Backend Engineer - AI for Business Department (AI4B)

Job Description:

Department Overview

Rakuten provides a variety of services in e-commerce, fintech, digital content, and communications to users worldwide. Our AI for Business Department operates as a center of excellence, focusing on developing and deploying innovative solutions that combine frontier technologies to enhance our products and services across business units. 
We are a diverse team of engineers who are passionate about applying their skills to make a significant difference and potentially change society for the better through AI. 

Within AI4B, our AI Engineering Section serves as the foundation that powers innovation, with the mission to design, build, and scale the technical backbone of our products. Focused on backend systems, DevOps practices, and cloud infrastructure, we ensure reliability, performance, and adaptability—enabling teams to move fast while maintaining operational excellence. Are you passionate about building scalable, high-performance AI application backends and APIs? Join us as a Backend Engineer and help shape the future of AI-powered solutions across our ecosystem.

Position:

Position Details

As a Backend Engineer, you will design and operate scalable systems that handle complex AI workloads, integrate with multiple LLM providers, and meet strict security, latency, and reliability requirements across Rakuten's businesses. You'll work with both cutting-edge AI technologies and proven solutions like Python, FastAPI, PostgreSQL, and Redis, while implementing secure authentication systems, comprehensive monitoring, and efficient caching strategies. 

Your work will involve building intelligent applications leveraging AI, designing data models for AI applications including vector storage and embeddings management, and developing comprehensive test suites with test-driven development (TDD) practices. 

This role requires 5+ years of backend development experience with strong expertise in microservices architecture, RESTful API design, and containerization technologies. You'll collaborate in an international environment requiring strong English communication skills, while having the opportunity to work with multiple LLM providers, orchestration frameworks, and cloud platforms (Azure, GCP, AWS). The position offers unique exposure to AI/ML model deployment, MLOps practices, and real-time data processing systems that power intelligent applications used by millions of users worldwide. 

Key Job Responsibilities

• Design and implement high-performance REST APIs, microservices, and event-driven architectures for AI applications 

• Develop intelligent applications leveraging AI with seamless switching and fallback mechanisms 

• Build AI service integration layers and implement complex workflows including Retrieval-Augmented Generation (RAG) and agent systems 

• Implement comprehensive monitoring, security, and testing strategies for production AI systems 

• Partner with cross-functional teams to translate AI product ideas into production-ready applications 

Mandatory Qualifications:

• Bachelor's degree or above in Computer Science, Software Engineering, or related technical discipline 

• 5+ years of backend development experience with scalable web applications and APIs 

• Strong expertise in Python, FastAPI, PostgreSQL, and Redis technologies 

• Strong English communication skills and experience with collaborative development workflows 

Desired Qualifications:

• Experience with multiple LLM providers (OpenAI, Anthropic, Google Vertex AI) and frameworks like LangChain 

• Knowledge of vector databases, embedding management, and cloud platforms (Azure, GCP, AWS) 

• Experience with monitoring tools (Prometheus, Grafana) and AI/ML model deployment practices 

• Knowledge and understanding of distributed systems and security best practices 

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Languages:

English (総合 - 3 - 上級)