OpenAPI Experts in Hamburg
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Meet FRATCH Experts in Hamburg, who have recently used OpenAPI
Rizwan B.
Last position:
Software Developer/Database/Devops at Sutor Bank
Project:
- Migration of existing VB6 applications to C# (Web API, web app)
- Report development and risk management systems (C#, API interfaces, databases)
- Document archive (API interfaces, databases)
Team size: 9 people
- Reports for Risk and Credit Management Development and maintenance of report structures to support risk and credit decisions, including data modeling, validation, and automated delivery for business departments.
- Migration and maintenance of a legacy system to the latest version of .NET for the asset management system Planning and execution of the modernization of an existing asset management system, including code refactoring, performance optimization, test automation, and sustainable maintenance after migration.
- Replacement of the COM-based archive connection with a REST API Redevelopment of the connection of the document archive (HYPARCHIV) to a REST API based on OpenAPI/Swagger, including archiving with index and stamp fields as well as PDF export into the customer portal.
- Redesign of payment receipt processing Replacement of an existing Oracle PL/SQL process with a new implementation in C# based on a business concept, including business alignment, data modeling, and test coverage.
- Design and implementation of tests, creation of deployment pipelines, and documentation
- Further development and implementation of change requests.
- Regular code reviews within the team as well as optimization of application scalability and stability
Technologies: C#, .NET 10, ASP.NET, Unit of Work, IoC, Three-Tier Architecture, Entity Framework Core, DB First, Web API, Swagger, OpenAPI, Python, Oracle, HYPARCHIV, Azure, PowerBI, DevOps, AKS (Azure Kubernetes Service), GIT, xUnit, NSubstitute, TypeScript, Vue.js, Pinia,
Methods: agile development using Scrum
Florian W.
Last position:
Software Engineer at micimo GmbH
- Developing a professional appointment planner for organizations with specific, detailed requirements
- Evaluating various existing software solutions
- Creating a list of technical requirements
- Implementing these requirements
- Technologies under consideration: WebDAV, CalDAV, Rust, Baikal, OAuth, Keycloak
Bastian I.
Last position:
Senior Backend Engineer Go & Technical Consultant at Regulated trading system for digital assets
- Technical consulting to meet regulatory MiCA requirements for a digital trading platform
- Design and development of backend components and APIs to comply with transparency and auditability requirements
- Optimization of core functions in order execution and market data delivery with a focus on performance and reliability under high load
- Conducting job interviews and structured onboarding of new developers
- Successful and timely implementation of the regulatory MiCA requirements, ensuring uninterrupted operations
- Reduction of user issues and common support requests by fixing complex backend problems
- Strengthening the development team through effective onboarding, mentoring, and knowledge sharing
Rainer D.
Last position:
Enterprise Architecture Management / Backend Software Developer at Polizei Hamburg
- Several projects in a police context
- Model and document police procedures/projects with Archimate (as-is/to-be) in the context of P20 (BKA)
- Create software architectures with microservices
- POC development with Springboot/Docker/Kubernetes
- Project size: 10 people
- Enterprise architecture management with Togaf and Archimate
- Backend software development Springboot
- DevOps with Kubernetes
- Implemented using: Java 17/21, Springboot 3, P20 architecture, Togaf, Archimate
Daniel S.
Last position:
Senior Software Engineer at energielenker solutions GmbH
- Designed and implemented a Python-based ETL pipeline with the Dagster framework to transform raw energy data from heterogeneous sources using InfluxDB and visualizations in Grafana
- Defined time-based and dependency-based jobs
- Deployed to managed Kubernetes clusters using Helm
- Integrated InfluxDB Cloud
- Prepared data for use in Grafana, including cleaning, normalization, and time-based resampling in Python
- Developed dashboards and visualizations in Grafana
- Developed unit tests with mocking using pytest
- Set up a CI/CD pipeline in GitLab
Technologies: Python, Dagster, InfluxDB, Grafana, pandas, pytest, REST, CI/CD, GitLab, Container, Kubernetes, Helm, Docker, Cloud
Rutger B.
