# Transaction

Artifact: [androidx.room:room-common](https://developer.android.com/jetpack/androidx/releases/room) [View Source](https://cs.android.com/search?q=file:androidx/room/Transaction.kt+class:androidx.room.Transaction) Added in [2.0.0](https://developer.android.com/jetpack/androidx/releases/room#2.0.0)

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```
@Target(allowedTargets = [AnnotationTarget.FUNCTION])
@Retention(value = AnnotationRetention.BINARY)
public annotation Transaction
```

<br />

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Marks a method in a `https://developer.android.com/reference/androidx/room/Dao` class as a transaction method.

When used on a non-abstract method of an abstract `https://developer.android.com/reference/androidx/room/Dao` class, the derived implementation of the method will execute the super method in a database transaction. All the parameters and return types are preserved. The transaction will be marked as successful unless an exception is thrown in the method body.

Example:

```
@Dao
abstract class SongDao {
    @Insert
    abstract fun insert(song: Song)
    @Delete
    abstract fun delete(song: Song)
    @Transaction
    fun insertAndDeleteInTransaction(newSong: Song, oldSong: Song) {
        // Anything inside this method runs in a single transaction.
        insert(newSong)
        delete(oldSong)
    }
}
```

When used on a `https://developer.android.com/reference/androidx/room/Query` method that has a `SELECT` statement, the generated code for the `https://developer.android.com/reference/androidx/room/Query` will be run in a transaction. There are 2 main cases where you may want to do that:

- If the result of the query is fairly big, it is better to run it inside a transaction to receive a consistent result. Otherwise, if the query result does not fit into a single android.database.CursorWindow, the query result may be corrupted due to changes in the database in between cursor window swaps.

- If the result of the query is a POJO with {@link Relation} fields, these fields are queried separately. To receive consistent results between these queries, you also want to run them in a single transaction.

Example:

```
data class AlbumWithSongs : Album (
    @Relation(parentColumn = "albumId", entityColumn = "songId")
    val songs: List<Song>
)

@Dao
public interface AlbumDao {
    @Transaction
    @Query("SELECT * FROM album")
    fun loadAll(): List<AlbumWithSongs>
}
```

If the query is asynchronous (e.g. returns a androidx.lifecycle.LiveData or RxJava Flowable), the transaction is properly handled when the query is run, not when the method is called.

Putting this annotation on an `https://developer.android.com/reference/androidx/room/Insert`, `https://developer.android.com/reference/androidx/room/Update` or `https://developer.android.com/reference/androidx/room/Delete` method has no impact because those methods are always run inside a transaction. Similarly, if a method is annotated with `https://developer.android.com/reference/androidx/room/Query` but runs an INSERT, UPDATE or DELETE statement, it is automatically wrapped in a transaction and this annotation has no effect. Room will only perform at most one transaction at a time, additional transactions are queued and executed on a first come, first serve order.