# RDB Overview The relational database (RDB) manages data based on relational models. With the underlying SQLite database, the OpenHarmony RDB provides a complete mechanism for managing local databases. To satisfy different needs in complicated scenarios, the RDB offers a series of methods for performing operations such as adding, deleting, modifying, and querying data, and supports direct execution of SQL statements. ## Basic Concepts - RDB A type of database created on the basis of relational models. The RDB stores data in rows and columns. - Predicate A representation of the property or feature of a data entity, or the relationship between data entities. It is mainly used to define operation conditions. - Result set A set of query results used to access data. You can access the required data in a result set in flexible modes. - SQLite database A lightweight open-source relational database management system that complies with Atomicity, Consistency, Isolation, and Durability (ACID). ## Working Principles The OpenHarmony RDB provides a common operation interface (**RdbStore**) for external systems. It uses the third-party open-source SQLite as the underlying persistent storage engine, which supports all SQLite database features. **Figure 1** How RDB works ![](figures/en-us_image_0000001115980740.png) ## Default Settings - The default database logging mode is write-ahead logging (WAL). - The default database flush mode is Full mode. - The default shared memory is 8 MB for the OpenHarmony database and 2 MB for a single query. ## Constraints - A maximum of four connection pools can be connected to an RDB to manage read and write operations. - To ensure data accuracy, the RDB supports only one write operation at a time.