Senior Java Engineer

Cosmose, Inc.; Warszawa; On-site; PLN 180,000 - 300,000; Full time

Job summary

Cosmose AI is seeking a strong Mid or Senior Java Engineer to scale the backend of our KaiKai Lock Screen and CMS, processing content for millions of users daily. You will design and maintain backend services, build scalable data pipelines, and collaborate with Product, Content, and Data teams to deliver meaningful features.

We value ownership, proactive problem solving, and fluency in English and Polish as we push the boundaries of AI-powered mobile content.

Qualifications

• Strong experience in Java backend development.
• Experience building scalable and high-perf backend systems.
• Familiarity with event-driven systems (Kafka) and modern architectures.
• Knowledge of relational databases and SQL.
• Ability to take ownership and drive solutions.
• Good communication within the team (Polish and English required).
• Experience with high-traffic systems is a plus.

Responsibilities

• Design, develop, and maintain backend services for KaiKai Lock Screen and CMS.
• Build scalable systems for large-scale data processing and real-time content delivery.
• Contribute to architecture and system design improvements.
• Maintain high code quality through testing, code reviews, and engineering best practices.
• Work with distributed and event-driven systems to build scalable backend solutions.
• Improve system performance, reliability, and efficiency for a high-traffic product.
• Support integrations with external services and third-party systems.
• Collaborate closely with Product, Content, and Data teams to deliver meaningful features.
• Share knowledge and contribute to continuous team improvement.

Skills

Java backend

Scalable backend

Event-driven systems (Kafka)

Relational databases

Ownership

Polish & English

High-traffic systems

Job description

Unicorn Foundations

At Cosmose AI, we’re building smart, innovative technology that’s redefining digital marketing and content distribution. Since launching in 2014, we’ve gone from transforming offline shopping to powering personalized content experiences through AI. Founded by Miron Mironiuk, our journey began with one bold idea - and it has grown into a feature that caters to what people care about most, right from their lock screens.

Exceptional Engineering

Our engineering team is far from average; it’s a powerhouse of talent. Comprising former FAANG employees and programming champions, they bring deep expertise and a shared commitment to building world‑class products. Our strength lies in engineering solutions that leverage the power of AI, Data, and distributed computing. We thrive in a fast‑paced startup environment, where agility, creativity, and quality drive everything we do.

Global Presence, Local Innovation

Cosmose AI is a global company with a presence in 8 countries, transforming content engagement through cutting‑edge technology. Our flagship product, KAIKAI Lock Screen, delivers AI‑powered, personalized content to users worldwide, unlocking new and exciting opportunities for product placement.

About the Role: Mid or Senior Java Engineer

We’re looking for a strong Mid or Senior Java Engineer to help shape and scale the backend of our core product, KaiKai Lock Screen, used by millions of users every day. You’ll build and improve the services behind both the Lock Screen and our internal CMS, working on systems that process and deliver content at scale and have a real impact on the end‑user experience.

We’re Looking For Someone Who

• Dreams Big: ambitious, passionate, and out to achieve massive goals
• Customer‑centric: We care about our customers and strive to provide them with the best solutions
• Celebrates both Success & Failure: Rejoicing the wins but also reflecting, learning & improving proactively when there are defeats
• Honesty and Integrity: We appreciate respectful feedback
• Problem Solver: You’re a fixer and not a complainer
• Actionable: Work and make SMART decisions fast with conviction

What You’ll Do

• Design, develop, and maintain backend services for KaiKai Lock Screen and CMS
• Build scalable systems for large‑scale data processing and real‑time content delivery
• Contribute to architecture and system design improvements
• Maintain high code quality through testing, code reviews, and engineering best practices
• Work with distributed and event‑driven systems to build scalable backend solutions
• Improve system performance, reliability, and efficiency for a high‑traffic product
• Support integrations with external services and third‑party systems
• Collaborate closely with Product, Content, and Data teams to deliver meaningful features
• Share knowledge and contribute to continuous team improvement

What You’ll Need

• Strong experience in Java backend development
• Experience building scalable and high‑performance backend systems
• Familiarity with event‑driven systems (Kafka) and modern architectures
• Knowledge of relational databases and SQL
• Ability to take ownership and drive solutions
• Good communication within the team (Polish and English required)
• Experience with high‑traffic systems is a plus

What You’ll Enjoy

Cosmose provides employees with a great platform to immerse and learn about the most exciting, innovative, and disruptive technologies in offline behavior solutions. We are unveiling untapped opportunities and profits in the industry – and we’re just getting started. At the core of our constant innovation is a rockstar engineering team, including four world championship programming medalists, working alongside driven, curious minds across all departments. You’ll work with a seasoned leadership team, backed with expertise across strategic domains – all within a culture that values learning, impact, and cross‑functional collaboration. Cosmose is where a strong culture and shared goals unite individuals and teams to achieve bold milestones together.

We’re building something big – and we’re just getting started. If you’re excited about shaping the future of AI‑powered mobile content and working with a team that’s pushing boundaries, we’d love to meet you.

Be part of the Cosmose story. Apply now!