Last position:
Partner & Managing Director at AI.IMPACT
- Building an AI & Data Consultancy Practice with the goal of helping European companies adopt Artificial Intelligence and modern data platforms
- End-to-end further development of a production system using modified coding agents (OpenCode). Tech stack: Kubernetes, Argo, Keycloak, Typescript, Grafana, GitOps, DevOps, Playwright
- Internal research project on the use of coding agents in the field of mathematical logic for creating formal models. Use of Cursor IDE and Codex, Codex CLI. Architecture design, quality control and refactoring, as well as writing code and tests. Repository (open source) available pre-launch
- Research on the role of mathematical logic as a formal language that connects IT and AI with business processes
- Project lead for collecting and deploying parking recommendations for rail vehicles with significant savings potential based on real-time data in a mobility and transport company
- Project lead for collecting and distributing process measurement points for real-time control in a mobility and transport company
- Deputy application owner for an app used for communication in the dispatching and provision of rail vehicles
Patrick V.
Last position:
Senior Director, Retail Media at EUROBAUSTOFF Handelsgesellschaft mbH & Co. KG
- Strategic consulting on marketing funds (WKZ) and retail media, focusing on monetization opportunities and data-driven business models
- Conducting a portfolio analysis of existing WKZ measures to assess the revenue and ROI impact of WKZ investments on supplier performance
- Potential analysis of digital WKZ products and initiatives to identify growth and efficiency levers
- Preparing and presenting the results to management and deriving a strategic move-forward plan
- Designing and facilitating several executive workshops to develop a holistic retail media vision and transformation roadmap
- Defining and prioritizing retail media business cases for data-driven evaluation of investment options
- Developing a technical target architecture considering heterogeneous ERP infrastructures and designing an integrated loyalty program
- Designing change management, including impact analysis on organizational structures and processes
- Creating and presenting C-level decision templates
- Establishing a clear retail media governance structure and technical foundation for data-driven marketing
- Developing a roadmap for implementation in 2026
Jonas T.
Last position:
IT Consultant at Bechtle AG
- Prequalified 200 demands within five weeks on the multi-cloud-commerce platform bechtle-clouds.com with three external consultants
- Served a dual role as project manager and requirements engineer for various enhancements
- Domain topics: reimplementation of Entra ID Sync, improvement of support processes, optimizations in the product catalog
- Demand and project management in ServiceNow
- Refinement of epics and stories with internal teams and external service providers
- Management of the external international service provider
Daniel Y.
Last position:
Senior Frontend Developer at node.energy
- Significant enhancement of the B2B React SPA software "opti.node" for commercial operations management for renewable energy plant operators
- Introduced features not previously available on the market, resulting in a significant revenue increase
- Comprehensive automated tests within Storybook
- Introduced a UI library to improve developer experience and speed up feature development by about 40%
- Clear presentation of complex commercial and legal issues, with intuitive UX and UI
- UX consulting to shorten essential interaction flows by 75%
Marcel S.
Last position:
Lead Developer / Software Architect at Rezeptprüfstelle Duderstadt GmbH
Responsible for the new development of billing and auditing software for prescriptions (prescriptions) to fully check and evaluate e-prescriptions for correctness (content, billing)
The system consists of several contexts that run as services (Docker containers):
Checking and processing data deliveries via FTP and email
Managing invoices, clearings, advance payments and deductions
Managing and running audit rules and audit folders
Evaluations based on Metabase
Developer Stack: Kotlin, Vue 3 / Vuetify 3, ANTLR, Spring Boot 3, REST API, Gradle, Docker, GitLab, PostgreSQL, Kafka, Keycloak, Scrum, Grafana, Loki, Testcontainers, Prometheus
Florian F.
