We’re looking for a Mid-Senior Backend Engineer - AdExchange (Inventory & Enrichment) to join our tech team and help build the future of neuro-contextual advertising at global scale. Who We Are At Seedtag, our mission is to transform advertising by proving that effectiveness and user privacy can truly coexist. As the leading Neuro-Contextual Advertising Company, we combine Artificial Intelligence, Natural Language Processing, Computer Vision, and neuroscience to understand not only what content is about, but how it makes people feel and what they intend to do next. Our proprietary AI, Liz, enables brands to connect with audiences across the open web and Connected TV without cookies or user tracking. Founded in 2014 by two ex-Googlers, Seedtag has grown to 700+ Seedtaggers in 17 countries, backed by €250M in funding, and operates today as a global ad-tech leader. If you enjoy solving complex engineering challenges and building AI-driven systems at scale, you’ll feel right at home here. The Role You will join the AdExchange Engineering Team, the "engine room" of Seedtag. Your primary focus will be on Inventory Management and Bid Enrichment. This is a high-stakes environment where you will build the logic that categorises global ad inventory and enriches bidding requests with real-time contextual data. You will work in a high-throughput ecosystem where we don't just "deliver code"—we form hypotheses, measure real business impact, and iterate with an agile mindset to optimise our auction win rates and revenue. Your Challenge As a Mid-Senior Backend Engineer - AdExchange (Inventory & Enrichment), you will: - Inventory & Bid Enrichment: Own the core services that manage our global ad inventory and enrich bids with contextual metadata. This is a critical "hot path" in our infrastructure where low latency is non-negotiable. - High-Performance Node.js: Utilise Node.js, TypeScript, and Fastify to build ultra-low-latency services capable of handling hundreds of millions of requests daily. - Architectural Excellence: Lead the way in applying Hexagonal Architecture and Domain-Driven Design (DDD) to ensure our AdExchange remains maintainable and scalable as we grow. - Event-Driven Systems: Design and evolve our data pipelines using Kafka, ensuring that bid enrichment and inventory updates are processed reliably and asynchronously. - Technical Ownership: You will be responsible for making key technical decisions, managing project timelines, and collaborating with Data and Product teams to drive the project forward. - Quality & Observability: Ensure our services are not only fast but also resilient, well-tested, and fully observable through robust monitoring and logging practices. Our Core Values Outcome over Output We measure success by impact and value, not by volume of features or lines of code. Failure Is Allowed, Learning Is a Must Experimentation is key to innovation. We test early, iterate often, and learn fast. We Are All Scouts We take ownership and leave things better than we found them. We Are Data-Driven Data informs our decisions and helps us continuously improve our systems and results. Tech Stack We operate at large scale, supporting up to 120k requests per second, with ML models responding in under 10 milliseconds and processing 20 TB of data daily. Our stack includes: - Primary: Node.js, TypeScript, Fastify - Secondary: Go (Golang) - Data & Messaging: Kafka, MongoDB, Redis - Infrastructure: Kubernetes, AWS, Terraform - Patterns: DDD, Hexagonal Architecture, Event-Driven Microservices What You’ll Need to Succeed - The 5-Year Milestone: You have 5+ years of demonstrable experience in software engineering, with a focus on building high-performance backend systems. - Node.js & TypeScript Expert: You have a deep understanding of the Node.js event loop and asynchronous programming. Experience with Fastify is a significant plus. - Architectural Practitioner: You don't just know the theory; you have experience implementing Hexagonal Architecture (Ports & Adapters) and DDD. You know how to define clear domain boundaries and isolate business logic from infrastructure. - Distributed Systems Knowledge: You are comfortable with Event-Driven Architectures and message brokers like Kafka. You understand the challenges of eventual consistency and distributed data. - The "Big Picture" Thinker: You understand the business context of your work. You can balance the need for "perfect code" with the pragmatic need to deliver value and iterate based on data. - Software Craftsmanship: You are an advocate for Clean Code, SOLID principles, and TDD. You take pride in producing code that is easy for your peers to read and maintain. - Curiosity for Go: While our core stack for this role is Node.js, we have services in Go (Golang). You don't need to be an expert, but you should be excited to learn and contribute to our Go codebase when needed. - AI-Driven Efficiency: You see AI as a partner in engineering. You actively use AI tools to increase your productivity, from generating unit tests to refactoring legacy patterns. You are curious about how AI can help us build more resilient systems faster and you stay updated on the best ways to integrate it into a professional SDLC. Why Join Seedtag? - A key moment of growth with real ownership and global impact. - Flexible work model with 100% remote or hybrid options. (Remote contracts available in Spain, Italy, the UK, Belgium, the Netherlands, and Germany.) - Continuous learning through a learning platform and optional language classes. - A supportive, trust-based culture that values well-being. - Team activities, offsites, and opportunities to connect beyond work. Additional Perks - Home office setup budget up to €1,000 - Paid trips to our HQ in Madrid - MacBook Pro M3 Ready to Join the Seedtag Adventure? At Seedtag, we create an environment where everyone can thrive. If you need accommodations during the hiring process, let us know and we’ll ensure a positive experience. Send us your CV and let’s build the future of neuro-contextual advertising together.