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 meanin