Staff Backend Engineer (d/f/m)

Description

At Kleinanzeigen, search is the beating heart of how millions of users discover listings every day. We're evolving our search platform — moving to Vespa, expanding semantic and vector search capabilities, and building ranking models grounded in real buyer and seller signals. We're looking for a Senior Search Engineer who bridges the gap between search infrastructure and data science: someone who can both architect a high-performance retrieval pipeline and run rigorous offline experiments to validate relevance improvements.

You'll join our Core Experience engineering team and work closely with Product, UX, and fellow engineers to shape search quality for one of Europe's largest classifieds platforms.

Requirements

What we're looking for

Must-have:

• 5+ years of hands-on experience in search engineering — production-scale deployments using Elasticsearch, Solr, Vespa, or similar.
• Strong data science fundamentals: experimental design (A/B testing, interleaving), relevance evaluation, and statistical analysis with Python (Pandas, NumPy, Jupyter).
• Solid software engineering skills in at least one JVM language (Kotlin, Java, or Scala) plus Python.
• Deep familiarity with vector/semantic search: dense retrieval, approximate nearest neighbour (ANN), embedding fine-tuning or prompting for query/document understanding.
• Track record of authoring and leading technical proposals based on data analysis — not just implementing specs written by others.

Nice-to-have:

• Experience with Vespa or migrating from Solr/Elasticsearch to Vespa.
• Familiarity with LLM-based approaches to data labelling, evaluation set generation, or query augmentation.
• Experience with big data tooling (Spark, Trino, DuckDB) for large-scale offline analysis.
• AWS cloud experience (S3, DynamoDB, Lambda, Kubernetes).
• Conference talks, published work, or open-source contributions in the search/IR space.