This article describes key concepts of Flyway and how we can use this framework to continuously remodel our application's database schema reliably and easily. At the end, we'll present an example of managing an in-memory H2 database using a Maven Flyway plugin.
Flyway updates a database from one version to a next using migrations. We can write migrations either in SQL with database specific syntax or in Java for advanced database transformations.
Migrations can either be versioned or repeatable. The former has a unique version and is applied exactly once. The latter does not have a version. Instead, they are (re-)applied every time their checksum changes.
Within a single migration run, repeatable migrations are always applied last, after pending versioned migrations have been executed. Repeatable migrations are applied in order of their description. For a single migration, all statements are run within a single database transaction.
In this article, we mainly focus on how we may use the Maven plugin to perform database migrations.
2. Flyway Maven Plugin
To install a Flyway Maven plugin, add following plugin definition to your pom.xml:
<plugin> <groupId>org.flywaydb</groupId> <artifactId>flyway-maven-plugin</artifactId> <version>4.0.3</version> </plugin>
You can check the latest version of the plugin available on Maven repository.
This Maven plugin may be configured in four different ways. Please refer to the documentation to get a list of all configurable properties.
2.1. Plugin Configuration
We may configure the plugin directly by use of the <configuration> tag in the plugin definition of the pom.xml:
<plugin> <groupId>org.flywaydb</groupId> <artifactId>flyway-maven-plugin</artifactId> <version>4.0.3</version> <configuration> <user>databaseUser</user> <password>databasePassword</password> <schemas> <schema>schemaName</schema> </schemas> ... </configuration> </plugin>
2.2. Maven Properties
We may also configure the plugin by specifying configurable properties as Maven properties in pom.xml:
<project> ... <properties> <flyway.user>databaseUser</flyway.user> <flyway.password>databasePassword</flyway.password> <flyway.schemas>schemaName</flyway.schemas> ... </properties> ... </project>
2.3. External Configuration File
We may also provide plugin configuration in a separate .properties file:
flyway.user=databaseUser flyway.password=databasePassword flyway.schemas=schemaName ...
The default configuration file name is flyway.properties and it should reside in the same directory as the pom.xml file. Encoding is specified by flyway.encoding (Default is UTF-8).
If you are using any other name (e.g customConfig.properties) as the configuration file, then it should be specified explicitly when invoking the Maven command:
$ mvn -Dflyway.configFile=customConfig.properties
2.4. System Properties
Finally, all configuration properties may also be specified as system properties when invoking Maven on the command line:
$ mvn -Dflyway.user=databaseUser -Dflyway.password=databasePassword -Dflyway.schemas=schemaName
Following is an order of precedence when a configuration is specified in more than one way:
- System properties
- External configuration file
- Maven properties
- Plugin configuration
3. Example Migration
In this section, we walk through the required steps to migrate a database schema to an in-memory H2 database using the Maven plugin. We use an external file to configure Flyway.
3.1. Update POM
Add an appropriate database driver dependency for the H2 database in the pom.xml:
<dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <version>1.4.196</version> </dependency>
You can check the latest version of the driver available on Maven repository. Add Flyway plugin to pom.xml as explained in Section 2 above.
3.2. Configure Flyway Using External File
Create a file myFlywayConfig.properties in $PROJECT_ROOT with the following content:
flyway.user=databaseUser flyway.password=databasePassword flyway.schemas=app-db flyway.url=jdbc:h2:mem:DATABASE flyway.locations=filesystem:db/migration
The above configuration specifies that our migration scripts are located in the db/migration directory. It connects to an in-memory H2 instance using databaseUser and databasePassword.
The application database schema is app-db. Please replace flyway.user, flyway.password, flyway.url with your database username, database password and database host/port appropriately.
3.3. Define First Migration
Flyway adheres to the following naming convention for migration scripts:
- <Prefix> – Default prefix is V, which may be configured in the above configuration file using the flyway.sqlMigrationPrefix property.
- <Version> – Migration version number. Major and minor versions may be separated by an underscore. Migration version should always start with 1.
- <Description> – Textual description of the migration. The description needs to be separated from the version numbers with a double underscore.
Create a directory db/migration in $PROJECT_ROOT with a migration script named V1_0__create_employee_schema.sql containing SQL instructions to create e.g. an employee table:
CREATE TABLE IF NOT EXISTS `employee` ( `id` int NOT NULL AUTO_INCREMENT PRIMARY KEY, `name` varchar(20), `email` varchar(50), `date_of_birth` timestamp )ENGINE=InnoDB DEFAULT CHARSET=UTF8;
3.4. Execute Migrations
Invoke the following Maven command from $PROJECT_ROOT to execute database migrations:
$ mvn clean flyway:migrate -Dflyway.configFile=myFlywayConfig.properties
This should result in a first successful migration. The database schema may now be depicted as follows:
employee: +----+------+-------+---------------+ | id | name | email | date_of_birth | +----+------+-------+---------------+
Repeat the steps from subsections 3.3. and 3.4. to define and run new migrations at will.
3.5. Define And Execute Second Migration
Create a second migration file with name V2_0_create_department_schema.sql containing the following two queries:
CREATE TABLE IF NOT EXISTS `department` ( `id` int NOT NULL AUTO_INCREMENT PRIMARY KEY, `name` varchar(20) )ENGINE=InnoDB DEFAULT CHARSET=UTF8; ALTER TABLE `employee` ADD `dept_id` int AFTER `email`;
Execute a similar migration as was mentioned in section 3.4 above. The database schema looks like following after successfully executing the second migration.
employee: +----+------+-------+---------+---------------+ | id | name | email | dept_id | date_of_birth | +----+------+-------+---------+---------------+
department: +----+------+ | id | name | +----+------+
We may now verify that both migrations were indeed successful by invoking the following Maven command:
$ mvn flyway:info -Dflyway.configFile=myFlywayConfig.properties
4. How Flyway Works
To keep track of which migrations have already been applied, when and by whom, it adds a special bookkeeping table to your schema. This metadata table also tracks migration checksums and whether or not the migrations were successful.
The framework performs the following steps to accommodate evolving database schemas:
- It checks a database schema to locate its metadata table (SCHEMA_VERSION by default). If the metadata table does not exist, it will create one
- It scans an application classpath for available migrations
- It compares migrations against the metadata table. If a version number is lower or equal to a version marked as current, it is ignored
- It marks any remaining migrations as pending migrations. These are sorted based on version number and are executed in order
- As each migration is applied, the metadata table is updated accordingly
Flyway supports the following basic commands to manage database migrations.
- Info: Prints current status/version of a database schema. It prints which migrations are pending, which migrations have been applied, what is the status of applied migrations and when they were applied.
- Migrate: Migrates a database schema to the current version. It scans the classpath for available migrations and applies pending migrations.
- Baseline: Baselines an existing database, excluding all migrations, including baselineVersion. Baseline helps to start with Flyway in an existing database. Newer migrations can then be applied normally.
- Validate: Validates current database schema against available migrations.
- Repair: Repairs metadata table.
- Clean: Drops all objects in a configured schema. All database objects are dropped. Of course, you should never use clean on any production database.
In this article, we've shown how Flyway works and how we can use this framework to remodel our application database reliably.
The code accompanying this article is available on Github.