Fullstack Developer (Java & React)

This role is suitable for a Middle-level Fullstack Developer with at least three years of commercial experience building Java and Spring Boot applications alongside React frontends. Required skills include Java 17, Spring Boot, REST APIs, TypeScript, Node.js, React, state management, automated testing, Git, Maven, Docker, software architecture, and English at B2 level or higher. The candidate should be familiar with designing scalable SaaS features, developing backend services and user interfaces, writing unit and integration tests, troubleshooting technical issues, reviewing code, and collaborating in agile Scrum teams.

1. How would you design a scalable RESTful API in Spring Boot for workflow automation?

I would separate controllers, services, and repositories, use DTOs for API contracts, validate input with Bean Validation, and centralize exception handling. I would also consider pagination, idempotency, authorization, observability, and backward-compatible versioning.

2. What Java 17 features would you use in a Spring Boot application, and why?

I would use records for immutable DTOs, sealed classes where restricted type hierarchies improve domain modeling, and pattern matching where it simplifies type checks. I would also apply modern Stream and Optional usage carefully to keep the code readable and maintainable.

3. How would you handle concurrency and consistency when multiple users update the same workflow?

I would use optimistic locking with a version field when conflicts are uncommon, returning a clear conflict response when an update is stale. For high-contention operations, I would evaluate transactional boundaries, database locking, idempotency, and retry strategies.

4. How do you secure a Spring Boot REST API?

I would use Spring Security with token-based authentication, enforce authorization at both endpoint and business-operation levels, and validate permissions against the requested resource. I would also apply input validation, secure headers, least-privilege access, audit logging, and protection against common threats such as injection and replay attacks.

5. When would you use caching in a SaaS workflow application, and what risks must be considered?

Caching is useful for frequently read, relatively stable data such as configuration or metadata. I would define clear TTL and invalidation rules, consider tenant isolation, avoid caching sensitive data improperly, and monitor hit rates, memory usage, and stale-data behavior.

6. How would you structure state management in a React and TypeScript application?

I would keep local UI state in React when possible, use a server-state solution such as TanStack Query for fetching, caching, and synchronization, and use Redux for genuinely shared client state. I would define strong TypeScript models, handle loading and error states consistently, and avoid duplicating server state unnecessarily.

7. What is the difference between server state and client state in React?

Server state is owned by the backend and requires fetching, caching, synchronization, and invalidation, while client state is controlled by the application, such as UI preferences or modal visibility. Treating them separately leads to simpler components and more predictable updates.

8. How would you test a feature spanning a React frontend and Java backend?

I would use JUnit 5 and Mockito for unit tests, Spring integration tests for API and persistence behavior, Vitest for frontend logic and components, and Playwright for critical end-to-end user flows. Tests should cover successful behavior, validation, authorization, failures, and important edge cases.

9. How can Node.js and serverless functions fit into this architecture?

Node.js and serverless functions can handle lightweight backend logic, API resolvers, integrations, or event-driven operations close to the frontend or cloud services. They should remain stateless, validate inputs, handle timeouts and retries, and be monitored for cold starts, failures, and resource usage.

10. How would you diagnose a production performance issue in a cloud-based SaaS product?

I would first use metrics, logs, traces, and recent deployment history to identify whether the bottleneck is in the frontend, API, database, external dependency, or infrastructure. I would reproduce the issue when possible, inspect latency and resource usage, apply a targeted fix, and verify it with monitoring and regression tests.

Appfire is a global software company with 800+ employees across 28 countries, building products that help teams collaborate and automate workflows. As a Fullstack Developer, you’ll design and maintain scalable SaaS features, REST APIs, backend services, and React applications while collaborating in an empowered agile team. The stack includes Java 17, Spring Boot, TypeScript, Node.js, React, Redux or TanStack Query, JUnit, Mockito, Vitest, Playwright, Git, Maven, and Docker. This is a fully remote role in Poland; salary is not disclosed, but the opportunity offers equity, strong health and wellbeing benefits, generous paid time off, learning support, and meaningful ownership in a growing global company.