Local Storage (Room Database)

Relax. We'll talk through this in plain words — no textbook voice.

What you'll walk away with

Understand Local Storage (Room Database) without the intimidation
Get comfortable running Android Studio/Compose code yourself
Apply this concept in a real project right away

Let's think about it this way for a second

Room is a library that wraps Android's built-in SQLite database in a type-safe, Kotlin-friendly way — there are three core concepts: Entity (`@Entity`, defines the table structure as a data class), DAO (`@Dao`, defines database operations - insert/query/delete - as an interface), and Database (`@Database`, the class that combines your Entities/DAOs). Pair Room with a ViewModel (Advanced lesson 1), and your data survives even an app restart — unlike RAM-only ViewModel state.

Let's connect this to a real scenario

Define `@Entity data class TodoEntity(@PrimaryKey val id: String, val title: String, val isDone: Boolean)`, then write `@Dao interface TodoDao { @Query("SELECT * FROM TodoEntity") suspend fun getAll(): List<TodoEntity>; @Insert suspend fun insert(todo: TodoEntity) }` — calling `todoDao.insert(newTodo)` from your ViewModel permanently saves the data to the device disk, so it's still there even if you force-close and reopen the app.

Room = a database library that wraps SQLite in a type-safe, Kotlin-friendly way
Entity (table), DAO (database operations), Database (ties them together) — the three core concepts
Permanently stores data on the device disk — survives an app restart

Let's walk through it together

kotlin

@Entity
data class TodoEntity(
    @PrimaryKey val id: String,
    val title: String,
    val isDone: Boolean
)

@Dao
interface TodoDao {
    @Query("SELECT * FROM TodoEntity")
    suspend fun getAll(): List<TodoEntity>

    @Insert
    suspend fun insert(todo: TodoEntity)
}

@Database(entities = [TodoEntity::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun todoDao(): TodoDao
}

You should see

Even after force-closing and reopening the app, you should see the todo list data still there (pulled back from Room).

5-minute try-it

Write `TodoEntity`, `TodoDao`, and `AppDatabase` yourself, then rewrite `TodoViewModel` (Advanced lesson 1) to call the Room DAO instead of using an in-memory list.

A quick word of caution

Using `fallbackToDestructiveMigration()` in a production app instead of a proper `Migration` strategy when the database schema changes can wipe out all of an existing user's data after an update.

Easy traps

Updating the database schema (an Entity field) but forgetting to bump the `version` number — this can crash existing users' apps after an update
Running a Room database operation directly on the main thread — calling it outside a `suspend` function can freeze the app

Now try it yourself

Write `TodoEntity`, `TodoDao`, and `AppDatabase` yourself, then rewrite `TodoViewModel` (Advanced lesson 1) to call the Room DAO instead of using an in-memory list.

You'll know it worked when: Even after force-closing and reopening the app, you should see the todo list data still there (pulled back from Room).