OpenAPI Experts in Cologne
to define better APIs with vetted, available freelancersHire experts who design OpenAPI specifications, generate client and server code, and connect API documentation with testing and CI workflows. Get precisely matched, vetted and available freelancers quickly for your project.
Meet FRATCH Experts in Cologne, who have recently used OpenAPI
Emre S.
Last position:
IT Project Manager | SAP E-Invoicing Outbound at Rohde & Schwarz
Project: Introduction of statutory-compliant electronic invoicing (outbound, Germany) in SAP for an international technology group.
- Project management: Planning, steering and reporting of the E-Invoicing outbound rollout across all affected business areas, including budget and resource coordination.
- UAT: Structured user acceptance test with key users per business area, test cases per invoice scenario, and consolidated test results for decision making.
- Defect management: Handover and tracking of defects in Jira, including validation of the generated invoices against the statutory schemas.
- Cutover, go-live and hypercare: Readiness checks and cutover planning aligned with the statutory deadlines, coordination of go-live and stabilization during hypercare.
- Stakeholder management and reporting: Alignment with Finance, Tax, business departments and IT, preparation of status reports and decision papers, and consistent follow-up of open items.
- Change management and documentation: Communicated process changes to the business departments, enabled key users, and created process and compliance documentation to ensure traceability and audit readiness.
Patrick D.
Last position:
Fullstack Developer
- SPA for automated communication of medical findings with role-based access (Sanctum)
- Server-side LLM integration (OpenRouter) with structured processing
- Automated sending via SMS/voice call (Twilio, ElevenLabs) with queue + status retry
- Full test coverage with 80+ documented test cases
Technologies: PHP, Laravel, LLM API (OpenRouter), Twilio, ElevenLabs, Laravel Sanctum, PHPUnit, Playwright, Docker, REST
Predrag P.
Last position:
Test Developer | Test Automation Engineer at CreditReform
Technologies: Java 25, Selenium, Insomnia, SSO, Gitlab, Jenkins, Docker, Kubernetes, Jira, IntelliJ, Nexus, Oracle, MariaDB, SpringBoot, Grafana, Concept Board, Vue.js, OpenApi, Kotlin. Implementation, maintenance, and enhancement of automated end-to-end regression tests for UI and API interfaces. Appropriate and targeted test automation involving multiple software products, including Java-based Windows applications. Automation of business processes with Selenium, reset jobs, and DB import jobs, as well as creation of test concepts, test case descriptions, and structured defect reporting.
Andreas B.
Last position:
Senior Software Developer at ELIONOVA AG
- Created an application to control measurement hardware with a Tauri application using React.JS and TypeScript on Raspberry Pi
- Developed a cloud application for storing and analyzing measurement results using Spring Boot and React.JS
- Built a Tauri application for printing ZPL labels on Windows
- Technologies: Java, JavaScript, Tauri, Rust, React.JS, AWS, Kubernetes, Docker, GitLab CI/CD, REST API (JSON), Maven, CI/CD, PostgreSQL
Rainer L.
Last position:
Senior IT Consultant, Senior Software Architect, Senior Software Developer, Senior DevOps Engineer at Techniker Krankenkasse
Automation of the database major / minor releases for the TKeasy project
Concept for database major / minor release automation
Assessment of the current state
Evaluation of Redgate Flyway functionalities
Concept development
Products: Redgate Flyway, GitHub, Quest, Erwin Data Modeller, Oracle, Atlassian Jira, Atlassian Confluence
Knowledge: Docker, Continuous Delivery, Continuous Integration, Redgate Flyway, Major Releases, Minor Releases
Andreas K.
Last position:
Backend Developer at Univention GmbH
- Developed a SCIM-compliant Python interface to connect external systems
- Expanded and enhanced UCS modules in Python
- Extended and newly developed Helm charts for container orchestration in Kubernetes
- Maintained and further developed CI/CD pipelines in GitLab
- Implemented automated tests with Pytest and Playwright
'Nubus' is based on a microservice architecture, relies on open source technologies, and runs in Docker containers.
UCS is an IAM complete solution based on Debian, used as an alternative to Microsoft's Active Directory. The work was characterized by the close integration of infrastructure and application logic.
Marc S.
