【Senior Software Engineer】English-Speaking | International AI SaaS

This global SaaS company made it its mission to automate the travel industry.In an industry chronically suffering under labour shortage - made even more severe through the recent pandemic and the current record numbers of international tourists, especially in Japan - their suite of AI products have been massively popular among their hospitality clients.Supporting more than 8000 clients, they had a successful IPO, acquired several competitors and are actively expanding across APAC, Europe and the US.With a profitable and stable business, they are currently evolving from a monolithic Ruby on Rails architecture to a Service Oriented Architecture with modern tech stack to support new products, global integrations and long-term scalability.As one of the leading figures of this transformation, they're now hiring for an experienced Senior Software Engineer.In this role, you will:Modernize the platform by transitioning from a Rails monolith to a scalable service-oriented architectureDefine domain boundaries, architecture patterns, data models, and external APIsEvaluate and recommend a future tech stack focused on concurrency, type-safety, and developer experienceWork hands-on within the monolith to ensure safe, incremental migration under high trafficDesign robust systems ensuring performance, reliability, payment integrity, and booking consistencyLead CI/CD improvements, including zero-downtime deployment and operational best practicesStrengthen observability through monitoring, alerting, logging, and honest post-mortemsMentor engineers, guide technical decisions, and foster strong engineering cultureCollaborate closely with product partners to clarify requirements and drive impactful deliveryRequirements:Proficiency with a statically-typed modern language (e.g., Go, Kotlin, Scala, Rust, or C#), with strong understanding of concurrency and asynchronous programming.Proven track record building and maintaining services within a service-oriented or microservices architecture, and refactoring or maintaining legacy/monolithic codebases.Hands-on experience implementing and maintaining CI/CD pipelines and zero-downtime deployment strategies.Experience with scalable databases/storage systems (e.g., Aurora, Cassandra) and designing high-availability data schemas.Solid knowledge of monitoring, logging, and alerting systems (e.g., Prometheus, Grafana, Datadog) to ensure system health under high traffic load.Nice-to-Haves:Experience with Ruby and Ruby on Rails (useful for understanding and refactoring the existing system).Familiarity with payment or transaction systems requiring strong consistency and reliability.Experience building high-load, distributed systems with a focus on performance and reliability.Strong English communication skills, for collaborating with international teams.Why join:Architectural ownership: Transform a core business platform used across Asia, from tech stack definition to implementationBusiness stability + startup agility: Profitable, fast-growing, and product-driven companyInfluence and visibility: Direct collaboration with founders and leadershipCareer growth: Build leadership experience in large-scale system designWork style flexibility: Hybrid model, ~2 days/week on-site in TokyoInternational Team and English as internal language