Transactions

• Creating and using transactions

  • Specifying Isolation Levels

• Using QueryRunner to create and control state of single database connection

Creating and using transactions

Transactions are created using DataSource or EntityManager. Examples:

await myDataSource.transaction(async (transactionalEntityManager) => {
    // execute queries using transactionalEntityManager
})

or

await myDataSource.manager.transaction(async (transactionalEntityManager) => {
    // execute queries using transactionalEntityManager
})

Everything you want to run in a transaction must be executed in a callback:

await myDataSource.manager.transaction(async (transactionalEntityManager) => {
    await transactionalEntityManager.save(users)
    await transactionalEntityManager.save(photos)
    // ...
})

The most important restriction when working in a transaction is to ALWAYS use the provided instance of entity manager - transactionalEntityManager in this example. DO NOT USE GLOBAL ENTITY MANAGER. All operations MUST be executed using the provided transactional entity manager.

Specifying Isolation Levels

Specifying the isolation level for the transaction can be done by supplying it as the first parameter:

await myDataSource.manager.transaction(
    "SERIALIZABLE",
    (transactionalEntityManager) => {},
)

Isolation level implementations are not agnostic across all databases.

The following database drivers support the standard isolation levels (READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE):

• MySQL

• Postgres

• SQL Server

SQLite defaults transactions to SERIALIZABLE, but if shared cache mode is enabled, a transaction can use the READ UNCOMMITTED isolation level.

Oracle only supports the READ COMMITTED and SERIALIZABLE isolation levels.

Using QueryRunner to create and control state of single database connection

QueryRunner provides a single database connection. Transactions are organized using query runners. Single transactions can only be established on a single query runner. You can manually create a query runner instance and use it to manually control transaction state. Example:

// create a new query runner
const queryRunner = dataSource.createQueryRunner()

// establish real database connection using our new query runner
await queryRunner.connect()

// now we can execute any queries on a query runner, for example:
await queryRunner.query("SELECT * FROM users")

// we can also access entity manager that works with connection created by a query runner:
const users = await queryRunner.manager.find(User)

// lets now open a new transaction:
await queryRunner.startTransaction()

try {
    // execute some operations on this transaction:
    await queryRunner.manager.save(user1)
    await queryRunner.manager.save(user2)
    await queryRunner.manager.save(photos)

    // commit transaction now:
    await queryRunner.commitTransaction()
} catch (err) {
    // since we have errors let's rollback changes we made
    await queryRunner.rollbackTransaction()
} finally {
    // you need to release query runner which is manually created:
    await queryRunner.release()
}

There are 3 methods to control transactions in QueryRunner:

• startTransaction - starts a new transaction inside the query runner instance.

• commitTransaction - commits all changes made using the query runner instance.

• rollbackTransaction - rolls all changes made using the query runner instance back.

Learn more about Query Runner.