I am learning DDD topic from scratch and on many DDD examples I see "repository interface" sitting in domain. What is real purpose of this repository actually/what does it do or solve exactly? Does this correspond or have some connection to persistence layer implementation?
Thank you.
What is real purpose of this repository actually/what does it do or solve exactly?
The (authoritative) reference for the REPOSITORY pattern in the context of domain-driven-design is chapter 6 of the "blue book" by Eric Evans. That is the chapter where he discusses lifecycle management patterns.
The repository is a facade, intended to support the illusion that DOMAIN ENTITIES (a chapter 5 pattern) are kept in an in-memory collection, somewhere. So when your process needs an entity, it asks the repository for it (usually offering an identifier as a hint).
It's a form of information hiding, in the Parnas 1971 sense; we can freely switch between transient collections stored in memory and durable collections stored on disk, or in the RDBMS, or in the key/value store. Only the implementation(s) of the repository need to worry about the specifics of the plumbing.
The motivation was largely to separate developer focus; the separation of ideas permits developers working on the "business rules" to concentrate on the domain, without being distracted by the concerns of storage and retrieval.