---
id: versioned-migrations
title: Versioned Migrations
---

If you are using the Atlas migration engine you are able to use the versioned migrations feature of it. Instead of
applying the computed changes directly to the database, it will generate a set of migration files containing the
necessary SQL statements to migrate the database. These files can then be edited to your needs and be applied by any
tool you like (like golang-migrate, Flyway, liquibase).

## Generating Versioned Migration Files

As mentioned above, versioned migrations do only work, if the new Atlas based migration engine is used. Migration files
are generated by computing the difference between two **states**. We call the state reflected by your Ent schema the **
desired** state, and the last state before your most recent changes the **current** state. There are two ways for Ent to
determine the current state:

1. Replay the existing migration directory and inspect the schema (default)
2. Connect to an existing database and inspect the schema

We emphasize to use the first option, as it has the advantage for you to not have to connect to a production database to
create a diff. In addition, this approach also works, if you have multiple deployments in different migration states.
The first step is to enable the versioned migration feature by passing in the `sql/versioned-migration` feature flag.
Depending on how you execute the Ent code generator, you have to use one of the two options:

![atlas-versioned-migration-process](https://entgo.io/images/assets/migrate-atlas-replay.png)

#### With Ent CLI

If you are using the default go generate configuration, simply add the `--feature sql/versioned-migration` to
the `ent/generate.go` file as follows:

```go
package ent

//go:generate go run -mod=mod entgo.io/ent/cmd/ent generate --feature sql/versioned-migration ./schema
```

#### With entc package

If you are using the code generation package (e.g. if you are using an Ent extension), add the feature flag as
follows:

```go
//go:build ignore

package main

import (
	"log"

	"entgo.io/ent/entc"
	"entgo.io/ent/entc/gen"
)

func main() {
	err := entc.Generate("./schema", &gen.Config{
		Features: []gen.Feature{gen.FeatureVersionedMigration},
	})
	if err != nil {
		log.Fatalf("running ent codegen: %v", err)
	}
}
```

After regenerating the project, you have two ways to generate new migration files. First, there will be a new top-level 
method in the generated `migrate` package: `NamedDiff`. All you have to do is provide an Atlas 
[dev database](https://atlasgo.io/dev-database) URL for the desired database dialect.

```go
package main

import (
	"context"
	"log"

	"<project>/ent/migrate"

	"ariga.io/atlas/sql/sqltool"
	"entgo.io/ent/dialect"
	"entgo.io/ent/dialect/sql/schema"
	_ "github.com/go-sql-driver/mysql"
)

func main() {
	ctx := context.Background()
	// Create a local migration directory able to understand golang-migrate migration files for replay.
	dir, err := sqltool.NewGolangMigrateDir("migrations")
	if err != nil {
		log.Fatalf("failed creating atlas migration directory: %v", err)
	}
	// Write migration diff.
	opts := []schema.MigrateOption{
		schema.WithDir(dir),                         // provide migration directory
		schema.WithMigrationMode(schema.ModeReplay), // provide migration mode
		schema.WithDialect(dialect.MySQL),           // Ent dialect to use
	}
	// Generate migrations using Atlas support for TiDB (note the Ent dialect option passed above).
	err = migrate.NamedDiff(ctx, "tidb://user:pass@localhost:3306/ent_dev", "my_migration", opts...)
	if err != nil {
		log.Fatalf("failed generating migration file: %v", err)
	}
}
```

You can then generate a new set of migration files by simply calling `go run -mod=mod main.go`.