Last position:
Solution Architect with Developer Focus at SymFinIT Solutions GmbH
- 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
Andreas S.
Last position:
Lead Developer at Software
- Extended the document management system with a standard CMIS (Content Management Interoperability Services) interface
- Implemented CMIS core services like navigation, access rights, search, CRUD operations, and versioning in Java
- Implemented based on RESTful / OpenAPI services
- Delivered as a fat-jar and native container image
- Deployed on-premises and serverlessly as an Azure Container Application using Terraform
- Improved team autonomy through infrastructure engineering and short feedback loops
- Established observability with OpenTelemetry, Azure Monitor, and Azure Logic Apps
- Introduced Terraform and trunk-based development processes
- Ensured quality with BDD tests in C# using SpecFlow and Testcontainers
- Created Azure DevOps pipeline integration tests
- Introduced cloud deployment processes
- Trained staff in cloud and Terraform
Jürgen B.
Last position:
Software Developer at Inform AG
- Coaching junior developers
- Further development and customer-specific extensions of a standard software product (microservices (DDD) and partly monolith)
- Interface extensions
- Web services
- Test automation
- Design
- Technologies used: Java8, Java17, TypeScript, Python, PostgreSQL, Spring (Integration, Data, Boot), JPA, AWS (Cloud, Lambda, EC2, SQS/SNS), Microservices, Fitnesse/Cucumber, Gitlab
Mark P.
Last position:
Full-Stack Software Developer, Product Data Import at Otto (GmbH & Co KG)
- 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
René S.
Last position:
CRM Developer at CiS GmbH
- Developed a greenfield CRM for managing incoming customer requests for a municipal utility provider
- Implemented with PHP/Symfony 5, EasyAdmin, Doctrine ORM and Oracle XE
- Handed over to CiS GmbH after initial groundwork as planned
Discover over 15,000 top freelancers
Statistics of experts using OpenAPI
Aggregated from the professional profiles of matched freelancers.
Experience
19 years
Position duration
1.7 years (Germany: 1.8 years)
Positions per freelancer
12 (Germany: 14)
Top business areas
Information Technology, Product Development, Quality Assurance
Top industries
Information Technology, Banking and Finance, Retail
Certification focus areas
Information Technology, Product Development, Project Management
Bachelor's degree or higher
91% (Germany: 89%)
Master's degree or higher
45% (Germany: 51%)
Doctorate
18% (Germany: 11%)
Certifications per freelancer
2
Most common languages
German, English, French
Speak two or more languages
93% (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 Hamburg 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 Hamburg 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 (100%)
- Banking and Finance (60%)
- Retail (53%)
- Transportation (47%)
- Healthcare (40%)
- Automotive (33%)
- Energy (33%)
- Insurance (33%)
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. A specification defines endpoints, parameters, request bodies, responses, authentication and data models before or alongside implementation. Teams use YAML or JSON documents to create a shared contract between services, web applications, mobile clients and external consumers.
Design and delivery
OpenAPI specialists turn business requirements into clear API contracts and maintain them as systems evolve. They establish resource names, operation behavior, error responses and backward-compatibility rules. The specification can support contract-first delivery, review workflows and consistent handovers between product, backend, frontend and quality teams.
- Define paths, schemas, parameters and response models
- Document OAuth 2.0, API keys and other security schemes
- Plan versioning, pagination, filtering and error handling
- Review changes for breaking API behavior
Tooling ecosystem
The ecosystem includes Swagger UI and Swagger Editor for viewing and editing specifications, along with validators, linters and documentation generators. Tools such as Redoc, openapi-generator and Stoplight are often part of the workflow. Strong specialists also connect specifications with Postman, CI pipelines, automated tests and service repositories.
When to bring expertise
Companies often need freelance support when an API is being designed from scratch, several services have inconsistent interfaces or public documentation is difficult to maintain. Hamburg teams can work with specialists on-site, remotely or in a hybrid setup, depending on security and collaboration needs. Clear English is common in distributed projects; German may matter for local stakeholders and documentation.
