Senior Backend Engineer (Python)

We are looking for a highly specialized Senior Backend Engineer to join our team as a technical cornerstone for one of our key UK-based fintech partners. In this role, you will be responsible for the development and maintenance of high-quality, production-grade server-side logic designed to empower and scale small businesses globally.

As a Senior specialist, you will view backend development as a craft. You will be embedded in a global product team building and operating the microservices that power financial services for hundreds of thousands of small businesses in the UK and India. The platform infrastructure already exists — your job is to build on top of it with the reliability, security, and correctness that fintech demands.

We need a professional who writes code that is correct in adversarial conditions, at scale, and in the context of financial data. You must be able to justify every architectural decision with depth: not just identifying what is wrong, but explaining why it breaks and how to fix it properly. Experience with distributed systems patterns such as idempotency, saga patterns, and circuit breakers is essential — these are daily tools, not theoretical concepts.

Requirements

Must Have

• 7+ years of professional experience: in backend development, with at least 5 years dedicated to the Python ecosystem as your primary language.

• Python Framework Mastery: Expert-level proficiency in Django, FastAPI, and Flask, understanding when to use each based on project needs (monolithic vs. high-performance microservices).

• Solid Engineering Fundamentals: Deep understanding of SOLID principles, design patterns, and ACID properties in database transactions — and the ability to justify their application with concrete reasoning.

• Distributed Microservices Patterns: Proven, practical experience designing and operating microservices at scale. This includes idempotency (implementing idempotency keys, safe retries, and explaining what breaks without them), saga and outbox patterns for distributed transactions, circuit breakers, retry/backoff strategies, and eventual consistency trade-offs in a financial context.

• Security Engineering Mindset: You catch security vulnerabilities unprompted during code review: authentication and authorization gaps, IDOR, injection vectors, and broken access control. Deep understanding of auth schemes, token validation, and why shortcut patterns are exploitable. Applies OWASP Top 10 as a default lens, not an afterthought.

• Defensive Programming: Systematic approach to input validation including edge cases (zero values, boundary conditions, negative inputs) — especially in payment and pricing flows. Structures exception handling intentionally: avoids swallowing generic exceptions, returns correct HTTP status codes, and prevents cascading 500 errors. Validates outputs from external service calls (payment gateways, inventory services, notification services) before acting on them.

• API Design: Extensive experience building, documenting, and versioning complex REST APIs with a focus on performance, security, and developer experience.

• Database & Transaction Management: Expert knowledge of PostgreSQL including data modeling, query optimization, and indexing. Deep understanding of transaction management: knows exactly what a transaction fixes, when to use locking strategies, and how to prevent race conditions in concurrent financial operations.

• Event-Driven Architecture: Practical experience with message brokers (Kafka, RabbitMQ, AWS SQS/SNS, or equivalent) to decouple services and handle asynchronous workloads reliably and safely.

• Hardcore Testing & Quality: Absolute commitment to automated testing (Pytest, Unittest) including edge case and failure path coverage, not just happy-path scenarios. Follows Scrum/Agile best practices.

• Technical Communication: Ability to lead technical design reviews and communicate effectively with stakeholders in the UK and globally. Can articulate the reasoning behind every technical decision with depth and precision (English B2–C1).

• Fintech Domain Awareness: Familiarity with payment processing concepts, financial transaction integrity, and the regulatory sensitivity of financial data.

Nice to have

• Salesforce Integration: Familiarity with integrating Python backends with Salesforce environments.

• Observability: Experience implementing monitoring and error tracking using Sentry.

• DevOps Mindset: Experience building CI/CD pipelines (GitHub Actions, AWS CodePipeline) and managing containerized applications.

• Security First: Familiarity with OWASP standards and AWS IAM best practices.