Building Event Sourcing with Spring Boot and MongoDB
In this tutorial, we will explore how to implement Event Sourcing using Spring Boot and MongoDB. Event Sourcing is a powerful architectural pattern that can be used to ensure data consistency, auditability, and scalability in complex systems. It enables you to store the state of an application as a series of events, capturing every change made to the data over time. Each event represents a specific action and can be replayed to reconstruct the state of the application at any point in time.
Table of Contents:
Before we begin, lets make sure we have the following prerequisites installed:
- Java Development Kit (JDK) 8 or later
- Spring Boot
- MongoDB database
- An Integrated Development Environment (IDE) of your choice (Eclipse, IntelliJ, etc.)
Setting up the Project
Let’s start by creating a new Spring Boot project. You can use the Spring Boot Initializr or your preferred IDE to create the project with the following dependencies:
- Spring Web
- Spring Data MongoDB
- Lombok (optional but recommended for reducing boilerplate code)
Domain Model
For this tutorial, let’s assume we’re building a simple banking application with the following domain model:
- Account: Represents a bank account with an account number, balance, and owner information.
- Transaction: Represents a transaction made on an account, containing details like the transaction ID, amount, timestamp, etc.
Configuring MongoDB for Event Sourcing
In order to use MongoDB with Spring Boot, we need to configure the database connection. Create a configuration class, MongoDBConfig, to do this:
import org.springframework.context.annotation.Configuration;
import org.springframework.data.mongodb.config.AbstractMongoClientConfiguration;
import org.springframework.data.mongodb.repository.config.EnableMongoRepositories;
@Configuration
@EnableMongoRepositories(basePackages = "com.example.repository")
public class MongoDBConfig extends AbstractMongoClientConfiguration {
@Override
protected String getDatabaseName() {
return "event_sourcing_demo";
}
@Override
protected String getMappingBasePackage() {
return "com.example.domain";
}
@Override
protected String getMongoClientDatabaseName() {
return "event_sourcing_demo";
}
}Implementing the Event Store
The Event Store is responsible for storing events related to the changes made in the system. Let’s create a simple event class, AccountEvent, representing the events for the Account domain:
import lombok.AllArgsConstructor;
import lombok.Data;
@Data
@AllArgsConstructor
public class AccountEvent {
private String accountId;
private String eventType;
private double amount;
}Next, we’ll create the AccountEventRepository, which will interact with the MongoDB to store and retrieve events:
import org.springframework.data.mongodb.repository.MongoRepository;
public interface AccountEventRepository extends MongoRepository<AccountEvent, String> {
// Add custom query methods if needed
}Implementing the Aggregate
An Aggregate is a representation of a domain object and its state. In our case, the AccountAggregate will represent an Account and its associated events. Create a class named AccountAggregate:
import lombok.Data;
import org.springframework.data.annotation.Id;
import org.springframework.data.annotation.PersistenceConstructor;
import org.springframework.data.mongodb.core.mapping.Document;
import java.util.ArrayList;
import java.util.List;
@Data
@Document(collection = "account_aggregate")
public class AccountAggregate {
@Id
private String accountId;
private double balance;
private List<AccountEvent> events;
@PersistenceConstructor
public AccountAggregate(String accountId) {
this.accountId = accountId;
this.balance = 0.0;
this.events = new ArrayList<>();
}
}Handling Commands with Event Sourcing
Commands represent actions that can be performed on the system. In our case, we’ll have a CreateAccountCommand to create a new account and a DepositMoneyCommand to deposit money into an existing account. Let’s create these classes:
import lombok.AllArgsConstructor;
import lombok.Data;
@Data
@AllArgsConstructor
public class CreateAccountCommand {
private String accountId;
private String accountHolder;
}
@Data
@AllArgsConstructor
public class DepositMoneyCommand {
private String accountId;
private double amount;
}Next, we’ll create the corresponding command handlers, which will handle these commands and update the state of the AccountAggregate accordingly:
