using gRPC for intra-service communication.

Movement Segment Processing & Routing

• Migrated services to new movement-segment event streams.
• Built replay and rebuild tooling for segment correction.
• Optimised throughput and reliability for high-volume event processing.

Condition Monitoring & Wagon Analytics

• APIs and backend services for wagon condition monitoring.
• Brake-wear prediction processing and wagon analytics functionality.
• PostgreSQL views and optimised query models for operational dashboards.

Operational Reliability - First Responder

• Investigated production incidents and distributed-system failures; DLQ analysis, replay and operational recovery.
• Tuned database performance and AWS infrastructure under production load.
• Improved observability, monitoring and operational tooling.
• Supported rollout strategies, monitoring and post-deployment stabilisation.

• Restructuring for product-led growth in the field of digital user-centric tech product development language learning experiences
• Scaling empowered tech product teams
• Anti-fragile software engineering

• Project lead ‘Cloud Migration’ for moving the on-premise production environment to Scaleway.
• Transformed a Docker-Swarm legacy setup to a modern Kubernetes-based cloud environment.
• Architected a GDPR-compliant cloud landscape and deployment setup – 100% European sovereign cloud.
• Hands-on bootstrapped the cloud environment with Terraform, ArgoCD, and GitLab Pipelines CI/CD.
• Replaced the legacy VPN with modern mTLS PKI and deep AD integration.
• Managed an 11-headed agile team using Kanban, moderating team meetings and plannings.
• Successfully finished the migration, moving infrastructure, services, and data, from planning to execution.

• Further development of the Android app "StarMoney"
• Major XML to Compose redesigns
• Introduction of new technologies and architecture
• Implementation of instant bank transfer
• Development following the agile Scrum process

• Solution for micro-service and frontend solution for the electric grid
• Leading of development teams
• Technologies: React-Native, TypeScript, Kotlin, AWS, Infrastructure As Code, Serverless Architecture, React, NoSQL, GraphQL, Angular, Playwright, Python, ML(Ops), Kubernetes, Docker

• Role: Developer, Architect
• Technologies: java, typescript, golang, spring (boot, web, security, data), Angular, AWS (OpenSearch, Aurora, SNS/SQS, CloudWatch, EC2, IAM), Kubernetes, Terraform, Helm, OAuth, Keycloak, CI/CD, gradlew, Liquibase, Test Driven Development, shell scripting

• Further development of a finance layer for subscription and one-time purchase billing and accounting.
• Technologies: Kotlin, AWS (SQS, S3, ElastiCache, Aurora), React, TypeScript, Coroutines, IntelliJ IDEA, Spring (Boot, Data, WebFlux), JUnit, Testcontainers, Cucumber, Liquibase, PostgreSQL, Webservices (REST), R2DBC, OpenAPI 3, Gradle, GIT, Docker, Netty, GitHub Actions, Jira/Confluence, arc42

• Migration of 3 DE/AT payment iOS/Android apps and web portal to global solution
• Performed a fit gap analysis for 6 consumer apps vs. global solution
• Managed the migration project to global CWC solution, developed end-to-end tests for backend systems, Pi Planning, release planning

• Manage and operate the product data import services for the Otto merchant
• Enhance and maintain the backend systems
• Optimize and maintain AWS infrastructure
• Build a new product data import API
• Design and plan stories and features
• Conduct code reviews to ensure code quality and best practices
• Analyze and fix bugs
• Technologies: Java, Spring Boot, Kafka, AWS, Fargate, Terraform, MongoDB, Mongo Atlas, OpenAPI, GitHub, GitHub Actions, GitHub Copilot, Akhq, Debezium, JUnit, Test Containers, Hexagonal Architecture

• Developed solution to filter heavily noisy analog sensor data by combining high-speed sampling with digital firmware filtering
• Increased battery life of a battery-powered medical device from about 10 days to about 18 months
• Supported the overall project up to initial clinical tests and TÜV approval

• Contributed to the development of the Rubix editorial system
• Implemented various microservices based on Java, AWS S3, AWS SQS, AWS SNS, and Spring Boot, deployed to AWS ECS and AWS Fargate
• Designed and developed AWS Lambdas using TypeScript
• Used PostgreSQL in an AWS RDS Aurora cluster and AWS DynamoDB
• Implemented continuous deployment with GitLab pipelines
• Built an Infrastructure as Code environment with AWS CDK
• Set up and maintained a monitoring platform using AWS CloudWatch
• Developed various frontend components with Vue.js
• Designed the microservice architecture applying Domain Driven Design and GraphQL interfaces

• Operation and further development of the inventory system for booksellers (Quimus). It is an in-house development by Libri that is sold to customers as Software as a Service. The software is developed in an agile way by two developer teams (about 5 developers each). The software consists of around 25 Java microservices that mainly communicate via messaging and share a common Angular frontend. The application runs on Kubernetes in AWS.
• Technologies used: Java 17 & 21, Spring Boot 2 & 3, Hibernate, MySQL, Spring Cloud AWS, Lombok, AWS SQS, AWS SNS, AWS RDS, AWS S3, DynamoDB, Kubernetes, Docker, OpenSearch, Hibernate Search, Liquibase, Gradle, Terraform, Helm, GitLab CI, Keycloak OAuth2, TypeScript, Angular
• My focus until December 2023: Connecting additional POS systems to the inventory system. Connecting the data warehouse for report display. Extending existing features (goods receipts, invoicing, item management, ...). Operations and DevOps tasks.
• Focus from January 2024: Extracting the product search from the inventory system into a global service to use in other applications. Integrating the product search into the booksellers' online shops (also run by Libri). Importing and providing digital items in the product search.

• Collaborating with developers, security, and operations teams to align requirements
• Supporting product owners and development teams in deploying logging and monitoring solutions based on the Elastic Stack
• Developing and standardizing log schemas as well as defining practical standards for observability
• Designing and further developing logging architectures for complex, distributed multi-tenant environments
• Connecting application and infrastructure logs as well as security tools and metrics
• Optimizing data flows and modeling for analysis and reporting purposes
• Building automated infrastructures using Terraform/Terragrunt and Ansible
• Maintaining and evolving CI/CD pipelines in GitLab as well as automated development environments in Hetzner Robot
• Operating and automating Proxmox clusters including Ceph storage
• Setting up and running Kubernetes (k3s) clusters on Fedora CoreOS including core services like Vault, OpenLDAP, and HAProxy
• Implementing security-critical infrastructures according to BSI basic protection and securing existing systems
• Supporting the operation of solutions in cloud environments (e.g., AWS)
• Working in agile teams following Scrum and Kanban
• Technologies/Tools: Elastic Stack (Elasticsearch, Logstash, Beats/Elastic Agent, Kibana), Terraform, Terragrunt, Ansible, GitLab CI/CD, GitOps, Proxmox, Proxmox Ceph, Kubernetes (k3s), OpenShift, Helm, Kustomize, Vault, OpenLDAP, HAProxy, Hetzner Robot systems, AWS, Prometheus, Grafana, Syslog Linux/Windows, Docker, NGINX, Apache Security & Compliance (BSI basic protection, SIEM/SOC)