Lead Senior Backend Engineer (m/f/d)

T‑Systems is part of the Deutsche Telekom Group, with around 30.000 employees worldwide. We create technology with purpose to generate a positive impact on society. We are looking for curious talent, eager to learn, take on challenges, and contribute ideas that transform our customers’ experience.

We trust people: we offer autonomy, continuous support, and a collaborative environment where you can grow without limits. We are one global team, guided by respect, integrity, and a passion for doing better every day.

T-Systems is at the forefront of innovation, delivering advanced technology solutions across sectors including automotive, healthcare, and public services. Our AI Foundation Services team _builds the platform infrastructure that powers AI inference at scale, including API gateways, authentication, billing, and multi-tenant serving._ designs and builds high-performance backend systems and APIs that power intelligent applications across diverse industries. We cultivate a collaborative engineering culture where _technical depth_ creativity and _ownership_ technical expertise drive real-world impact.

Role Overview

We are looking for a _senior backend engineer_ highly skilled Python Developer with _strong system design skills and experience building production infrastructure_ deep experience designing, building, and scaling distributed systems and APIs. In this role, _Y_ y _You will_ ou will _own_ architect and _drive the evolution of the_ maintain backend _platform behind our AI inference endpoints - a multi-tenant system handling authentication, API key management, usage metering, billing and request proxy_ services that enable high-availability, data-intensive AI-powered solutions._The role is high-ownership and you will shape architecture decisions, raise the bar of engineering quality and build systems that are performant, secure and observable at scale._ You’ll collaborate with experts in AI infrastructure and data engineering to build robust, secure, and efficient systems that scale to millions of requests.

Responsibilities and Duties

• _Design and build core platform services: API gateway, authentication, authorization, key rotation, and multi-tenant isolation._ Design, implement, and optimize APIs and backend systems using Python frameworks such as primarily FastAPI (or Flask, or Django).
• _Architect and implement usage metering, billing integration, and rate limiting for inference endpoints._ Build and maintain scalable, fault-tolerant microservices for data processing and AI integration.
• _Build and operate a high-throughput proxy/routing layer for AI model serving traffic._ Collaborate with cross-functional teams to design system architecture and ensure seamless system interoperability.
• _Design telemetry and observability into the platform from the ground up — structured logging, distributed tracing, metrics, and alerting._ Implement robust CI/CD pipelines, monitoring, and observability for high-performance production systems.
• _Drive technical decisions on architecture, data modeling, and technology choices._ Identify performance bottlenecks and drive improvements in reliability, scalability, and latency.
• _Establish engineering standards for the backend codebase: testing, code review, CI/CD, and deployment practices._ Ensure best practices for security, compliance, and maintainability across the API lifecycle.
• _Collaborate closely with the ML infrastructure team to integrate with model serving systems (e.g., NVIDIA Dynamo, vLLM, SGlang, TensorRT-LLM)._

Must-have requirements:

• Degree in Computer Science, Software Engineering, or equivalent professional experience.
• 5+ years of experience _building and operating_ developing backend systems _in production_ ,_with a meaningful scale_ with a focus on API design and high-scale production environments.
• Strong proficiency in Python _and at least one systems-oriented language (Go, Rust, Java, C++)_ ,_willingness to work across the current stack which includes Python (FastAPI) - we value the right tool for the jobengineer_ with deep knowledge of asynchronous programming, modern Python tooling and profiling.
• Proven experience designing and maintaining REST APIs (_REST, gRPC, WebSockets, SSE_ Including websockets, SSE, Webhooks etc.) in production environments _and the tradeoffs between them_.
• Solid understanding of distributed systems _: consistency, fault tolerance, concurrency, performance and networking_ , data pipelines, and performance optimization.
• _Experience designing authentication/authorization systems (OAuth2, JWT, API key management, RBAC/ABAC)._
• Experience with cloud infrastructure (AWS, GCP, or Azure) and container _s_ ized environments (Docker _) and orchestration (_ , Kubernetes).
• Understanding of database design (SQL, ORMs etc.) and data modeling at scale.
• Fluent in English (written and verbal) and comfortable collaborating in international teams.

Nice-to-have requirements:

• Experience with high-throughput _, low-latency_ or real-time systems (e.g., message queues, event-driven architectures)_._
• Familiarity with generative AI or large language models (e.g., integration of AI APIs, RAG workflows, or vector databases).
• _Experience_ Knowledge of _in_ modern DevOps practices, infrastructure as code,_GitOps workflows_ and observability tools _(Prometheus, Grafana, OpenTelemetry)_.
• _Exposure to ML serving infrastructure (model servers, GPU scheduling, inference optimization) is a plus but not required._
• Comfortable occasionally touching or contributing to frontend code when necessary.
• Experience in stress/load-testing and evaluating performance of production systems.
• Strong problem-solving mindset and commitment to code quality and performance excellence.
• _Track record of improving engineering culture: mentoring junior engineers, introducing better practices, leading technical discussions._

What do we offer you?

Work environment & flexibility

• International, dynamic and collaborative environment
• T-Social: social initiatives (sports, community, health, ...)
• Hybrid work model (remote/on-site)
• Flexible working hours

Growth & development

• Customized training: access to Coursera to learn whatever you want, whenever you want
• Weekly language classes (English, Spanish & German)
• International Mentoring Sessions & Experience Days

Compensation & benefits

• Flexible compensation plan (health insurance, meal vouchers, childcare, transport)
• Telemedicine
• Life and accident insurance
• Social fund

Wellbeing & time off

• 26+ working days of vacation per year
• Free access to specialist services (medical, legal, wellness)
• 100% salary coverage during medical leave

And many more advantages of being part of T-Systems!

If you are looking for a new challenge, do not hesitate to send us your CV! Please send CV in English. Join our team!

_T-Systems Iberia will only process the CVs of candidates who meet the requirements specified for each offer._