Last position:
Fullstack Developer at PLANT-MY-TREE
PLANT-MY-TREE®-per-order
The application enables Shopify merchants to automatically place tree-planting orders for every incoming order. By integrating ecological contributions directly into the purchase process, the manual effort for tracking and billing reforestation initiatives is eliminated. The system increases transparency for end customers through real-time visualizations of the ecological impact directly in the storefront. The architecture is based on a modular monolith with Spring Boot in the backend and an integrated React app inside the Shopify admin area. The solution uses webhooks to capture order data in an event-driven way and integrates the weclapp ERP system for automated monthly invoicing. An app proxy mechanism provides dynamic statistics such as CO2 compensation and planted trees without any performance loss for the merchant shop.
Tasks:
- Design of the modular software architecture based on Spring Modulith to ensure high maintainability
- Development of the event-driven business logic for evaluating Shopify orders via webhooks
- Implementation of automated invoicing by connecting the weclapp REST API
- Building the frontend using React Router and Shopify App Bridge for native integration
- Design of the database model and implementation of the persistence layer with JPA/Hibernate and Prisma
- Integration of internationalization processes for global use in the frontend and email communication
- Automation of deployment processes using Docker and GitLab CI/CD
Project skills: Java 25, Spring Boot, Spring Security, Spring Modulith, Hibernate, JPA, REST API, PostgreSQL, Maven, Liquibase, React, TypeScript, React Router, Vite, Node.js, Prisma, Zod, Docker, Docker Compose, GitLab CI/CD, Shopify CLI, Shopify App Bridge, Polaris, weclapp, i18next, Lombok, Vitest
Allal K.
Last position:
Java Senior Full Stack Developer at Insurance ÖRAG
- Further development of a policy administration system (contract/claims) for the legal expenses insurer ÖRAG.
- My role: Senior Software Developer.
- The team consists of 12 developers.
- Technologies used: Java 8/21, Java EE, Quarkus, WebLogic, JPA, RabbitMQ, JTA, CI, CD, Jenkins, DB2, Maven, Jenkins, GIT (bitbucket) later GitLab, Junit, Elasticsearch, Mockito, Jira, SonarQube, Scrum, React, Workflow
- Migration and modernization of legacy systems: rewrote C and C++ code in Java to improve maintainability, scalability, and performance. Refactored Python into Java, including optimization and integration into modern architectures.
- Study on the use of AI (Codex) for implementing a Java feature. The goal is to compare the time spent and productivity of AI-supported development with a classic implementation by a developer. The results should show how strongly the use of AI affects development time and efficiency.
- Development of the following tariff calculators for the years 2024/2026: private, companies, doctors, farmers, owner-occupied G + H, landlords, traffic, club, top managers, sales representatives, special criminal law, savings bank, municipal, BayGT, doctors, Dehoga
Markus G.
Last position:
Full Stack Developer at REWE Digital
- A warehouse valuation system was reimplemented using Java, Spring Boot, and Camunda. The backend solution focuses on integration and batch calculations, the frontend on managing formulas and reviewing results.
- Java 21
- Spring Boot
- JPA
- Maven
- REST
- Kafka
- PostgreSQL
- DB2
- Liquibase
- Google Cloud Storage
- Keycloak
- GitLab CI/CD
- Helm
- Terragrunt
- SonarQube
- Angular
- IntelliJ
- JUnit 5
- Mockito
- Open API
Jens H.
Last position:
#52 Test Manager in Safety-Critical Infrastructure at Telecommunications Company
- Network elements, network, operational and operator-related systems
- Supporting around 20 different products (managed agile)
- Requirements analysis and management (functional and non-functional)
- Test concept and planning (scope, test phases, effort, schedule, resources, organization, etc.)
- Aligning approach with clients and service providers
- Managing and assisting in creation of test cases for system, integration and end-to-end tests
- Guiding and supporting test execution
- Defect management and planning bug fixes with external providers
- Preparing and facilitating workshops
- Regular reporting and creating automated test reports
- Further development of agile testing methodology and configuration of supporting tools
- Conceptual support in developing a quality seal for rating product and service quality of internal and external providers
- Requirements engineering based on ISO/IEC 25010, Scrum, SaFE
- BMC Remedy, Atlassian Confluence, Atlassian Jira (including plugins Xray, Structure), Microsoft Office (incl. Visio), Obsidian, doxis document management system, Gitlab, Obsidian
Louis N.
