Staff Backend Engineer (d/f/m)

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.

Responsibilities

What you'll do

- Design and implement search features end-to-end: from indexing pipeline changes and schema design to query rewriting, ranking, and autocomplete.

- Lead data-driven technical initiatives - write RFCs grounded in query log and user behaviour analysis, then own delivery from design to production.

- Champion semantic and vector search: design embedding-based retrieval components, integrate LLMs for query understanding or evaluation, and measure real impact via A/B experiments.

- Build and maintain offline evaluation pipelines (relevance judgment, NDCG, MRR) to accelerate the feedback loop between experimentation and deployment.

- Improve indexing reliability and throughput - reduce re-ingestion times, ensure consistency, and tighten SLAs.

- Contribute to the Vespa migration: design index schemas, relevance profiles, and deploy ML ranking models within the platform.

- Collaborate cross-functionally with Data Science, Product, and other engineering teams; represent the team at technical forums and occasionally at external conferences.

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.

Benefits

At Kleinanzeigen, we care about our people - their wellbeing, growth, and life beyond work. Our benefits are designed to support you holistically, giving you flexibility and opportunities to thrive.

Flexibility

• 28 business days of paid annual holidays
• 20 days “work from anywhere”
• Flexible working time (hybrid)
• Paid additional parental leave for both parents
• Extra paid days off for special occasions (e.g. your wedding or a move)

Wellbeing & Support

• Private health insurance
• Gym Pass (may cover up to 70% of gym membership fees)
• 40€ per month as a remote working allowance.

Growth & Development

• Personal development budget
• Learning opportunities to grow your skills and career
• A unique position in an international and dynamic environment
• A flat, collaborative culture where your feedback actively shapes our processes

Perks & Everyday Benefits

• Competitive compensation package (Base salary + annual bonus)
• Flexible Remuneration Program (tax-efficient options for transport, restaurant vouchers and childcare)
• Corporate benefits and exclusive discounts
• An attractive referral program

Kleinanzeigen is an equal opportunity employer that values diversity and inclusion. We welcome people of all backgrounds, races, religions, colours, national origins, gender identities, sexual orientation, ages, marital status, or disability status.

If this role sparks your interest but you’re not sure you meet every single requirement, we’d still love to hear from you anyway!