1# RDB Overview 2 3The 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. 4 5## Basic Concepts 6 7- RDB 8 9 A type of database created on the basis of relational models. The RDB stores data in rows and columns. 10 11- Predicate 12 13 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. 14 15- Result set 16 17 A set of query results used to access data. You can access the required data in a result set in flexible modes. 18 19- SQLite database 20 21 A lightweight open-source relational database management system that complies with Atomicity, Consistency, Isolation, and Durability (ACID). 22 23## Working Principles 24The 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. 25 26**Figure 1** How RDB works 27 28 29 30## Default Settings 31- The default database logging mode is write-ahead logging (WAL). 32- The default database flush mode is Full mode. 33- The default shared memory is 8 MB for the OpenHarmony database and 2 MB for a single query. 34 35## Constraints 36- A maximum of four connection pools can be connected to an RDB to manage read and write operations. 37- To ensure data accuracy, the RDB supports only one write operation at a time. 38
