Principal Engineer - Software Architecture and Modernization-3

abcfinancial; Poland; On-site; PLN 260,000 - 420,000; Full time

Job summary

ABC Fitness is seeking a Principal Engineer to lead the architecture and hands-on modernization of critical software systems. You will steer the transformation from legacy, database-centric apps to domain-oriented software while keeping compatibility and business continuity intact.

This is a hands-on individual-contributor role, where you’ll influence teams through architecture, production-ready code, and strategic collaboration across engineering, product, security, and data teams.

Qualifications

• Extensive experience designing, building, and operating production backend systems.
• Proven ability to implement and ship features with tangible impact.
• Experience with distributed architectures, scalability, and reliability.

Responsibilities

• Lead software architecture and modernization for legacy systems.
• Contribute hands-on in backend coding across Java, Go, or Node/TS.
• Define domain models, APIs, and service contracts with clear boundaries.
• Guide framework and runtime upgrades, modernization of dependencies.
• Diagnose performance, concurrency, and data-access issues in production.
• Improve data access, persistence strategies, and messaging patterns.
• Establish rigorous testing and code-quality practices.
• Translate architectural risk into prioritized, team-wide execution plans.

Skills

Backend systems

Distributed systems

Hands-on implementation

Tools

Java/Spring Boot

Go (Golang)

Node.js/TypeScript

Job description

Join ABC Fitness and become part of a culture that's as ambitious as it is authentic. Let's transform the future of fitness-together!

Our Values

Best Life

We believe great work begins with great people. That's why our culture is built on respect, trust, and belonging. We create an inclusive environment where every team member can bring their authentic self to work-because diverse perspectives drive innovation and meaningful impact.

Growth Mindset

We are doers, thinkers, and dreamers. At ABC Fitness, your growth is our investment. Through continuous learning, mentorship, and professional development opportunities, we empower you to reach new heights-personally and professionally.

One Team

From day one, you'll be part of a team that collaborates, celebrates, and cares. We move fast, support one another, and have fun along the way. Because when you thrive, we all thrive.

Lead the architecture and hands-on modernization of critical software systems.

ABC Fitness is modernizing critical technology from legacy, database-centric applications toward well-designed, domain-oriented software. This role will provide principal-level technical leadership for that transformation while protecting compatibility, correctness, and business continuity.

As a Principal Engineer, you will set software architecture and engineering direction and remain close to implementation. You will work across application code, domain models, APIs, data access, messaging, runtime behavior, and tests to turn complex technical risk into durable software.

This is a hands-on individual-contributor role. You will influence teams through technical judgment, clear architecture, working software, production insight, and partnership not through direct people management.

WHAT YOU'LL DO

• Lead software architecture and modernization – Shape the evolution of legacy applications into well-structured, domain-oriented software. Define target architecture, module and service boundaries, migration seams, compatibility strategies, and incremental paths that teams can implement safely.
• Stay deeply hands-on in code – Contribute directly to critical implementations, prototypes, refactoring, code reviews, and production fixes. Work comfortably in backend code and frameworks such as Java/Springboot, Golang, Node/TypeScript, or similar backend ecosystems, and help teams improve application structure, runtime behavior, and maintainability.
• Design strong domain models and APIs – Create clear domain abstractions, ownership boundaries, service contracts, and APIs. Improve how business rules are represented in code and keep consumers insulated from legacy storage and implementation details.
• Modernize application foundations – Guide framework and runtime upgrades, dependency modernization, concurrency and lifecycle improvements, and the decomposition of tightly coupled code. Use practical refactoring strategies that keep large systems releasable throughout the work.
• Engineer for correctness, performance, and reliability – Diagnose difficult behavior through code, runtime evidence, database interactions, and dependency flows. Fix root causes in implementation and design, including concurrency defects, inefficient data access, resource contention, failure propagation, and recovery behavior.
• Improve data access and integration design – Move business logic out of database-centric implementations and into testable application code. Design effective persistence, transaction, caching, messaging, and event-consumer patterns while preserving historical, reporting, and audit needs.
• Raise the bar for code quality and testing – Establish strong practices for unit, integration, contract, and functional testing. Improve code review, static analysis, dependency management, feature controls, and validation so teams can change complex software with confidence.
• Create reusable engineering patterns – Turn successful designs into clear patterns, reference implementations, libraries, and architecture decisions that help teams solve recurring software problems consistently.
• Turn technical strategy into cross-team execution – Translate architectural risk into clear code-level priorities, decisions, ownership, and sequenced implementation plans. Partner with engineering, product, security, and data teams to move complex software initiatives from design through working code.

WHAT THIS IS NOT

This is not a people-management or architecture-only role. The engineer is expected to contribute directly to implementation and production outcomes.

This is not limited to greenfield design or modernization for its own sake. Decisions must balance long-term software quality with migration safety, business value, and the realities of changing production applications.

WHAT YOU BRING

• Deep backend and distributed-systems engineering – You have substantial experience designing, building, operating, and evolving production backend systems, services, APIs, and distributed applications.
• Credible hands-on impl