Haken Senior Backend Engineer - Checkout APIs Tokyo

Join one of Japan's leading internet and e-commerce companies as a Senior Backend Engineer, building and operating high-traffic checkout APIs and backend systems that support millions of online transactions.

This role focuses on the architecture, development, operation, and continuous improvement of mission-critical e-commerce services, including RESTful APIs, message consumers, and batch processing systems. You will work with Java-based technologies and modern containerized infrastructure while helping maintain the reliability and scalability of a large-scale digital commerce platform.

You will also take a technical leadership role, partnering with Product Managers, QA engineers, and cross-functional development teams to deliver reliable backend solutions and drive engineering improvements.

Key Responsibilities

• Design, develop, test, and release backend services and checkout API features aligned with business and product requirements.
• Operate and maintain mission-critical RESTful APIs, message consumers, and batch processing systems supporting high-volume e-commerce transactions.
• Investigate production incidents, perform root cause analysis, troubleshoot complex system issues, and implement effective solutions.
• Identify and address technical debt while continuously improving backend performance, scalability, stability, and reliability.
• Lead engineering initiatives and provide technical direction across backend development projects.
• Collaborate closely with Product Managers, QA engineers, and cross-functional teams throughout the software development lifecycle.
• Contribute to operational improvements and engineering practices that strengthen the reliability of large-scale checkout systems.

Required Skills and Qualifications

Experience:

• 5+ years of full-stack or backend engineering experience using Java EE.
• Proven experience designing, developing, and operating RESTful Web APIs for large-scale online or web-based systems.
• Hands-on experience with Docker or other containerization technologies.
• Experience supporting production systems, including troubleshooting, root cause analysis, and incident resolution.
• Strong understanding of backend application development and the software development lifecycle.
• Ability to contribute to the development and operation of high-traffic, business-critical systems.

Soft Skills:

• Strong sense of ownership and accountability when managing mission-critical production systems.
• Proactive leadership skills with the ability to drive engineering initiatives and technical improvements.
• Strong problem-solving and troubleshooting capabilities.
• Effective communication skills for collaborating across multidisciplinary and international teams.
• Team-oriented mindset with the ability to work effectively with Product Management, QA, and engineering stakeholders.

Language Requirements:

• English: Conversational level, approximately TOEIC 600+ equivalent, for daily technical collaboration.
• Japanese: Basic level.

Preferred Skills & Qualifications

• Design and development experience using Spring Framework.
• Practical experience with RDB, KVS, or NoSQL databases.
• Hands-on experience designing or maintaining CI/CD pipelines.
• Experience using monitoring and observability tools such as Grafana.
• Knowledge of scalable backend architectures for high-traffic online platforms.
• Experience using AI technologies and AI-assisted development tools to improve the software development lifecycle.

About the Company

Our client is one of Japan's leading internet services and e-commerce technology companies, operating large-scale digital platforms that support millions of users and transactions.

Engineers work in an innovative and internationally connected technology environment where backend engineering, API development, scalable architecture, AI-enabled workflows, and modern DevOps practices play an important role in delivering reliable customer experiences.

For experienced Java and backend engineers, this is an opportunity to work directly on high-concurrency systems where engineering decisions can have a significant impact at scale.

Why You'll Love Working Here

• Build and operate high-traffic checkout APIs and backend systems supporting millions of transactions.
• Gain hands-on experience with large-scale Java backend architecture, REST APIs, containerization, and distributed systems.
• Take ownership of mission-critical services from development through production operation and continuous improvement.
• Strengthen your technical leadership by driving engineering initiatives across multidisciplinary teams.
• Explore the use of AI technologies to improve engineering productivity and software development workflows.
• Work in an international technology environment with opportunities for daily cross-functional collaboration.
• Benefit from a hybrid working model with flexible work-from-home arrangements.
• Access an employee food hall with meals provided and receive a transportation allowance.

Don't Miss Out - Apply Now!