MongoDB Transactions

@Service
@RequiredArgsConstructor
public class PaymentService {

    private final MongoTemplate mongoTemplate;
    private final MongoDatabaseFactory dbFactory;

    public void transferFunds(String fromId, String toId, BigDecimal amount) {
        MongoTransactionManager txManager = new MongoTransactionManager(dbFactory);
        TransactionTemplate txTemplate = new TransactionTemplate(txManager);

        txTemplate.execute(status -> {
            // Debit
            mongoTemplate.updateFirst(
                Query.query(Criteria.where("_id").is(fromId)
                    .and("balance").gte(amount)),
                new Update().inc("balance", amount.negate()),
                Account.class
            );

            // Credit
            mongoTemplate.updateFirst(
                Query.query(Criteria.where("_id").is(toId)),
                new Update().inc("balance", amount),
                Account.class
            );

            return null;
        });
    }
}

// application.properties
spring.data.mongodb.uri=mongodb://localhost:27017/mydb

// Configuration
@Configuration
public class MongoConfig extends AbstractMongoClientConfiguration {
    @Bean
    MongoTransactionManager transactionManager(MongoDatabaseFactory dbFactory) {
        return new MongoTransactionManager(dbFactory);
    }
}

// Service
@Service
public class OrderService {

    @Transactional  // Works with MongoTransactionManager
    public Order createOrder(CreateOrderRequest request) {
        Order order = orderRepository.save(new Order(request));
        inventoryService.decrementStock(request.items()); // same transaction
        return order;
    }
}

// MongoDB shell / driver
const session = client.startSession();
session.startTransaction({
    readConcern: { level: 'snapshot' },
    writeConcern: { w: 'majority' }
});

try {
    await accounts.updateOne(
        { _id: fromId }, { $inc: { balance: -amount } }, { session }
    );
    await accounts.updateOne(
        { _id: toId }, { $inc: { balance: amount } }, { session }
    );
    await session.commitTransaction();
} catch (error) {
    await session.abortTransaction();
    throw error;
} finally {
    session.endSession();
}

// Strongest consistency for financial operations
MongoTransactionManager txManager = new MongoTransactionManager(
    dbFactory,
    TransactionOptions.builder()
        .readConcern(ReadConcern.SNAPSHOT)
        .writeConcern(WriteConcern.MAJORITY)
        .build()
);