OpenAPI Experts in Cologne
for reliable API contracts, matched in minutes with vetted, available freelancersHire experts who design clear API contracts, maintain OpenAPI documents, and connect specifications with Swagger tooling, code generation, and automated tests. Find precise matches with vetted, available freelancers who can work remotely or alongside your Cologne team.
Meet FRATCH Experts in Cologne, who have recently used OpenAPI
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
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
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
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
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
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.
Predrag P.
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 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.
Andreas B.
Last position:
Senior Frontend Developer at Roadex
- Development of an app
- Technologies: React Native, Expo, SupaBase, TypeScript, GitHub, Trello
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
21 years (Germany: 19 years)
Position duration
1.6 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
89%
Master's degree or higher
67% (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 19 Sep 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 19 Sep 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 (83%)
- Retail (67%)
- Banking and Finance (58%)
- Transportation (58%)
- Government and Administration (58%)
- Automotive (50%)
- Telecommunication (50%)
- Healthcare (42%)
Please note that freelancers can work across multiple industries, so percentages overlap.
About the technology
API contracts
OpenAPI is a standard for describing HTTP APIs in a machine-readable format. An OpenAPI document defines paths, parameters, request bodies, responses, authentication and reusable schemas. Teams use the OpenAPI Specification, or OAS, to align product, frontend and backend work before implementation.
Services and systems
OpenAPI supports REST APIs behind web applications, mobile products, partner integrations and internal services. It also helps document gateways and microservices so consumers can understand available operations without reading implementation code. Clear contracts reduce ambiguity as APIs evolve.
Ecosystem and tooling
The ecosystem includes Swagger UI for interactive documentation, Swagger Editor for authoring, and tools that validate documents or generate client and server code. Strong specialists also work with JSON Schema, YAML, Postman, API gateways and contract-testing workflows. They understand how tooling choices affect maintainability.
When expertise matters
Companies often bring in freelance OpenAPI expertise when API documentation has drifted from production or several teams need one reliable contract. Typical work includes:
- Creating or restructuring OpenAPI documents
- Converting legacy Swagger definitions to current OAS practices
- Adding validation, linting and contract tests
- Integrating specifications with CI/CD and code generation
Delivery in Cologne
Cologne companies can use OpenAPI specialists for logistics, media, commerce, health and business software projects, while keeping collaboration remote or partly on site. The right professional can review an existing API, establish contribution rules and help teams communicate clearly in English or German. They should fit the team’s delivery process, not only produce a document.
What strong specialists do
Good OpenAPI professionals model real business behavior rather than copying endpoint names into a file. They handle versioning, backward compatibility, error formats, security schemes and reusable components with care. They also verify that generated documentation matches deployed behavior and leave teams with practical standards they can maintain.
Frequently asked questions
Everything clients usually want to know about OpenAPI, in one place.
OpenAPI is used to describe HTTP APIs in a structured format that people and tools can read. Companies use it for documentation, design reviews, validation, client generation, server scaffolding and contract testing.
OpenAPI is the open specification maintained by the OpenAPI Initiative, while Swagger is the name associated with the earlier specification and a family of tooling. Many teams still say Swagger when they mean an OpenAPI document or tools such as Swagger UI.
A strong OpenAPI specialist usually understands REST design, HTTP semantics, JSON Schema, YAML or JSON, authentication and API versioning. Useful adjacent skills include Swagger UI, automated validation, Postman, API gateways, CI/CD and contract testing.
The right level depends on the work. A small documentation update may need focused specification knowledge, while designing contracts across microservices requires experience with compatibility, governance, security and team workflows. Assess the specialist against the API’s scope and risks rather than a generic time threshold.
OpenAPI work is often well suited to remote collaboration because documents, reviews and validation run through shared repositories and CI pipelines. A Cologne team may choose remote delivery, on-site workshops or a hybrid setup, with language expectations agreed before the engagement.
A high-quality OpenAPI document is accurate, complete and understandable to an API consumer. Check whether examples validate, schemas describe real payloads, errors and security are explicit, reusable components are applied sensibly, and automated checks prevent drift from the running API.
Hire an OpenAPI freelancer when teams disagree about API behavior, documentation is unreliable, or generated clients and tests cannot be trusted. External expertise can also help establish a specification-first workflow before a new integration or service landscape grows.
Working with OpenAPI teams involves more than editing YAML. Specialists need to discuss domain behavior with product and technical stakeholders, review changes constructively, understand the implementation stack and explain trade-offs around versioning, code generation and governance.
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 746 € based on an 8-hour working day.
Of the freelancers in Cologne, Germany who have used OpenAPI in their recent projects, 89% hold at least a Bachelor's degree and 67% 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 (42%).
The most common industries among freelancers in Cologne, Germany who have used OpenAPI in their recent projects are Information Technology (83%), Retail (67%), and Banking and Finance (58%).
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 (67%).
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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We always have the time for a call or email!
Philipp Thomaschewski
FRATCH CEO