- Align APIs across microservices or product teams
- Modernize Swagger documentation into a maintained contract
- Generate SDKs, server stubs or test scenarios
- Prepare an API for partners, customers or public consumption
Skills around OpenAPI
Practical expertise usually includes REST principles, HTTP semantics, JSON Schema, authentication and API lifecycle management. Depending on the project, specialists may also work with Java, Spring Boot, .NET, Node.js, Python or Go, plus gateways and cloud delivery systems. They understand that generated artifacts need review and integration rather than blind acceptance.
Signs of quality
A strong professional makes specifications precise enough for humans and tools without hiding important behavior in prose. They test examples, validate schemas and check that implementation and documentation remain aligned. Ask for evidence of API governance, change management, generated client use and clear handling of compatibility, failures and security boundaries.
Frequently asked questions
What clients ask us most about OpenAPI — answered in short.
OpenAPI is used to describe HTTP APIs in a structured format that people and tools can read. Companies use it for API design, documentation, validation, code generation, testing and collaboration between teams.
OpenAPI is the name of the specification maintained by the OpenAPI Initiative, while Swagger is the earlier name and remains associated with tools such as Swagger UI and Swagger Editor. A specialist should be comfortable with both the specification and the Swagger tooling commonly found in existing projects.
OpenAPI work benefits from solid knowledge of HTTP, REST design, JSON Schema, authentication and API versioning. Depending on the assignment, useful adjacent skills include Java with Spring Boot, .NET, Node.js, Python, API gateways, CI pipelines and automated contract tests.
OpenAPI experience should match the project’s complexity rather than a fixed time period. A simple internal API may need specification and documentation skills, while a public or distributed API requires judgment around compatibility, security, governance, generated clients and long-term change management.
OpenAPI work is often well suited to remote collaboration because specifications, reviews and tooling are digital. On-site sessions in Hamburg can still help when teams need workshops, architecture decisions or close coordination with local product and compliance stakeholders.
OpenAPI deliverables may include a validated specification, interactive documentation, reusable schemas, examples, linting rules, generated SDKs or server stubs and contract tests. Agree in advance how the specification will be reviewed, versioned and kept in sync with the implementation.
OpenAPI quality is visible in precise schemas, realistic examples, consistent responses and explicit security behavior. Reviewers should also check for breaking changes, accurate implementation alignment, useful error definitions and documentation that helps consumers complete real tasks.
OpenAPI is a strong choice when several teams, services or external consumers depend on the same API contract. Compared with prose-only documentation, it gives validation, tooling and generation workflows, while still allowing specialists to explain business rules that a schema cannot express fully.
The average hourly rate of freelancers in Hamburg, Germany who have used OpenAPI in their recent projects is 102 €, which corresponds to a daily rate of about 818 € based on an 8-hour working day.
Of the freelancers in Hamburg, Germany who have used OpenAPI in their recent projects, 91% hold at least a Bachelor's degree, 45% hold at least a Master's degree, and 18% hold a doctorate.
On average, freelancers in Hamburg, Germany who have used OpenAPI in their recent projects have 19 years of professional experience, with a single engagement typically lasting around 1.7 years.
The most common languages among freelancers in Hamburg, Germany who have used OpenAPI in their recent projects are German (100%), English (93%), and French (13%).
The most common industries among freelancers in Hamburg, Germany who have used OpenAPI in their recent projects are Information Technology (100%), Banking and Finance (60%), and Retail (53%).
The most common business areas among freelancers in Hamburg, Germany who have used OpenAPI in their recent projects are Information Technology (100%), Product Development (100%), and Quality Assurance (87%).
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 
Munich 
Cologne 
Frankfurt 
Stuttgart 
Dusseldorf Countries:
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FRATCH CEO
