5 steps to Use Mongock for MongoDB Changelogs

Powerful Java Tool for Microservices & Distributed Environments NoSQL Data Migrations

Avatar Posted by Omar YAYA on February 04, 2022 · 10 mins read



If you find yourself often having to run scripts on your MongoDB to keep your data in sync, this article might be for you. We will talk about Mongock, a Java-based migration tool as part of your application code for Distributed environments (Source: Official Mongock Docs). I will go through how to set up & configure Mongock to run at startup with your SpringBoot app.

Prerequisites & Who This Article is For

I recommend having a Java Spring Boot app that connects to MongoDB up & running to get the best out of this tutorial.

This article is for you if:

  • Your app is built with Java, Spring, & Mongodb.
  • You want to migrate data in a distributed environment and/or microservices architecture.

What we will do

We will work on a demo application to migrate your users’ data to support new fields. Assume you had an application that had the following Users table structure:

    "_id": "ObjectID()"
    "username": "johndoe",
    "fullName": "John Doe",
    "dob" : "1990-01-01",

Now you want to add firstName and lastName fields to your object model for personalization. We will write a utility method to generate firstName and lastName based on the user’s fullName, so our object model will look like this:

class User {
    private Long id;
    private String username;
    private String fullName;
    private String firstName;
    private String lastName;
    private Date dob;

    // ...

For simplicity, we will ignore users who had multiple spaces in their name and would only focus on full names formatted as “FirstName LastName”. Of course, you’ll need to take into account the different formats for the data you want to migrate.

Step 1: Configurations

  • Go to your build.gradle file and add the following lines to include Mongock in your app:

Note: The latest Mongock version when writing this article was "5.0.32". Feel free to use later versions if this one seems outdated.

implementation "io.mongock:mongock:5.0.32"
implementation "io.mongock:mongock-springboot:5.0.32"
implementation "io.mongock:mongodb-springdata-v3-driver:5.0.32"
  • Go to your app’s main class (e.g., SpringApplication.java) and enable Mongock
import io.mongock.runner.springboot.EnableMongock;
// ... Your Imports

@EnableMongock  // <---- Add this line
public class SpringApplication implements CommandLineRunner {
    // Do your magic

Step 2: Create Changelogs Directory

Create a folder in your app directory to include the changelog classes (e.g., com.demo.app.changelogs)

Step 3: App Startup Scan

You need to tell your application to scan the directory created in Step 2 at startup. To do so, head to your application.yaml (or application.properties) file and add the following:


# mongock
  migration-scan-package: com.demo.app.changelogs 

Alternatively, application.properties

# mongock

Step 4: Migration Code

We will create a class called FirstLastNameChangelog.java in the directory we created in Step 2. This class will include the business logic necessary to fill the firstName & lastName fields in the User.java class. Here is our algorithm:

  1. Count the total number of users at the beginning of execution
  2. Fetch users in batches (e.g., limit 100)
  3. For each batch:
    1. For each user in the batch:
      1. Compute the firstName and lastName properties from the fullName property by using String.split()
      2. Set the fields in the User object.
      3. Persist the object to the database.
      4. Increment successful count or log errors/exceptions.
  4. Log the number of successful updates & compare it with the expected number calculated in step 1.
import io.mongock.api.annotations.ChangeUnit;
import io.mongock.api.annotations.Execution;
import io.mongock.api.annotations.RollbackExecution;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.data.mongodb.core.query.Query;
import org.springframework.data.mongodb.core.query.Update;
import org.springframework.util.ObjectUtils;

import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

import static org.springframework.data.mongodb.core.query.Criteria.where;

@ChangeUnit(id="FirstLastNameChangelog", order = "1", author = "omaryaya")
public class FirstLastNameChangelog {

    Logger logger = LoggerFactory.getLogger(FirstLastNameChangelog.class);
    AtomicInteger successfulUserUpdatesCounter = new AtomicInteger();

    public void setFirstAndLastNameToUsers(MongoTemplate mongoTemplate) {
        Query query = new Query(

        query.fields().include("_id", "fullName");

        long usersWithoutFirstAndLastName = mongoTemplate.count(query, User.class);
        query.limit(100); // set after counting all users to avoid always getting 100 as the maximum number of users

        List<User> users = mongoTemplate.find(query, User.class);
        while(users != null || users.getSize() != 0) {
            users.forEach(user -> {
                try {
                    Criteria criteria = where("_id").is(user.getId());
                    String[] names = splitNamesForUser(user);
                    String firstName = names[0], lastName = names[1];
                    Update update = new Update()
                                        .set("firstName", firstName)
                                        .set("lastName", lastName);
                    mongoTemplate.findAndModify(new Query(criteria), update, User.class);
                } catch (Exception ex) {
                    logger.error(String.format("Faield to set firstName & lastName for user with id %s", user.getId()), ex);

            users = mongoTemplate.find(query, User.class);

        logger.info("First and last names set for {} users out of {} total.", successfulUserUpdatesCounter,  usersWithoutFirstAndLastName);


    private String[] splitNamesForUser(User user) {
        if(user.getFullName() == null || user.getFullName().isEmpty() || !user.getFullName().contains(" ")) {
            throw new ParseNameException("Failed to parse the user's name");
        return user.getFullName().split(" ");

    public void rollback() {
        // Our change is backward-compatible; we don't need to implement a rollback mechanism.


Step 5: Validate

Congratulations, the changelog is done! When you run your application, Mongock will run at startup & will update your Users table. Check your database to ensure that the firstName and lastName properties have been filled, and check your application logs to see how many users have been successfully updated.

Please feel free to tweet me @OmarYayaa if you have any questions.