The Service Layer: Where Separate Components Become a System

This is Part 4 of a series building a production-ready semantic search API with Java, Spring Boot, and pgvector. Part 1 covered the architecture. Part 2 defined the schema. Part 3 handled the embeddings — how text becomes vectors. Each piece worked in isolation. But systems don't fail in isolation — they fail at the boundaries. If you've ever built a feature that worked perfectly on its own but broke the moment you connected it to everything else — this article is about preventing that. At this point, we have a schema that can store documents and an embedding layer that can generate vectors. But nothing connects them. A document has nowhere to go. A query has no pipeline. This is where the service layer comes in. This is a production-style implementation — not a demo. The full project structure, tests, and configuration are available on GitHub . What Does the Service Layer Actually Do? The database stores state, but it doesn't understand it. PENDING , READY , and FAILED only become mea