What Is Flyway

Flyway is an open-source tool, licensed under Apache License 2.0 and a commercial license, that helps you implement automated and version-based database migrations. 1 It allows you to define the required update operations in an SQL script or as Java code. You can then run the migration from a command line client or automatically as part of your build process or integrated into your Java application. The good thing about this process is that Flyway detects the required update operations and executes them. So, you don’t need to know which SQL update statements need to be performed to update your current database. You and your co- workers just define the update operations to migrate the database from one version to the next. And Flyway detects the current version and performs the necessary update operations to get the database to the latest version. To be able to do that, Flyway uses a metadata table to document the current database version and all executed updates. By default, this table is called SCHEMA_VERSION. OK, enough theory. Let’s implement the first automated database migration with Flyway. 2 As described earlier, you can use SQL scripts or Java classes to define your database migration. In most cases, you should define all migration steps in an SQL script. But if you need to implement complex migrations, e.g., read information from a BLOB and store it in a new set of columns, you can do that in Java. Let’s stick to the most common approach and use the following SQL statements to create a small database. This is a script for a PostgreSQL database that creates a custom database function, a book table and Hibernate’s default sequence hibernate_sequence. Please be aware that SQL scripts are often database- specific. You, therefore, might need multiple SQL scripts, if you need to support multiple databases. I store these SQL statements in a file called V1__create_database. The file name follows Flyway’s default naming convention: The letter V followed by the version number, 2 underscores and a short description. So, this file contains the SQL statements for database version 1 and Flyway will store it with the description “create_database” in the SCHEMA_VERSION table. That’s all you need to do to define your first database migration. You now just have to 3 trigger it. 3 You can integrate Flyway into your application, or you can run it automatically as part of your build process or manually from the command line. In this video, I want to show you the command line client and the Java integration. The command line client is easy to use. You just need to download the latest version from https://flywaydb.org, I will add the link to it to the description and extract the archive to your local file system. After that, you should find the following files and folders on your hard drive. In this guide, you just need the 2 selected folders, conf and sql. 4 In its default configuration, Flyway processes all SQL files located in the sql folder. So, you should put you V1__create_database.sql file there. And the conf folder contains the flyway.conf configuration files. That file contains lots of comments which document all configuration parameters in great detail. So, I will only point you to the most important ones. These are: flyway.url, flyway.user and flyway.password The flyway.url parameter defines the JDBC URL which Flyway shall use to connect to the database. For most databases, Flyway will detect the required JDBC driver based on the flyway.url. So, you don’t need to provide it. The parameters flyway.user and flyway.password are optional. The command line client will prompt you for the user information, if you don’t provide them in the configuration file. 5 And here is the configuration that I use for this example. I tell Flyway to connect to my PostgreSQL database on localhost and to use the user postgres with the password postgres. OK, so let’s run Flyway and initialize the database to version 1. You can do that by calling the command line client with the migrate command. And you’re done. As you can see in the log output, Flyway found an empty database and applied the migration script for version 1. Running the migration process from the command line is OK if your operations team updates your application manually. But for all automated deployments or applications that your customers host themselves, you might prefer an integrated migration that updates the database at application startup. 6 Flyway is implemented in Java and extremely easy to integrate. You just have to add the flyway-core jar file to your project. If you’re using Maven, you can do that with the following dependency. If you’re using Spring Boot, that’s all you need to do. Spring Boot automatically integrates with Flyway and triggers the migration automatically. But doing that yourself for any other Java application is also not a big deal. You can easily trigger the Flyway database migration from your Java code. 7 You can do that by creating a new Flyway instance, configuring the datasource and calling the migrate() method. You can either call the dataSource method with a DataSource object or provide the JDBC connection, username and password as Strings. In general, I recommend to provide a DataSource object because you can easily get it from your connection pool. But in my simple test environment, I don’t use a connection pool and, therefore, provide the required information as Strings. OK, you’re almost done. The only thing that’s missing is the migration script. I’m reusing the SQL script from the previous example and copy it to the src/main/resources/db/migration folder. Similar to the command line client, Flyway’s API integration checks all files in that folder and uses them to update the database if necessary. That’s it! You implemented your first database migration with Flyway. For all future updates of your application, you now just need to create an SQL script and drop it to the sql folder of the command line client or the src/main/resources/db/migration directory of your Java project. 8.

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