Senior Backend Engineer

Summary

Join KOMOJU, a leading cross-border payment gateway in Japan, as a Senior Backend Engineer. You will build backend systems using Ruby on Rails, translate product goals into working software, and lead large technical projects. Collaborate with product managers, provide code reviews, mentor junior developers, and ensure high-quality, reliable software. The ideal candidate has experience with large web applications, building scalable APIs, and database performance. KOMOJU offers a product-focused environment, a flat and inclusive culture, and opportunities for professional growth. The company is international, with English as the primary language, and offers language training.

Requirements

• Experience with building performant and scalable APIs
• Solid understanding of database performance characteristics and data access patterns
• Ability to design data models and system architectures spanning multiple data stores (experience working with OLAP and OLTP databases)
• Hands-on experience with observability practices, including metrics, logging and tracing
• Strong technical communication skills - including the ability to document designs and decisions clearly and concisely
• Experience with integrating with third party APIs in a secure and fault-tolerant manner
• Experience and understanding of common web vulnerabilities and how to mitigate them
• Strong understanding of software architecture, design patterns and best practices

Responsibilities

• Build backend systems using Ruby on Rails stack
• Understand product goals and feature requests and translate them into working software
• Lead large technical projects from design through implementation and delivery
• Support team members through code review and architecture discussions
• Ensure high-quality, reliable software - including thorough testing, proactive monitoring, observability, and continuous improvement
• Identify high-leverage refactoring opportunities that improve team velocity, code clarity, and long-term maintainability

Preferred Qualifications

• Experience with TypeScript, Vue.js and/or React
• Experience with payments or payment systems
• Business-level spoken Japanese

Benefits

• 10 days regular vacation, additional 5 days summer and 5 days winter vacation
• Paid birthday holiday
• Budget for self-learning allowance, to ensure our employees’ skills remain current
• Language training for Japanese