# 5.6 Databases

### Creating the Database

The database does the heavy lifting of storing and retrieving your entities through the abstract functions of your DAO. 

The following code defines an `GroceryDatabase`class to hold the database. `GroceryDatabase`defines the database configuration and serves as the app's main access point to the persisted data. The database class must satisfy the following conditions:

* The class must be annotated with a [`@Database`](https://developer.android.com/reference/kotlin/androidx/room/Database) annotation that includes an [`entities`](https://developer.android.com/reference/kotlin/androidx/room/Database#entities) array that lists all of the data entities associated with the database.
* The class must be an abstract class that extends [`RoomDatabase`](https://developer.android.com/reference/kotlin/androidx/room/RoomDatabase).
* For each DAO class that is associated with the database, the database class must define an abstract method that has zero arguments and returns an instance of the DAO class.

```kotlin
@Database(entities = [GroceryItem::class], version = 1)
abstract class GroceryDatabase: RoomDatabase() {
    abstract fun groceryItemDao(): GroceryItemDao
}
```

### Accessing the Database

After you have defined the data entity, the DAO, and the database object, you can use the following code to create an instance of the database:

```kotlin
val db = Room.databaseBuilder(
            applicationContext,
            GroceryDatabase::class.java, "grocery-database"
        ).build()
```

{% hint style="danger" %}
If you are using your database across your app, you should only need one instance of the Room Database class. Having the number of instances of a Database can lead to memory leaks. Therefore one should use the [singleton ](/chapters/7.-persistent-storage/6.1-singletons.md)patterns in the Database to keep one unified instance!
{% endhint %}

From there, you can call any of the methods defined in your DAO:

```kotlin
val groceryItemDao = db.groceryItemDao()
groceryItemDao.insert(groceryItem)
val groceryItems = groceryItemDao.getAll()
...
```

Note: since database access can take a long time, we want to run it in the background--otherwise the UI thread can be blocked, causing the app to freeze or just be generally slow. The Rooms library can be combined with [Coroutines ](/chapters/5.5-concurrency/7.2-coroutines.md)to help mitigate this issue!


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