import org.springframework.stereotype.Component;
@Component
public class AccountCommandHandler {
private final AccountAggregateRepository accountAggregateRepository;
public AccountCommandHandler(AccountAggregateRepository accountAggregateRepository) {
this.accountAggregateRepository = accountAggregateRepository;
}
public void handle(CreateAccountCommand command) {
AccountAggregate accountAggregate = new AccountAggregate(command.getAccountId());
accountAggregateRepository.save(accountAggregate);
}
public void handle(DepositMoneyCommand command) {
AccountAggregate accountAggregate = accountAggregateRepository.findById(command.getAccountId()).orElseThrow(
() -> new RuntimeException("Account not found")
);
accountAggregate.getEvents().add(new AccountEvent(command.getAccountId(), "DEPOSIT", command.getAmount()));
accountAggregate.setBalance(accountAggregate.getBalance() + command.getAmount());
accountAggregateRepository.save(accountAggregate);
}
}Implementing the Command Controller
The Command Controller will expose endpoints to execute the commands. Create a class named AccountController:
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.*;
@RestController
@RequestMapping("/accounts")
public class AccountController {
private final AccountCommandHandler accountCommandHandler;
public AccountController(AccountCommandHandler accountCommandHandler) {
this.accountCommandHandler = accountCommandHandler;
}
@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public void createAccount(@RequestBody CreateAccountCommand command) {
accountCommandHandler.handle(command);
}
@PostMapping("/{accountId}/deposit")
@ResponseStatus(HttpStatus.OK)
public void depositMoney(@PathVariable String accountId, @RequestBody DepositMoneyCommand command) {
command.setAccountId(accountId);
accountCommandHandler.handle(command);
}
}Querying Data using Event Sourcing – Implementing the Query Controller
The Query Controller will expose endpoints to fetch account information. Create a class named AccountQueryController:
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
@RequestMapping("/accounts")
public class AccountQueryController {
private final AccountQueryService accountQueryService;
public AccountQueryController(AccountQueryService accountQueryService) {
this.accountQueryService = accountQueryService;
}
@GetMapping("/{accountId}")
public ResponseEntity<AccountAggregate> getAccount(@PathVariable String accountId) {
AccountAggregate accountAggregate = accountQueryService.getAccount(accountId);
if (accountAggregate != null) {
return ResponseEntity.ok(accountAggregate);
} else {
return ResponseEntity.notFound().build();
}
}
}Implementing the Query Service
The Query Service will interact with the MongoDB and return the aggregated account information. Create a class named AccountQueryService:
import org.springframework.stereotype.Service;
@Service
public class AccountQueryService {
private final AccountAggregateRepository accountAggregateRepository;
public AccountQueryService(AccountAggregateRepository accountAggregateRepository) {
this.accountAggregateRepository = accountAggregateRepository;
}
public AccountAggregate getAccount(String accountId) {
return accountAggregateRepository.findById(accountId).orElse(null);
}
}Testing the Application
Now that we’ve implemented the Event Sourcing architecture using Spring Boot and MongoDB, it’s time to test our application. You can use tools like Postman or cURL to interact with the REST endpoints exposed by the AccountController.
Create a new account:
POST /accounts
Content-Type: application/json
{
"accountId": "12345",
"accountHolder": "John Doe"
}Deposit money into the account:
POST /accounts/12345/deposit
Content-Type: application/json
{
"amount": 500.0
}Retrieve account information:
GET /accounts/12345You should see the updated account information with the deposited amount.
In this tutorial, we’ve explored how to implement Event Sourcing using Spring Boot and MongoDB. Event Sourcing provides a powerful mechanism for storing and reconstructing application state and can be particularly beneficial in systems requiring auditing, scalability, and data consistency. By following the steps and code examples provided, you now have the foundation to build more complex applications using Event Sourcing and expand on this pattern to suit your specific use cases.
By Kurukshetran
Read more on Introduction to CQRS Pattern in Spring Boot and MongoDB.