:::info Note
If you want to inspect an existing database and compute the diff against your Ent schema, pass in the `ModeInspect` 
migration mode.
:::

### A Word on Global Unique IDs

**This section only applies to MySQL users using the [global unique id](migrate.md/#universal-ids) feature.**

When using the global unique ids, Ent allocates a range of `1<<32` integer values for each table. This is done by giving
the first table an autoincrement starting value of `1`, the second one the starting value `4294967296`, the third one
`8589934592`, and so on. The order in which the tables receive the starting value is saved in an extra table
called `ent_types`. With MySQL 5.6 and 5.7, the autoincrement starting value is only saved in
memory ([docs](https://dev.mysql.com/doc/refman/8.0/en/innodb-auto-increment-handling.html), **InnoDB AUTO_INCREMENT
Counter Initialization** header) and re-calculated on startup by looking at the last inserted id for any table. Now, if
you happen to have a table with no rows yet, the autoincrement starting value is set to 0 for every table without any
entries. With the online migration feature this wasn't an issue, because the migration engine looked at the `ent_types`
tables and made sure to update the counter, if it wasn't set correctly. However, with versioned migration, this is no
longer the case. In oder to ensure, that everything is set up correctly after a server restart, make sure to call
the `VerifyTableRange` method on the Atlas struct:

```go
package main

import (
	"context"
	"log"

	"<project>/ent"
	"<project>/ent/migrate"
	"entgo.io/ent/dialect/sql"
	"entgo.io/ent/dialect/sql/schema"

	_ "github.com/go-sql-driver/mysql"
)

func main() {
	drv, err := sql.Open("mysql", "user:pass@tcp(localhost:3306)/ent")
	if err != nil {
		log.Fatalf("failed opening connection to mysql: %v", err)
	}
	defer drv.Close()
	// Verify the type allocation range.
	m, err := schema.NewMigrate(drv, nil)
	if err != nil {
		log.Fatalf("failed creating migrate: %v", err)
	}
	if err := m.VerifyTableRange(context.Background(), migrate.Tables); err != nil {
		log.Fatalf("failed verifyint range allocations: %v", err)
	}
	client := ent.NewClient(ent.Driver(drv))
	// ... do stuff with the client
}
```

:::caution Important
After an upgrade to MySQL 8 from a previous version, you still have to run the method once to update the starting 
values. Since MySQL 8 the counter is no longer only stored in memory, meaning subsequent calls to the method are no 
longer needed after the first one.
:::

## Apply Migrations

The Atlas migration engine does not support applying the migration files onto a database yet, therefore to manage and
execute the generated migration files, you have to rely on an external tool (or execute them by hand). By default, Atlas
generates one "up" and one "down" migration file for the computed diff. These files are compatible with the popular
[golang-migrate/migrate](https://github.com/golang-migrate/migrate) package, and you can use that tool to manage the
migrations in your deployments.

```shell
migrate -source file://migrations -database mysql://root:pass@tcp(localhost:3306)/test up
```

## Moving from Auto-Migration to Versioned Migrations

In case you already have an Ent application in production and want to switch over from auto migration to the new
versioned migration, you need to take some extra steps.

### Create an initial migration file reflecting the currently deployed state

To do this make sure your schema definition is in sync with your deployed version(s). Then spin up an empty database and
run the diff command once as described above. This will create the statements needed to create the current state of
your schema graph. If you happened to have [universal IDs](migrate.md#universal-ids) enabled before, any deployment will 
have a special database table named `ent_types`. The above command will create the necessary SQL statements to create 
that table as well as its contents (similar to the following):

```sql
CREATE TABLE `users` (`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT);
CREATE TABLE `groups` (`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT);
INSERT INTO sqlite_sequence (name, seq) VALUES ("groups", 4294967296);
CREATE TABLE `ent_types` (`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT, `type` text NOT NULL);
CREATE UNIQUE INDEX `ent_types_type_key` ON `ent_types` (`type`);
INSERT INTO `ent_types` (`type`) VALUES ('users'), ('groups');
```

In order to ensure to not break existing code, make sure the contents of that file are equal to the contents in the
table present in the database you created the diff from. For example, if you consider the migration file from
above (`users,groups`) but your deployed table looks like the one below (`groups,users`):

| id  | type   |
|-----|--------|
| 1   | groups |
| 2   | users  |

You can see, that the order differs. In that case, you have to manually change both the entries in the generated
migration file.

