database-relational-overview.md

# RDB Overview

The relational database (RDB) manages data based on relational models. With the underlying SQLite database, the 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. A RDB is also called RDB store.

- **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 RDB provides common operation APIs for external systems. It uses the SQLite as the underlying persistent storage engine, which supports all SQLite database features.

**Figure 1** How RDB works

![how-rdb-works](figures/how-rdb-works.png)

## Default Settings

- The default RDB logging mode is Write Ahead Log (WAL).
- The default data flushing mode is **FULL** mode.
- The default size of the shared memory used by an OpenHarmony database is 2 MB.

## 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.