OpenAPI Experts in Düsseldorf
for reliable API contracts, matched in minutes with vetted professionalsHire experts who design OpenAPI specifications, document REST interfaces and connect API contracts with testing and code generation workflows. Get fast, precise matching with vetted, available freelancers who fit your project and collaboration needs.
Meet FRATCH Experts in Düsseldorf, 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.
Marijn S.
Last position:
Senior Software Engineer at Puls Security GmbH
Optimizing and acceleration of our Gitlab CI pipeline
Conceptual work for the PoC of the Zero Trust system
Extension of the policy-engine backend in Go
Extension of the policy-testing mechanism in Python
Architectural design of the PEP component of Zero Trust
Documentation of the product
Technologies: Zero Trust, Go, Python, Gitlab CI, Docker, JWT, Domain-Driven Design
Andreas G.
Last position:
Senior Fullstack .NET (Core) & Angular Developer at IT Service Provider
- Supporting several agile teams in the new product development of an enterprise CRM and ERP application
- Web frontend development with Angular 17–21 (microfrontends, Domain-Driven Design, MDB-Bootstrap)
- Backend development with C# and .NET 7–10, MongoDB, MassTransit, RabbitMQ, Entity Framework Core, ElasticSearch (microservices, DDD)
- Technical feature design for assigned tasks
- Technical architecture design including forward-looking planning of AI-supported extension options
- Proof-of-concept implementation of AI-supported workflows with n8n and a locally hosted LLM (Ollama)
- Coordination with stakeholders and Product Owners, support with sprint planning
Result: Delivery of production-ready microservices and frontend modules in agile sprint cycles.
Environment: .NET 7–10 · Angular 17–21 · C# · TypeScript · OpenAPI · Microfrontends · DDD · Kubernetes · Docker · Helm · MS SQL Server · PostgreSQL · MongoDB · MassTransit · RabbitMQ · EF Core · ElasticSearch · Kibana · Azure DevOps · KeyCloak · n8n · Ollama
Ingo K.
Last position:
Co-Founder & Managing Director at Elterngeld Buddy GmbH
- Responsible for the entire software architecture of the parental allowance portal
- iOS and Android app with React Native and Expo, incl. push and OTA updates
- Store releases via App Store Connect and Google Play Console
- TypeScript monorepo for app, website and backend, web with Next.js and Tailwind
- Own promo, voucher and discount engine with contact and segment logic
- Payments via RevenueCat and in-app purchases, authentication with BetterAuth
- REST APIs based on OpenAPI, PostgreSQL with Prisma and Drizzle
- Self-hosting on Hetzner with Docker, Coolify, Terraform and GitHub Actions
- Tests with Vitest, Playwright and visual regression via Chromatic
- Observability via Grafana Cloud and error tracking with Sentry
- Performance marketing via Meta Ads, measured with custom event tracking
- Grew the product to 20,000 app users with a purchase rate of almost 12 %
- Managing Director since 04.2025, responsible for product and technology
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
Jan G.
Last position:
Master Data Application at confidential
Visualization, search, and maintenance of various types of master data, providing them via a REST API, and implementing a variety of import and export formats.
Implementation, architecture, project management.
Technologies and tools: Angular (Signals, Signal Store, swagger-codegen, standalone components), Java, PostgreSQL, ActiveMQ, JavaScript, Spring Boot (Spring Core, Spring MVC, Spring Data/JPA via Hibernate, Spring Security, Spring Modulith), JUnit, Flyway, QueryDSL, GraphQL, REST API, Node, npm, Maven, SonarQube, GitLab CI/CD pipelines, Docker, Nexus, OpenAPI + Swagger UI, Keycloak, OAuth2, OpenID Connect, IntelliJ IDEA, Jira, Confluence.
Discover over 15,000 top freelancers
Statistics of experts using OpenAPI
Aggregated from the professional profiles of matched freelancers.
Experience
13 years (Germany: 19 years)
Position duration
1.2 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
Bachelor's degree or higher
100% (Germany: 89%)
Master's degree or higher
67% (Germany: 51%)
Certifications per freelancer
16 (Germany: 2)
Most common languages
German, English, Spanish
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 Düsseldorf 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 Düsseldorf 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 (83%)
- Transportation (83%)
- Retail (67%)
- Telecommunication (67%)
- Energy (50%)
- Banking and Finance (50%)
- Manufacturing (50%)
- Government and Administration (50%)
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. Teams use OpenAPI specifications to define paths, parameters, request bodies, responses, authentication and reusable schemas before or alongside implementation. The format supports clear communication between product, frontend and backend teams.