### Configure the tool you use to manage migrations to consider this file as applied

In case of `golang-migrate` this can be done by forcing your database version as
described [here](https://github.com/golang-migrate/migrate/blob/master/GETTING_STARTED.md#forcing-your-database-version)
.

## Use a Custom Formatter

Atlas' migration engine comes with great customizability. By the use of a custom `Formatter` you can generate the
migration files in a format compatible with other migration management tools and Atlas has built-in support for the
following four:

1. [golang-migrate/migrate](https://github.com/golang-migrate/migrate)
2. [pressly/goose](https://github.com/pressly/goose)
3. [Flyway](https://flywaydb.org/)
4. [Liquibase](https://www.liquibase.org/)

Please be aware, that migration directory replay us currently only support for `golang-migrate/migrate` formatted files.
Attempting to replay migration files of other tools might work, but is not officially supported (yet).

:::note
You need to have the latest master of Ent installed for this to be working.

```shell
go get -u entgo.io/ent@master
```

:::

```go
package main

import (
	"context"
	"log"

	"<project>/ent/migrate"

	atlas "ariga.io/atlas/sql/migrate"
	"ariga.io/atlas/sql/sqltool"
	"entgo.io/ent/dialect"
	"entgo.io/ent/dialect/sql/schema"
	_ "github.com/go-sql-driver/mysql"
)

func main() {
	ctx := context.Background()
	// Create a local migration directory.
	dir, err := atlas.NewLocalDir("migrations")
	if err != nil {
		log.Fatalf("failed creating atlas migration directory: %v", err)
	}
	// Write migration diff.
	opts := []schema.MigrateOption{
		schema.WithDir(dir),                         // provide migration directory
		schema.WithMigrationMode(schema.ModeReplay), // provide migration mode
		schema.WithDialect(dialect.MySQL),           // Ent dialect to use
		// highlight-start
		// Choose one of the below.
		schema.WithFormatter(sqltool.GolangMigrateFormatter),
		schema.WithFormatter(sqltool.GooseFormatter),
		schema.WithFormatter(sqltool.FlywayFormatter),
		schema.WithFormatter(sqltool.LiquibaseFormatter),
		// highlight-end
	}
	// Generate migrations using Atlas support for TiDB (note the Ent dialect option passed above).
	err = migrate.NamedDiff(ctx, "tidb://user:pass@localhost:3306/ent_dev", "my_migration", opts...)
	if err != nil {
		log.Fatalf("failed generating migration file: %v", err)
	}
}
```

### Note for using golang-migrate

If you use golang-migrate with MySQL, you need to add the `multiStatements` parameter to `true` as in the example below
and then take the DSN we used in the documents with the param applied.

```
"user:password@tcp(host:port)/dbname?multiStatements=true"
```

## Atlas migration directory integrity file

### The Problem

Suppose you have multiple teams develop a feature in parallel and both of them need a migration. If Team A and Team B do
not check in with each other, they might end up with a broken set of migration files (like adding the same table or
column twice) since new files do not raise a merge conflict in a version control system like git. The following example
demonstrates such behavior:

![atlas-versioned-migrations-no-conflict](https://entgo.io/images/assets/migrate/no-conflict.svg)

Assume both Team A and Team B add a new schema called User and generate a versioned migration file on their respective
branch.

```sql title="20220318104614_team_A.sql"
-- create "users" table
CREATE TABLE `users` (
    `id` bigint NOT NULL AUTO_INCREMENT,
    // highlight-start
    `team_a_col` INTEGER NOT NULL,
    // highlight-end
    PRIMARY KEY (`id`)
) CHARSET utf8mb4 COLLATE utf8mb4_bin;
```

```sql title="20220318104615_team_B.sql"
-- create "users" table
CREATE TABLE `users` (
    `id` bigint NOT NULL AUTO_INCREMENT,
    // highlight-start
     `team_b_col` INTEGER NOT NULL,
    // highlight-end
     PRIMARY KEY (`id`)
) CHARSET utf8mb4 COLLATE utf8mb4_bin;
```

