OpenAPI Experts in Cologne
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Meet FRATCH Experts in Cologne, who have recently used OpenAPI
Marc Smyk
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
Rainer Langbehn
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
As-is analysis
Evaluation of Redgate Flyway functionality
Concept creation
Products: Redgate Flyway, GitHub, Quest, Erwin Data Modeller, Oracle, Atlassian Jira, Atlassian Confluence
Skills: Docker, Continuous Delivery, Continuous Integration, Redgate Flyway, Major Releases, Minor Releases
Markus Glagla
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 Hagemeyer-Lee
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
Predrag Perovic
Last position:
Test Developer | Test Automator at CreditReform
Technologies: Java 25, Selenium, Insomnia, SSO, GitLab, Jenkins, Docker, Kubernetes, Jira, IntelliJ, Nexus, Oracle, MariaDB, Spring Boot, Grafana, Concept Board, Vue.js, OpenAPI, Kotlin. Implementation, maintenance, and extension of automated end-to-end regression tests for UI and API interfaces. Appropriate and targeted test automation across multiple software products, including Java-based Windows applications. Automating business processes with Selenium, reset jobs, and DB import jobs, as well as creating test concepts, test case descriptions, and structured bug reporting.
Louis Nguenkam
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 Thees
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.
Andreas Kosubek
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.
Andreas Beckers
Last position:
Senior Frontend Developer at Roadex
- Development of an app
- Technologies: React Native, Expo, SupaBase, TypeScript, GitHub, Trello
Allal Kharaz
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
Jacqueline Krech
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
21 years (Germany: 19 years)
Position duration
1.6 years (Germany: 1.8 years)
Positions per freelancer
15 (Germany: 14)
Top business areas
Information Technology, Product Development, Quality Assurance
Top industries
Information Technology, Transportation, Retail
Certification focus areas
Information Technology, Product Development, Marketing
Bachelor's degree or higher
100% (Germany: 89%)
Master's degree or higher
75% (Germany: 50%)
Certifications per freelancer
2
Most common languages
German, English, French
Speak two or more languages
100% (Germany: 93%)
Based on our profile pool as of 30 Aug 2026.
Daily rate distribution
The chart shows how the daily rates of freelancers in this technology in Cologne are distributed, based on recent contracts on our platform. Each bar covers a rate range — its height shows how many freelancers charge 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 30 Aug 2026. Actual rates may vary depending on seniority level, experience, skill specialization, project complexity, and engagement length.
About the technology
OpenAPI basics
OpenAPI is the standard for describing REST APIs in a machine-readable way. It turns endpoint definitions, request bodies, responses, and auth details into a contract that teams can share across development, testing, and documentation. Many people still call it Swagger, especially when they mean the older toolset around the same spec.
What experts deliver
- API specifications for new services and public APIs
- documentation that matches the real endpoints
- client and server stubs from the spec
- review of existing specs for gaps and drift
- support for versioning, security, and error models
Strong specialists keep the contract clear enough for engineers and readable enough for product and support teams.
Tooling and ecosystem
OpenAPI usually sits with Swagger UI, Swagger Editor, Redoc, and code generators. It also connects well with linting, contract testing, CI checks, and API gateways. A good professional knows how to fit the spec into the delivery flow instead of treating it as a one-time document.
When companies need help
Companies often bring in freelance OpenAPI experts when an API grows fast, teams disagree on contract details, or old documentation no longer matches the live service. That is common in SaaS, e-commerce, logistics, insurance, and internal platform work. In Cologne, this is also useful when local teams need clear handover documents for mixed on-site and remote collaboration.
What strong specialists do
A strong OpenAPI professional writes precise schemas, keeps examples realistic, and handles edge cases without bloating the spec. They know how to model pagination, auth, errors, webhooks, and reusable components. They also spot inconsistencies early, before front-end, back-end, and QA teams diverge.
How to judge fit
Look for experts who can explain why a field belongs in the contract, not just how to export a file. They should be comfortable reading existing YAML or JSON, improving naming, and aligning the spec with implementation. For Cologne teams, it helps if they can work clearly in English and adapt to German-speaking stakeholders where needed.
Frequently asked questions
Everything clients usually want to know about OpenAPI, in one place.
OpenAPI is used to describe REST APIs in a shared contract that people and tools can read. It helps teams design endpoints, generate documentation, validate requests, and keep implementation aligned with the spec. That makes it useful for public APIs, partner integrations, and internal services.
OpenAPI is the current specification name, while Swagger is the older name many people still use in conversation. Today, Swagger usually refers to the surrounding tools such as Swagger UI or Swagger Editor. If a freelancer says they work with Swagger, ask whether they mean the spec itself, the docs, or the tooling around it.
A strong OpenAPI specialist usually knows REST design, JSON Schema, YAML, and contract-first API design. It also helps if they understand testing, versioning, authentication, and how backend changes affect documentation. If the project includes client generation or gateways, they should be comfortable with those workflows too.
A good OpenAPI freelancer can start with an existing API, rough endpoint notes, or a codebase that needs proper documentation. The more useful input you provide, the faster they can turn it into a reliable contract. If you already have naming rules, auth patterns, or error handling conventions, share them early.
Bring in OpenAPI help when documentation is stale, multiple teams interpret endpoints differently, or a new API needs to launch cleanly. It is also a good fit when internal staff are busy and the spec must be cleaned up without slowing delivery. Freelance support is often the fastest way to get a contract back under control.
Most OpenAPI work can be done remotely because the key deliverables are specs, reviews, and feedback on API contracts. On-site time in Cologne can still help when several teams need workshop-style alignment or when stakeholders want faster decisions. Many projects use a mix of remote drafting and short in-person sessions.
A strong OpenAPI spec is consistent, complete, and close to the real API behavior. Look for clear naming, useful examples, reusable components, and accurate error responses instead of shallow endpoint lists. The best specs are easy to read, easy to validate, and easy for other specialists to use.
An OpenAPI assignment goes more smoothly when the freelancer knows the API style, the target audience, and whether the spec should drive implementation or document an existing service. They should also understand review cycles, approval steps, and who owns final sign-off. In Cologne-based teams, clear communication across technical and non-technical stakeholders is often important.
The average hourly rate of freelancers in Cologne, Germany who have used OpenAPI in their recent projects is 93 €, which corresponds to a daily rate of about 747 € based on an 8-hour working day.
Of the freelancers in Cologne, Germany who have used OpenAPI in their recent projects, 100% hold at least a Bachelor's degree and 75% hold at least a Master's degree.
On average, freelancers in Cologne, Germany who have used OpenAPI in their recent projects have 21 years of professional experience, with a single engagement typically lasting around 1.6 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 (45%).
The most common industries among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are Information Technology (82%), Transportation (64%), and Retail (64%).
The most common business areas among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are Information Technology (100%), Product Development (100%), and Quality Assurance (64%).
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 Request a free demo
Get in touch with the FRATCH team and we will get back to you within 4 hours.
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Philipp Thomaschewski
FRATCH CEO