Last position:
Specialist in Adobe Experience Manager & Microservices at Nguenkam Consulting & Engineering
Java 11/21
Springboot
Swagger/OpenApi
Azure Pipelines
Trivy Integration
Apache Sling
OSGI
Java Content Repository (JCR)
Apache Jackrabbit Oak
Adobe HTL
GraphQL
Maven
GitLab
IntelliJ
Azure Cloud
Azure DevOps
Terraform
AEM specialist in the Scrum team
Development and deployment of microservices via Open API interfaces
Operation of microservices in the Azure Cloud
Implementation of AEM modules and integration with microservices
Member of the knowledge transfer team and a backend coordination team
Sebastian T.
Last position:
Software Engineer / Cloud Engineer at OC Fulfillment GmbH
- Hybrid role as the first software engineer in the company to bootstrap the organization, laying out technical foundation and advising management on organizational aspects.
- Set up the SaaS platform architecture, selected languages and tools, and designed processes to deliver customer value quickly and reliably.
- Designed and implemented build and continuous deployment pipeline with multi-tenant approach enabling extreme scaling of the SaaS platform.
- Established initial ITSM and support processes, provided monitoring and alarming, and participated in the on-call team.
- Proposed new approaches to scale the platform and improve backend interfaces to mitigate bugs and communication overhead.
- Mentored new employees and served as a trustful team member and lateral leader.
- Conducted interviews for technical roles.
- Used technologies: Typescript, Node.js, Bash, Google Cloud Platform (App Engine, Cloud Run, Cloud Functions, Google Endpoints, Firebase), GitHub Actions, Docker, Terraform, MongoDB, Firebase, REST APIs, GraphQL.
Jacqueline K.
Last position:
OTTO
- Design, development, and monitoring of microservices
- Migrating functions from a monolithic system into separate microservices
- Designing and implementing new microservices (backend and frontend)
- Setting up and maintaining a CI/CD pipeline (GitHub Actions)
- Configuring cloud infrastructure with AWS CDK and Terraform
- Designing and implementing event-driven communication with AWS SNS/SQS and Apache Kafka
- Ensuring code quality through Test Driven Development (TDD), static code analysis, and Clean Code
Discover over 15,000 top freelancers
Statistics of experts using OpenAPI
Aggregated from the professional profiles of matched freelancers.
Experience
20 years (Germany: 19 years)
Position duration
1.7 years (Germany: 1.8 years)
Positions per freelancer
14
Top business areas
Information Technology, Product Development, Quality Assurance
Top industries
Information Technology, Retail, Banking and Finance
Certification focus areas
Information Technology, Product Development, Marketing
Bachelor's degree or higher
90% (Germany: 89%)
Master's degree or higher
70% (Germany: 51%)
Certifications per freelancer
2
Most common languages
German, English, French
Speak two or more languages
100% (Germany: 94%)
Based on our profile pool as of 29 Oct 2026.
Daily rate distribution
The chart shows how the daily rates of experts in this technology in Cologne are distributed, based on recent contracts on our platform. Each bar covers a rate range — its height shows the share of experts charging within that range.
Average rates of experts in Cologne using OpenAPI
Rates are based on recent contracts and do not include FRATCH margin.
The average daily rate is the mean of all daily rates from recent contracts of comparable freelancers on our platform.
The median daily rate is the middle value of all daily rates — half of comparable freelancers charge less, half charge more. Unlike the average, it is barely affected by outliers.
Calculated based on our freelancers’ daily rates as of 29 Oct 2026. Actual rates may vary depending on seniority level, experience, skill specialization, project complexity, and engagement length.
OpenAPI experts industry focus
See which industries our matched freelancers work in most often — every figure is calculated live from the freelancers on FRATCH.
- Information Technology (77%)
- Retail (69%)
- Banking and Finance (62%)
- Transportation (54%)
- Government and Administration (54%)
- Automotive (46%)
- Telecommunication (46%)
- Healthcare (38%)
Please note that freelancers can work across multiple industries, so percentages overlap.
About the technology
What OpenAPI defines
OpenAPI is a machine-readable standard for describing HTTP APIs. An OpenAPI document records endpoints, parameters, request bodies, responses, authentication and reusable data models in YAML or JSON. Teams use the specification as a shared contract between product, frontend, backend and operations specialists.
