Senior Backend Engineer

About the Team

The Serenity team is part of the Trust & Tools crew at leboncoin. We operate on the critical path of ad publication: if our systems are not available, no ads are published. Every day, our platforms process:

• 1.5 million ads
• 15 million messages All content is analyzed and decided automatically via rule engines and Machine Learning models. Our system is a pillar of leboncoin's trust system. Historically positioned as a central multi-marketplace team, Serenity is now fully integrated into the leboncoin ecosystem. A structuring merger is planned with the MIM team, in charge of the back-office, particularly on the processing of reports. The objective is to strengthen the integration between automated moderation, human workflows, and internal tools, with major technical and product challenges. We collaborate closely with Customer Relations and the fraud unit to further reduce risks. The team is based in Paris, operates in a multicultural environment, and includes Backend Engineers, Machine Learning Engineers, Frontend Engineers, QA, Product Manager, Engineering Manager, and SRE.

Job Responsibilities

We are looking for a Senior Backend Engineer capable of taking ownership of critical services with high business impact. You will be involved in:

• High-volume distributed architectures
• Real-time decision systems
• Advanced fraud issues
• A strategic technological modernization project
• Deep integration of our platform into the leboncoin ecosystem The role combines technical expertise, strong autonomy, and a structuring contribution to the team's technical directions.

Current Challenges

• Ensure maximum availability on a system that blocks publication
• Further reduce fraud
• Support the technical merger with the MIM team (back-office & reporting)
• Lead the progressive migration of Java/Kotlin services to Go
• Elevate performance, resilience, and observability standards Some upcoming developments are strategic and cannot be detailed publicly.

Architecture & Scalability Responsibilities

• Design and evolve high-volume distributed systems
• Ensure high availability, resilience, and performance
• Actively contribute to architectural decisions
• Identify, prioritize, and reduce technical debt
• Participate in the definition and implementation of the migration to Go

Development & Modernization Responsibilities

• Develop and maintain backend services in Python (central language)
• Contribute to the progressive migration of Java/Kotlin services to Go, with a view to improving performance and maintainability
• Design and evolve event-driven architectures (Kafka)
• Maintain high quality standards (tests, code reviews, documentation)
• Contribute to the continuous improvement of CI/CD (GitHub, GitHub Actions)

Production & Reliability Responsibilities

• Participate in on-call and incident management
• Define and improve SLOs/SLAs
• Optimize latency, throughput, and AWS costs
• Collaborate closely with SREs to enhance reliability

Technical Leadership & Collaboration Responsibilities

• Be a driving force in technical discussions and architecture reviews
• Mentor engineers and elevate the collective level
• Work closely with the PM to transform business needs into robust solutions
• Contribute to the technical roadmap
• Actively collaborate with the MIM team and business teams (reporting, customer relations, fraud)

Stack & Environment

• Languages: Python (central), Java/Kotlin (legacy), Go (strategic target)
• Cloud: AWS
• Messaging: Kafka
• Data: Postgres, DynamoDB, Redis, ElasticSearch
• Orchestration: Kubernetes
• CI/CD: GitHub, GitHub Actions
• Observability: Datadog
• Architecture: microservices, event-driven systems, real-time decisioning

Job Benefits

• Pleasant working conditions (central location, remote work…)
• Opportunities for rapid and tailored professional development
• A Swile meal voucher card (€9/day)
• Effective and competitive health and provident insurance coverage