Dropwizard Migrations

Adding Bundle

To add a new migration bundle, you must add the following code in the 'initialize' method.

public class Configure extends Configuration {
    @Valid
    @NotNull
    private DataSourceFactory db = new DataSourceFactory();

    @JsonProperty("database")
    public DataSourceFactory getDB() {
        return db;
    }
}

Explanation - 

• Override the 'initialize' method.
• Use the 'addBundle' method to add the migration bundle. Pass the object of the MigrationBundle class(pre-defined class) to it.
• Override the 'getDB()' method and return the DataSourceFactory object.

To make large data migration, register and retrieve ‘SessionFactory’ in the ‘CustomChange’ method. 

@Override
public Map<String, Object> getObjects() 
{
    Map<String, Object> m = new HashMap<>();
    m.put("Example session", hibernateBundle.getSessionFactory());
    return m;
}

public void execute(Database db) throws CustomChangeException {
    Scope.getCurrentScope().get("Example session", SessionFactory.class);
}

GetObjects() method stores the session name and session factory in a map. The 'getSessionFactory()' method returns the session factory. W can then return the map.

Invoke the ‘execute’ method for retrieving the SessionFactory.

Defining Migrations

The ‘migration.xml’ file contains the dropwizard migration of your project. For example, if you want to create a 'student' table, your migration.xml will look like this. 

<?xml version="1.0" encoding="UTF-8"?>

<databaseChangeLog
        xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
         http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.1.xsd">

    <changeSet id="1" author="coding_ninjas">
        <createTable tableName="student">
            <column name="RollNumber" type="int" autoIncrement="true">
            <constraints primaryKey="true" nullable="false"/>
            </column>

            <column name="StudentName" type="varchar(255)">
            <constraints nullable="false"/>
            </column>

            <column name="Course" type="varchar(255)">
            <constraints nullable="false"/>
            </column>
        </createTable>
    </changeSet>
</databaseChangeLog>
?>

This .xml file contains a student table having three-row. All rows must be non-empty, so set the nullable constraint to false. 

Working on the Database

Dropwizard migration module helps in retracting liquibase databases. Various operations performed are: 

Checking Database’s State 

To check the database state, the ‘db status’ command. 

java -jar <jar-file>.jar db status <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

JAR files are Java Archive files packed in zip format. It is a collection of many java files and their associated resources and metadata. 

There are various database states mentioned below:

Dumping the Schema 

Use the 'db dump' command to dump an existing schema in your database.

java -jar <jar-file>.jar db dump <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

This command will produce a Liquibase change log. Using this log you can recreate your database. 

Tagging the Schema 

To put a tag in your schema at a particular point, you can use the ‘db tag’ command. This makes the rollback operation easy. 

java -jar  <jar-file>.jar db tag <db-name>.yml <date> - <previous_move>

Here replace 

• <jar-file> with your jar file name. 
• <db-name> with your database name. 
• <date> with the date you want to put a tag. The date should be in YYYY-MM-DD format. 
• <previous_move> with the last user move. 

Migrating the Schema 

Migrating the schema means applying the pending change to your database. This can be done using the ‘db migrate’ command. 

java -jar <jar-file>.jar db migrate <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

The changes done will be irreversible. 

Rollback the Schema

Use the 'db rollback' command to roll back the already applied changes. For this, you need to mention the date, tag, or the number of changes to roll back. 

java -jar <jar-file>.jar db migrate <db-name>.yml –<tag> <date>-<previous_move>

Here replace 

• <jar-file> with your jar file name.
• <db-name> with your database name.
• <tag> with a created tag.
• <date> with the date you want to put a tag. The date should be in YYYY-MM-DD format. 
• <previous_move> with the last user move. 

The changes done will be irreversible. 

Testing Migration 

The ‘db test’ command is used to check if a set of pending changes can be rolled back fully or not.

java -jar <jar-file>.jar db test <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

Preparing Rollback Script

To create a rollback script of the pending change, use the 'db prepare-rollback' command. 

java -jar <jar-file>.jar db prepare-rollback <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

This will result in a DDL(Data Definition Language) script of all unapplied changes. 

Dumping the objects

To drop all the database objects, call  The 'db drop-all' command. Along with this command, you need to specify '--confirm-delete-everything'. As this command will delete every object so, confirmation is necessary. 

java -jar <jar-file>.jar db drop-all --confirm-delete-everything <db-name>.yml

Here replace <jar-file> with your jar file name and <db-name> with your database name. 

Support For Adding Multiple Migration Bundles

We need two different data source factories to perform migration on two different databases. To do so, follow the below-mentioned steps.

Create two different data source factories. 

public class Configure extends Configuration {
    @Valid
    @NotNull
    private DataSourceFactory source1 = new DataSourceFactory();

    @Valid
    @NotNull
    private DataSourceFactory source2 = new DataSourceFactory();

    @JsonProperty("database source 1")
    public DataSourceFactory getSource1() {
        return source1;
    }

    @JsonProperty("database source 2")
    public DataSourceFactory getSource2() {
        return source2;
    }
}

Give unique names to both migration bundles.

@Override
public void initialize(Bootstrap<Configure> bootstrap) {
    bootstrap.addBundle(new MigrationsBundle<Configure>() {
        @Override
        public DataSourceFactory getDB(Configure c) {
            return c.getSource1();
        }

        @Override
        public String name() {
            // Return name of 1st migration bundle
            return "Database1";
        }
    });

    bootstrap.addBundle(new MigrationsBundle<Configure>() {
        @Override
        public DataSourceFactory getDB(Configure c) {
            return c.getSource2();
        }

        @Override
        public String name() {
            // Return name of 2nd migration bundle
            return "Database2";
        }
    });
}

Migrate the schema using the ‘db migrate’ command. 

java -jar index.jar Database1 migrate index.yml
java -jar index.jar Database2 migrate index.yml

This was all about Dropwizard Migration. Let's now discuss some of the frequently asked questions related to our topic. 

Frequently Asked Questions

What is Dropwizard? 

Dropwizard is an open-source framework of Java. It is used for the fast development of high-performance RESTful web services. 

What are the different database states?

There are a total of seven states in the database. These are: online, offline, recovery, restore, suspect, emergency, and recovery pending.

Name the libraries gathered in Dropwizard.

Dropwizard gathers many popular libraries. These are Jersey, Jackson, Jetty, JUnit, Guava, etc,. These libraries enable the creation of lightweight packages. Furthermore, it uses its own library called Metrics.

What is a liquibase database?

Liquibase is an open-source platform using which you can create changes in the database. It supports various databases and file formats for defining the database structure. 

What is bootstrap in Dropwizard?

The bootstrap is a pre-start (temporary) application environment. It contains all of the components needed to bootstrap a Dropwizard command.

Conclusion 

This article contains basic information about the Dropwizard Migration module. In this article, we have discussed how to define and test migration. Along with the discussion of working on the database using the dropwizard migration module. The reader can carry out a thorough understanding of the topic by referring to the Official Documentation. For more information, Refer-

• Dropwizard JDBI3
• Dropwizard Forms
• Dropwizard Example
   

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