Best practice for User Provisioning between Microservice DB and Keycloak in a DDD/Hexagonal Architecture

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Summary

The problem at hand involves synchronizing user provisioning between a microservice database and Keycloak in a Domain-Driven Design (DDD) and Hexagonal Architecture setup. The goal is to create a record in the customer-service PostgreSQL database and provision the user in Keycloak upon registration. This article explores the best practices for achieving this synchronization, focusing on robustness, data consistency, and error handling.

Root Cause

The root cause of the challenge lies in the distributed nature of the system, where two separate systems (the customer-service database and Keycloak) need to be updated in a consistent manner. The key issues include:

  • Ensuring data consistency across both systems
  • Handling errors and failures during the provisioning process
  • Determining the best approach for synchronizing the two systems

Why This Happens in Real Systems

This issue arises in real systems due to the following reasons:

  • Microservice architecture: With multiple services involved, ensuring data consistency and synchronization becomes more complex
  • Distributed transactions: Handling transactions across multiple systems can be challenging, especially when dealing with event-driven architectures
  • Error handling and recovery: Implementing robust error handling and recovery mechanisms is crucial to maintain data consistency and prevent data loss

Real-World Impact

The impact of not addressing this issue can be significant, including:

  • Data inconsistencies: Inconsistent data between the customer-service database and Keycloak can lead to authentication issues and authorization problems
  • Security vulnerabilities: Failing to provision users correctly can expose the system to security risks and data breaches
  • System downtime: Errors during the provisioning process can cause system downtime and loss of revenue

Example or Code

// Example of using the Transactional Outbox Pattern
@Service
public class CustomerService {

    @Autowired
    private CustomerRepository customerRepository;

    @Autowired
    private KeycloakAdminClient keycloakAdminClient;

    @Transactional
    public CustomerResponse createCustomer(@Valid CreateCustomerRequest request) {
        Customer customer = customerMapper.toDomain(request);
        customer = customerRepository.save(customer);

        // Publish a CustomerCreated event
        CustomerCreatedEvent event = new CustomerCreatedEvent(customer.getId(), customer.getUsername());
        eventRepository.save(event);

        return customerMapper.toResponse(customer);
    }
}

// Example of an event handler that provisions the user in Keycloak
@Component
public class CustomerCreatedEventHandler {

    @Autowired
    private KeycloakAdminClient keycloakAdminClient;

    @Autowired
    private EventRepository eventRepository;

    @Transactional
    public void handleCustomerCreatedEvent(CustomerCreatedEvent event) {
        // Provision the user in Keycloak
        keycloakAdminClient.createUser(event.getUsername(), event.getPassword());

        // Mark the event as processed
        eventRepository.markEventAsProcessed(event.getId());
    }
}

How Senior Engineers Fix It

Senior engineers address this issue by:

  • Implementing the Transactional Outbox Pattern to ensure data consistency and handle errors
  • Using event-driven architecture to decouple the customer-service database and Keycloak
  • Implementing robust error handling and recovery mechanisms to prevent data loss and ensure system uptime
  • Monitoring and logging the provisioning process to detect and resolve issues quickly

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience with distributed systems and microservice architecture
  • Insufficient understanding of event-driven architecture and transactional patterns
  • Limited knowledge of error handling and recovery mechanisms
  • Inadequate testing and quality assurance practices to detect and resolve issues before they become critical

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