Couchbase Server 4.5 introduces a new Sub-Document API, which enhances performance and usability by allowing users to access or modify parts of a document without retrieving the entire document. This server-side feature supports CRUD operations based on paths, improves network efficiency, and facilitates maintaining key references between documents. The API allows for specific path queries and modifications within documents, supporting atomic operations that ensure changes are applied entirely or not at all, although it does not inherently manage consistency conflicts. The Sub-Document API simplifies client-side operations and is particularly advantageous for handling larger documents. Additionally, Couchbase employs an optimistic locking mechanism through a Compare-And-Swap (CAS) value, enabling users to manage concurrent access without a full ACID-compliant transaction manager.