APIs and deliverables
OpenAPI supports the design and delivery of REST-style services, public APIs, internal platform interfaces and partner integrations. Typical deliverables include:
- OpenAPI specifications for new or existing services
- Consistent schemas, status codes and error models
- Interactive reference documentation for API consumers
- Generated client libraries, server stubs and validation rules
- Versioning and migration guidance for changing contracts
Ecosystem and tooling
The OpenAPI Specification works with tools such as Swagger UI, Swagger Editor, Redoc and openapi-generator. Specialists may also use Postman, Insomnia, Spectral, Stoplight, contract-testing tools and CI pipelines to validate documents and publish documentation. Strong practice includes reusable components, security schemes, examples and linting rules that fit the wider delivery process.
When companies need specialists
Companies often bring in freelance OpenAPI expertise when an API estate has inconsistent documentation, unclear ownership or breaking changes. Specialists can define a contract before implementation, document legacy services, prepare an API program for external consumers or improve governance across teams. In Cologne, this work can support software, logistics, media, manufacturing and other businesses that expose services to customers or partners.
Working across locations
OpenAPI projects suit remote collaboration because specifications, review comments and automated checks live in shared repositories. Freelancers can work with teams in Cologne on a fully remote basis or join workshops and architecture sessions on site when product decisions require close coordination. Clear written communication in English or German, disciplined reviews and agreed ownership matter more than physical proximity.
What strong professionals bring
A strong OpenAPI professional understands API design, HTTP semantics, JSON Schema concepts and practical service architecture. They know how to balance consistency with the needs of real consumers, and they can explain trade-offs to technical and non-technical stakeholders. Look for evidence of validated specifications, useful examples, reliable documentation, backward-compatible changes and automation that keeps the contract accurate as the system evolves.
Frequently asked questions
Everything clients usually want to know about OpenAPI, in one place.
OpenAPI is used to describe HTTP APIs in a structured YAML or JSON document. Companies use it to align teams, publish reference documentation, validate requests and responses, and generate client or server code.
OpenAPI Specification is the open standard for describing APIs, while Swagger is a related set of tools and a former name closely associated with that standard. Swagger UI and Swagger Editor can work with OpenAPI documents, but the specification itself is not limited to those tools.
OpenAPI work benefits from strong knowledge of HTTP, REST design, JSON, JSON Schema and API security. Useful adjacent skills include automated testing, CI pipelines, version control, API gateways and tools such as Swagger UI, Redoc, Spectral or openapi-generator.
OpenAPI assignments vary widely in scope. A small service may need careful specification and documentation work, while a larger API estate requires experience with reusable components, governance, versioning, consumer needs and migration from incomplete or outdated documents.
OpenAPI collaboration works well remotely because documents, reviews and validation checks can be managed in shared repositories. Teams in Cologne may still choose on-site workshops for domain modelling, API design decisions or coordination with product and service teams.
OpenAPI quality is visible in clear resource names, accurate schemas, consistent responses, meaningful examples and explicit security definitions. Reviewers should also check whether automated linting and contract tests catch drift between the document and the running API.
OpenAPI is a strong fit for HTTP APIs that need broad consumer access, human-readable documentation and established REST tooling. GraphQL centres on a typed query schema, while gRPC uses protocol buffers and generated interfaces, so the right choice depends on transport, clients, performance and service boundaries.
OpenAPI specialists should clarify whether the assignment covers design-first work, documentation of an existing API, code generation, governance or testing. They should also confirm the source of truth, target OpenAPI version, publication process, compatibility expectations and who approves contract changes.
The average hourly rate of freelancers in Cologne, Germany who have used OpenAPI in their recent projects is 91 €, which corresponds to a daily rate of about 727 € based on an 8-hour working day.
Of the freelancers in Cologne, Germany who have used OpenAPI in their recent projects, 90% hold at least a Bachelor's degree and 70% hold at least a Master's degree.
On average, freelancers in Cologne, Germany who have used OpenAPI in their recent projects have 20 years of professional experience, with a single engagement typically lasting around 1.7 years.
The most common languages among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are German (100%), English (100%), and French (38%).
The most common industries among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are Information Technology (77%), Retail (69%), and Banking and Finance (62%).
The most common business areas among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are Information Technology (100%), Product Development (92%), and Quality Assurance (69%).
Main locations of FRATCH Experts, who have recently used OpenAPI
Our freelancers and interim experts are at home across the DACH region — available on-site in the major business hubs or fully remote. Choose a location to discover matched specialists, local market insights and up-to-date availability.
Berlin 
Hamburg 
Munich 
Frankfurt 
Stuttgart 
Dusseldorf Countries:
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