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sidebar_position: 4
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# Migrations

## Migration files

Migration files should look like this:

```ts
import { Kysely } from 'kysely'

export async function up(db: Kysely<any>): Promise<void> {
  // Migration code
}

export async function down(db: Kysely<any>): Promise<void> {
  // Migration code
}
```

The `up` function is called when you update your database schema to the next version and `down` when you go back to previous version. The only argument for the functions is an instance of `Kysely<any>`. It's important to use `Kysely<any>` and not `Kysely<YourDatabase>`.

Migrations should never depend on the current code of your app because they need to work even when the app changes. Migrations need to be "frozen in time".

Migrations can use the `Kysely.schema` module to modify the schema. Migrations can also run normal queries to read/modify data.

## Execution order

Migrations will be run in the alpha-numeric order of your migration names. An excellent way to name your migrations is to prefix them with an ISO 8601 date string.

By default, Kysely will also ensure this order matches the execution order of any previously executed migrations in your database. If the orders do not match (for example, a new migration was added alphabetically before a previously executed one), an error will be returned. This adds safety by always executing your migrations in the correct, alphanumeric order.

There is also an `allowUnorderedMigrations` option. This option will allow new migrations to be run even if they are added alphabetically before ones that have already executed. Allowing unordered migrations works well in large teams where multiple team members may add migrations at the same time in parallel commits without knowing about the other migrations. Pending (unexecuted) migrations will be run in alpha-numeric order when migrating up. When migrating down, migrations will be undone in the opposite order in which they were executed (reverse sorted by execution timestamp).

To allow unordered migrations, pass the `allowUnorderedMigrations` option to Migrator:

```ts
const migrator = new Migrator({
  db,
  provider: new FileMigrationProvider(...),
  allowUnorderedMigrations: true
})
```

## Single file vs multiple file migrations

You don't need to store your migrations as separate files if you don't want to. You can easily implement your own MigrationProvider and give it to the Migrator class when you instantiate one.

## PostgreSQL migration example

```ts
import { Kysely, sql } from 'kysely'

export async function up(db: Kysely<any>): Promise<void> {
  await db.schema
    .createTable('person')
    .addColumn('id', 'serial', (col) => col.primaryKey())
    .addColumn('first_name', 'varchar', (col) => col.notNull())
    .addColumn('last_name', 'varchar')
    .addColumn('gender', 'varchar(50)', (col) => col.notNull())
    .addColumn('created_at', 'timestamp', (col) =>
      col.defaultTo(sql`now()`).notNull(),
    )
    .execute()

  await db.schema
    .createTable('pet')
    .addColumn('id', 'serial', (col) => col.primaryKey())
    .addColumn('name', 'varchar', (col) => col.notNull().unique())
    .addColumn('owner_id', 'integer', (col) =>
      col.references('person.id').onDelete('cascade').notNull(),
    )
    .addColumn('species', 'varchar', (col) => col.notNull())
    .execute()

  await db.schema
    .createIndex('pet_owner_id_index')
    .on('pet')
    .column('owner_id')
    .execute()
}

export async function down(db: Kysely<any>): Promise<void> {
  await db.schema.dropTable('pet').execute()
  await db.schema.dropTable('person').execute()
}
```

## SQLite migration example

```ts
import { Kysely, sql } from 'kysely'

export async function up(db: Kysely<any>): Promise<void> {
  await db.schema
    .createTable('person')
    .addColumn('id', 'integer', (col) => col.primaryKey())
    .addColumn('first_name', 'text', (col) => col.notNull())
    .addColumn('last_name', 'text')
    .addColumn('gender', 'text', (col) => col.notNull())
    .addColumn('created_at', 'text', (col) =>
      col.defaultTo(sql`CURRENT_TIMESTAMP`).notNull(),
    )
    .execute()

  await db.schema
    .createTable('pet')
    .addColumn('id', 'integer', (col) => col.primaryKey())
    .addColumn('name', 'text', (col) => col.notNull().unique())
    .addColumn('owner_id', 'integer', (col) =>
      col.references('person.id').onDelete('cascade').notNull(),
    )
    .addColumn('species', 'text', (col) => col.notNull())
    .execute()

  await db.schema
    .createIndex('pet_owner_id_index')
    .on('pet')
    .column('owner_id')
    .execute()
}

export async function down(db: Kysely<any>): Promise<void> {
  await db.schema.dropTable('pet').execute()
  await db.schema.dropTable('person').execute()
}
```

## CLI (optional)

Kysely offers a CLI you can use for migrations (and more). It can help you create and run migrations.
It is not part of the core, and your mileage may vary.

For more information, visit https://github.com/kysely-org/kysely-ctl.

## Running migrations

You can then use:

```ts
const migrator = new Migrator(migratorConfig)
await migrator.migrateToLatest()
```

to run all migrations that have not yet been run. See the Migrator class's documentation for more info.

You will probably want to add a simple migration script to your projects like this:

```ts
import * as path from 'path'
import { Pool } from 'pg'
import { promises as fs } from 'fs'
import {
  Kysely,
  Migrator,
  PostgresDialect,
  FileMigrationProvider,
} from 'kysely'
import { Database } from './types'

async function migrateToLatest() {
  const db = new Kysely<Database>({
    dialect: new PostgresDialect({
      pool: new Pool({
        host: 'localhost',
        database: 'kysely_test',
      }),
    }),
  })

  const migrator = new Migrator({
    db,
    provider: new FileMigrationProvider({
      fs,
      path,
      // This needs to be an absolute path.
      migrationFolder: path.join(__dirname, 'some/path/to/migrations'),
    }),
  })

  const { error, results } = await migrator.migrateToLatest()

  results?.forEach((it) => {
    if (it.status === 'Success') {
      console.log(`migration "${it.migrationName}" was executed successfully`)
    } else if (it.status === 'Error') {
      console.error(`failed to execute migration "${it.migrationName}"`)
    }
  })

  if (error) {
    console.error('failed to migrate')
    console.error(error)
    process.exit(1)
  }

  await db.destroy()
}

migrateToLatest()
```

The migration methods use a lock on the database level and parallel calls are executed serially. This means that you can safely call migrateToLatest and other migration methods from multiple server instances simultaneously and the migrations are guaranteed to only be executed once. The locks are also automatically released if the migration process crashes or the connection to the database fails.

## Reference documentation

[Migrator](https://kysely-org.github.io/kysely-apidoc/classes/Migrator.html)