If they both merge their branch into master, git will not raise a conflict and everything seems fine. But attempting to
apply the pending migrations will result in migration failure:

```shell
mysql> CREATE TABLE `users` (`id` bigint NOT NULL AUTO_INCREMENT, `team_a_col` INTEGER NOT NULL, PRIMARY KEY (`id`)) CHARSET utf8mb4 COLLATE utf8mb4_bin;
[2022-04-14 10:00:38] completed in 31 ms

mysql> CREATE TABLE `users` (`id` bigint NOT NULL AUTO_INCREMENT, `team_b_col` INTEGER NOT NULL, PRIMARY KEY (`id`)) CHARSET utf8mb4 COLLATE utf8mb4_bin;
[2022-04-14 10:00:48] [42S01][1050] Table 'users' already exists
```

Depending on the SQL this can potentially leave your database in a crippled state.

### The Solution

Luckily, the Atlas migration engine offers a way to prevent concurrent creation of new migration files and guard against
accidental changes in the migration history we call **Migration Directory Integrity File**, which simply is another file
in your migration directory called `atlas.sum`. For the migration directory of team A it would look similar to this:

```text
h1:KRFsSi68ZOarsQAJZ1mfSiMSkIOZlMq4RzyF//Pwf8A=
20220318104614_team_A.sql h1:EGknG5Y6GQYrc4W8e/r3S61Aqx2p+NmQyVz/2m8ZNwA=

```

The `atlas.sum` file contains the checksum of each migration file (implemented by a reverse, one branch merkle hash
tree), and a sum of all files. Adding new files results in a change to the sum file, which will raise merge conflicts in
most version controls systems. Let's see how we can use the **Migration Directory Integrity File** to detect the case
from above automatically.

:::note
Please note, that you need to have the Atlas CLI installed in your system for this to work, so make sure to follow
the [installation instructions](https://atlasgo.io/cli/getting-started/setting-up#install-the-cli) before proceeding.
:::

In previous versions of Ent, the integrity file was opt-in. But we think this is a very important feature that provides
great value and safety to migrations. Therefore, generation of the sum file is now the default behavior and in the 
future we might even remove the option to disable this feature. For now, if you really want to remove integrity file 
generation, use the `schema.DisableChecksum()` option.

In addition to the usual `.sql` migration files the migration directory will contain the `atlas.sum` file. Every time
you let Ent generate a new migration file, this file is updated for you. However, every manual change made to the
mitration directory will render the migration directory and the `atlas.sum` file out-of-sync. With the Atlas CLI you can
both check if the file and migration directory are in-sync, and fix it if not:

```shell
# If there is no output, the migration directory is in-sync.
atlas migrate validate --dir file://<path-to-your-migration-directory>
```

```shell
# If the migration directory and sum file are out-of-sync the Atlas CLI will tell you.
atlas migrate validate --dir file://<path-to-your-migration-directory>
Error: checksum mismatch

You have a checksum error in your migration directory.
This happens if you manually create or edit a migration file.
Please check your migration files and run

'atlas migrate hash --force'

to re-hash the contents and resolve the error.

exit status 1
```

If you are sure, that the contents in your migration files are correct, you can re-compute the hashes in the `atlas.sum`
file:

```shell
# Recompute the sum file.
atlas migrate hash --dir file://<path-to-your-migration-directory> --force
```

Back to the problem above, if team A would land their changes on master first and team B would now attempt to land
theirs, they'd get a merge conflict, as you can see in the example below:

![atlas-versioned-migrations-no-conflict](https://entgo.io/images/assets/migrate/conflict.svg)

You can add the `atlas migrate validate` call to your CI to have the migration directory checked continuously. Even if
any team member would now forget to update the `atlas.sum` file after a manual edit, the CI would not go green,
indicating a problem.