Repository vs DAO: differences and when to use each pattern

Common confusion

Repository and DAO are often used interchangeably, but they are different patterns with different intentions. DAO persists complete objects. Repository abstracts storage and only works with domain aggregates.

DAO (Data Access Object)

DAO encapsulates access to a data source. There's typically one DAO per database entity. It exposes CRUD operations directly.

interface UserDao {
  findById(id: number): UserRow;
  save(user: UserRow): void;
  delete(id: number): void;
}

Repository

Repository acts as an in-memory collection of domain aggregates. Only aggregate roots have a repository. The language is domain-oriented, not database-oriented.

interface UserRepository {
  findByEmail(email: Email): User;
  add(user: User): void;
  remove(userId: UserId): void;
}

When to use each

Use DAO when working with ActiveRecord or simple CRUD without complex domain logic. Use Repository when you have DDD, Clean Architecture, or rich business logic.

You can combine them

It's common for a Repository to internally use one or more DAOs to populate complex aggregates from multiple tables.

Unsure which pattern to use? At Vynta we advise on data access architecture.