android-data-persistence

Android Data Persistence

Overview

Reliable data persistence is critical for Android apps. This skill covers Room (SQLite abstraction), DataStore (key-value and proto), offline-first architecture, Paging3 for large datasets, and the repository pattern that abstracts data sources from the rest of the app.

When to Use

• Setting up or modifying a Room database
• Writing database migrations
• Implementing offline-first data access
• Adding pagination for large datasets
• Choosing between Room, DataStore, and SharedPreferences
• Implementing the repository pattern

Skip when: Data is purely in-memory or comes only from a remote API with no caching.

Data Storage Decision Guide

Requirement	Solution
Structured data with relations	Room
User preferences (key-value)	DataStore (Preferences)
Typed settings with schema	DataStore (Proto)
Large datasets with pagination	Room + Paging3
Simple flags or tokens	DataStore (Preferences)
Never use	SharedPreferences (for new code)

Core Process

Step 1: Room Setup

1. Define entities:

@Entity(
    tableName = "tasks",
    indices = [Index(value = ["created_at"])],
)
data class TaskEntity(
    @PrimaryKey
    val id: String,
    @ColumnInfo(name = "title")
    val title: String,
    @ColumnInfo(name = "description")
    val description: String?,
    @ColumnInfo(name = "completed")
    val completed: Boolean = false,
    @ColumnInfo(name = "created_at")
    val createdAt: Long = System.currentTimeMillis(),
    @ColumnInfo(name = "updated_at")
    val updatedAt: Long = System.currentTimeMillis(),
)

1. Define DAOs:

@Dao
interface TaskDao {
    @Query("SELECT * FROM tasks ORDER BY created_at DESC")
    fun observeAll(): Flow<List<TaskEntity>>

    @Query("SELECT * FROM tasks WHERE id = :taskId")
    suspend fun getById(taskId: String): TaskEntity?

    @Query("SELECT * FROM tasks WHERE completed = :completed ORDER BY created_at DESC")
    fun observeByStatus(completed: Boolean): Flow<List<TaskEntity>>

    @Upsert
    suspend fun upsert(task: TaskEntity)

    @Upsert
    suspend fun upsertAll(tasks: List<TaskEntity>)

    @Query("DELETE FROM tasks WHERE id = :taskId")
    suspend fun deleteById(taskId: String)

    @Query("DELETE FROM tasks WHERE completed = 1")
    suspend fun deleteCompleted()

    @Transaction
    suspend fun replaceAll(tasks: List<TaskEntity>) {
        deleteAll()
        upsertAll(tasks)
    }

    @Query("DELETE FROM tasks")
    suspend fun deleteAll()
}