MongoDB Reactive driver

# Introduction

This sections covers the Mongock implementation for MongoDB Java Reactive Streams driver.

If you are curious about why mongock provides the mongodb-reactive-driver, if it's synchronous by definition, please take a look to this faq entry

# Important note

Despite this deriver, the Mongock's nature is still synchronous: A migration consists in a list of changes, which are executed in order, one after the other. If one of them fails, the migration aborts and Mongock will take it from that point in the next execution.

Developers must block all the database calls inside a changeUnit

This is a bit cumbersome, as it requires creating a blocking subscriber that allows the developer to block the call.

Luckily, Mongock provides such a util class: MongoSubscriberSync. You can see an example here

# MongoDB driver options and compatibility

# MongoDB common configuration

All the MongoDB drivers share the same configuration.

When setting configuration via properties file, it must be prefixed by mongock.mongo-db

# Properties

# Get started

Following the get started section, this covers steps 3 and 5 and 6.

# - Add maven dependency for the driver (step 2)

<dependency>
  <groupId>io.mongock</groupId>
  <artifactId>mongodb-reactive-driver</artifactId>
</dependency>

# - Build the driver (step 5)

These classes provide the same two static initializers

• withDefaultLock(mongoClient, databaseName)
• withLockStrategy(mongoClient, databaseName, lockAcquiredForMillis, lockQuitTryingAfterMillis, lockTryFrequencyMillis)

// For mongodb-sync-v4-driver
MongoReactiveDriver driver = MongoReactiveDriver.withDefaultLock(mongoClient, databaseName);
// For mongodb-v3-driver
//MongoCore3Driver driver = MongoCore3Driver.withDefaultLock(mongoClient, databaseName);
driver.setWriteConcern(WriteConcern.MAJORITY.withJournal(true).withWTimeout(1000, TimeUnit.MILLISECONDS));
driver.setReadConcern(ReadConcern.MAJORITY);
driver.setReadPreference(ReadPreference.primary());

# - Driver extra configuration (step 6)

# Transactions

Due to the MongoDB API design, to work with transactions the ClientSession object is required in every MongoDB driver operation.

Mongock makes this very simple. The developer only needs to specify a ClientSession parameter in the contructor or method of the @ChangeUnit and use it in the MongoDB operations. Mongock takes care of everything else.

The following code shows how to save documents inside the transaction using the ClientSession object.

    @Execution
    public void execution(ClientSession clientSession, MongoDatabase mongoDatabase) {
        SubscriberSync<InsertManyResult> subscriber = new MongoSubscriberSync<>();
        mongoDatabase.getCollection(CLIENTS_COLLECTION_NAME, Client.class)
                .insertMany(clientSession, IntStream.range(0, INITIAL_CLIENTS)
                        .mapToObj(ClientInitializerChange::getClient)
                        .collect(Collectors.toList()))
                .subscribe(subscriber);
        InsertManyResult result = subscriber.getFirst();
    }