Senior Backend Engineer – Python/FastAPI, AWS, AI

Oscar Associates Limited; Greater London; Hybrid; GBP 85,000 - 105,000; Full time

Job summary

Oscar Associates Limited is hiring a Backend Engineer to build scalable SaaS platforms from London. You will design REST and asynchronous services in Python 3.11 with FastAPI, interfacing across Java/Spring boundaries and PostgreSQL persistence.

You’ll ensure tenant isolation, access control and robust testing, while deploying on AWS (EKS/ECS, RDS, S3) and integrating OpenTelemetry/logging.

Qualifications

• At least 5 years of back-end engineering experience in production SaaS or platform systems.
• Experience with multi-tenant or enterprise systems, including isolation, permissions and auditability
• Strong Python skills and experience with FastAPI or comparable frameworks
• Solid SQL and PostgreSQL skills, with the ability to work at Java/Spring service boundaries
• Production AWS experience and confidence with Git, CI/CD and Linux
• Daily use of AI coding tools such as Cursor or Claude Code, with a considered view of where they help
• A degree in engineering, computer science or a related field, or equivalent experience
• We value Java/Spring depth alongside Python, event-driven architecture, queues and workflow engines, financial services or regulated-industry experience, and LLM service integration patterns
• For interview preparation, be ready to discuss an API contract evolved without breaking consumers, a tenant or permission boundary implemented and tested, and a production quality, reliability or cost problem diagnosed and improved

Responsibilities

• Design REST and asynchronous services in Python 3.11 and FastAPI
• Work at Java/Spring boundaries with PostgreSQL and async SQLAlchemy persistence
• Implement tenant isolation, context, role-based access and configuration
• Version and evolve contracts between core and AI services without breaking consumers
• Maintain comprehensive unit and integration tests with pytest and pytest-asyncio, and uphold code review standards
• Use OpenTelemetry traces, structured logs, metrics and actionable alerts to expose model-call latency, failure rates and token costs
• Build validation, schema enforcement and safe-failure behaviour for outputs that may be well-formed yet incorrect
• Apply authentication, authorisation, secrets management, input validation, OWASP awareness and audit trails in everyday engineering
• Deploy and operate services on AWS using EKS/ECS patterns, RDS and S3 through Jenkins quality gates
• Improve queries, caching, asynchronous throughput and resource right-sizing
• Build queueing, rate limits and cost-aware orchestration for model-calling services
• Ship through disciplined CI/CD alongside the India-based delivery organisation
• Create evaluations, visibility and safeguards to catch mistakes early, reduce them over time and prevent recurring failures

Skills

Back-end engineering

Python

PostgreSQL

AWS

CI/CD

Linux

AI tooling

Multi-tenant/enterprise

REST API design

Education

Engineering degree or equivalent

Tools

FastAPI

SQLAlchemy

OpenTelemetry

Jenkins

AWS EKS/ECS

Job description

Oscar Associates Limited is hiring a Backend Engineer to build scalable SaaS platforms from London. You will design REST and asynchronous services in Python 3.11 with FastAPI, interfacing across Java/Spring boundaries and PostgreSQL persistence.

You’ll ensure tenant isolation, access control and robust testing, while deploying on AWS (EKS/ECS, RDS, S3) and integrating OpenTelemetry/logging.