Table of Contents

Schema Migration with Flyway

Learn how to use Flyway to manage your schema migrations

Authors: Sergio del Amo

Micronaut Version: 4.9.0

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

You use Flyway:

Flyway is an open-source database migration tool. It strongly favors simplicity and convention over configuration.

2. What you will need

To complete this guide, you will need the following:

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide \
   --features=data-jdbc,mysql,flyway \
   --build=gradle --lang=kotlin
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

If you use Micronaut Launch, select "Micronaut Application" as application type and add mysql, data-jdbc, and flyway as features.

Setup IntelliJ IDEA to develop Micronaut Applications.

3.1. Create Entity

Create a @MappedEntity to save persons. Initially, consider name and age required. Use int primitive for the age.

src/main/kotlin/example/micronaut/Person.kt
package example.micronaut;

import io.micronaut.data.annotation.GeneratedValue
import io.micronaut.data.annotation.Id
import io.micronaut.data.annotation.MappedEntity
import io.micronaut.data.annotation.Version

@MappedEntity (1)
class Person(
    val name: String,

    val age: Int

) {

    @Id (2)
    @GeneratedValue (3)
    var id: Long? = null

    @Version (4)
    var version: Long? = null
}
1 Annotate the class with @MappedEntity to map the class to the table defined in the schema.
2 Specifies the ID of an entity
3 Specifies that the property value is generated by the database and not included in inserts
4 Annotate the field with @Version to enable optimistic locking for your entity.

3.2. Database Migration with Flyway

We need a way to create the database schema. For that, we use Micronaut integration with Flyway.

Flyway automates schema changes, significantly simplifying schema management tasks, such as migrating, rolling back, and reproducing in multiple environments.

Add the following snippet to include the necessary dependencies:

build.gradle
implementation("io.micronaut.flyway:micronaut-flyway")

We will enable Flyway in the Micronaut configuration file and configure it to perform migrations on one of the defined data sources.

src/main/resources/application.properties
(1)
flyway.datasources.default.enabled=true
1 Enable Flyway for the default datasource.
Configuring multiple data sources is as simple as enabling Flyway for each one. You can also specify directories that will be used for migrating each data source. Review the Micronaut Flyway documentation for additional details.

Flyway migration will be automatically triggered before your Micronaut application starts. Flyway will read migration commands in the resources/db/migration/ directory, execute them if necessary, and verify that the configured data source is consistent with them.

Create the following migration files with the database schema creation:

src/main/resources/db/migration/V1__create-person.sql
CREATE TABLE person(
    id   bigint primary key not null,
    name varchar(255)       not null,
    age  int                not null
)

During application startup, Flyway executes the SQL file and creates the schema needed for the application.

If you check the database schema, there are two tables:

  • person

  • flyway_scheme_history

Flyway uses the table flyway_scheme_history to keep track of database migrations.

The person table looks like:

Column Nullable

id

NO

version

NO

name

NO

age

NO

3.3. Drop Not Null Constraint

Applications change. Make age optional:

src/main/kotlin/example/micronaut/Person.kt
    val age: Int?

Add a new migration to drop the null constraint:

src/main/resources/db/migration/V2__nullable-age.sql
ALTER TABLE person MODIFY age int default null;

After the migration, the person table looks like:

Column Nullable

id

NO

version

NO

name

NO

age

YES

4. Flyway endpoint

To enable the Flyway endpoint, add the management dependency on your classpath.

build.gradle
implementation("io.micronaut:micronaut-management")

Enable the Flyway endpoint:

src/main/resources/application.properties
endpoints.flyway.enabled=true
endpoints.flyway.sensitive=false

4.1. Test

Create a test that invokes the Flyway endpoint.

src/test/kotlin/example/micronaut/FlywayEndpointTest.kt
package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpStatus.OK
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import jakarta.inject.Inject
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertNotNull
import org.junit.jupiter.api.Test

@MicronautTest (1)
class FlywayEndpointTest(@Client("/") val httpClient: HttpClient) { (2)

    @Test
    fun migrationsAreExposedViaAndEndpoint() {
        val client = httpClient.toBlocking()

        val response = client.exchange(HttpRequest.GET<Any>("/flyway"), Argument.listOf(FlywayReport::class.java))
        assertEquals(OK, response.status())

        val flywayReports = response.body()
        assertNotNull(flywayReports)
        assertEquals(1, flywayReports!!.size)

        val flywayReport = flywayReports[0]
        assertNotNull(flywayReport)
        assertNotNull(flywayReport.migrations)
        assertEquals(2, flywayReport.migrations!!.size)
    }

    internal class FlywayReport {
        var migrations: List<Migration>? = null
    }

    internal class Migration {
        var script: String? = null
            private set

        fun setId(script: String?) {
            this.script = script
        }
    }
}
1 Annotate the class with @MicronautTest so the Micronaut framework will initialize the application context and the embedded server. More info.
2 Inject the HttpClient bean and point it to the embedded server.

5. Test Resources

When the application is started locally — either under test or by running the application — resolution of the datasource URL is detected and the Test Resources service will start a local MySQL docker container, and inject the properties required to use this as the datasource.

For more information, see the JDBC section or R2DBC section of the Test Resources documentation.

6. Testing the Application

To run the tests:

./gradlew test

Then open build/reports/tests/test/index.html in a browser to see the results.

7. Running the Application

To run the application, use the ./gradlew run command, which starts the application on port 8080.

You can run a cURL command to test the application:

curl http://localhost:8080/flyway

You will see information about migrations.

8. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, an advanced JDK with ahead-of-time Native Image compilation, to generate a native executable of this Micronaut application.

Compiling Micronaut applications ahead of time with GraalVM significantly improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

8.1. GraalVM Installation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 21
sdk install java 21.0.5-graal

For installation on Windows, or for a manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.

Alternatively, you can use the GraalVM Community Edition:

Java 21
sdk install java 21.0.2-graalce

8.2. Native Executable Generation

To generate a native executable using Gradle, run:

./gradlew nativeCompile

The native executable is created in build/native/nativeCompile directory and can be run with build/native/nativeCompile/micronautguide.

It is possible to customize the name of the native executable or pass additional parameters to GraalVM:

build.gradle
graalvmNative {
    binaries {
        main {
            imageName.set('mn-graalvm-application') (1)
            buildArgs.add('-Ob') (2)
        }
    }
}
1 The native executable name will now be mn-graalvm-application
2 It is possible to pass extra build arguments to native-image. For example, -Ob enables the quick build mode.

You can run a cURL command to test the application:

curl http://localhost:8080/flyway

You will see information about migrations.

9. Next Steps

Explore more features with Micronaut Guides.

Check Micronaut Flyway integration.

Learn more about Flyway.

10. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.

11. License

All guides are released with an Apache license 2.0 license for the code and a Creative Commons Attribution 4.0 license for the writing and media (images…​).