Design and Delivery
OpenAPI specialists help shape APIs that are consistent, understandable and stable over time. They create contract-first designs, refine resource models and document error responses so teams can build against a shared interface. They also review existing specifications for ambiguity, duplication and breaking changes.
Ecosystem and Tooling
OpenAPI works across many languages, frameworks and API workflows. Strong professionals commonly connect specifications with:
- Swagger Editor and Swagger UI for authoring and interactive documentation
- OpenAPI Generator for client libraries, server stubs and models
- Contract testing, linting and validation in CI pipelines
- API gateways, authentication schemes and versioned documentation
When Expertise Helps
Companies often bring in freelance OpenAPI expertise when an API program is starting, an older specification needs restructuring or several teams must coordinate a shared contract. It is also useful when generated code is unreliable, documentation has drifted from production or a public API needs a clear onboarding path.
- Establishing conventions across multiple API teams
- Migrating Swagger specifications to current OpenAPI practices
- Introducing review gates for compatibility and quality
Skills Around OpenAPI
The best specialists understand more than YAML or JSON syntax. They can reason about REST resource design, HTTP semantics, JSON Schema, OAuth or other security schemes, pagination, versioning and backwards compatibility. Experience with tools such as Postman, Insomnia, Spectral and CI systems helps turn a specification into a dependable delivery process.
Choosing a Specialist
Look for professionals who can show clear specifications, useful examples and decisions that reflect real consumer needs. Ask how they handle nullable fields, polymorphism, error models, security requirements and changes that affect existing clients. For Düsseldorf teams, remote collaboration can work well when reviews, terminology and ownership are explicit; on-site workshops may help when several local stakeholders must align quickly.
Frequently asked questions
Questions about OpenAPI? Start with the answers below.
OpenAPI is used to describe HTTP APIs in a structured format that people and tools can understand. Companies use it for API design, documentation, validation, contract testing and generating client or server code.
OpenAPI commonly describes REST-style HTTP APIs, while GraphQL uses a schema and query language and gRPC relies on remote procedure calls with Protocol Buffers. The right choice depends on client needs, transport requirements, tooling and the level of control required over requests and responses.
OpenAPI work benefits from knowledge of REST principles, HTTP status codes, JSON Schema, authentication and API lifecycle management. Strong specialists may also bring experience with Swagger tooling, API gateways, contract testing, CI pipelines and backend or frontend integration.
OpenAPI deliverables can include a reviewed specification, reusable schemas, examples, security definitions and published documentation. Depending on the project, the specialist may also configure validation, generate SDKs, add contract tests or establish rules for future changes.
OpenAPI tasks vary from improving a small specification to coordinating contracts across many services. The needed level depends on API complexity, existing documentation, compatibility constraints and whether the specialist must influence architecture, tooling and team processes.
OpenAPI collaboration is well suited to remote work because specifications, reviews and tooling can be shared asynchronously. Düsseldorf companies should agree on working language, review routines, access requirements and meeting times; on-site sessions can still help with workshops involving several stakeholders.
OpenAPI quality shows in consistent naming, precise schemas, realistic examples and responses that match the actual service. A useful specification also explains authentication, errors and compatibility expectations without forcing consumers to guess.
OpenAPI projects often fail when documentation is written after implementation and never checked against running services. Other problems include vague schemas, inconsistent error responses, missing examples, weak security definitions and generated code being accepted without review.
The average hourly rate of freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects is 101 €, which corresponds to a daily rate of about 812 € based on an 8-hour working day.
Of the freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects, 100% hold at least a Bachelor's degree and 67% hold at least a Master's degree.
On average, freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects have 13 years of professional experience, with a single engagement typically lasting around 1.2 years.
The most common languages among freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects are German (100%), English (100%), and Spanish (33%).
The most common industries among freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects are Information Technology (83%), Transportation (83%), and Retail (67%).
The most common business areas among freelancers in Dusseldorf, Germany who have used OpenAPI in their recent projects are Information Technology (100%), Product Development (83%), 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 
Cologne 
Frankfurt 
Stuttgart Request a free demo
Get in touch with the FRATCH team and we will get back to you within 4 hours.
Would you rather directly get in touch?
We always have the time for a call or email!
Philipp Thomaschewski
FRATCH CEO
