Microservices and DDD-Aggregates: Achieving High Scalability and Data Consistency in Complex Systems

Introduction Microservice architecture and Domain-Driven Design (DDD) are becoming increasingly popular in the development of modern banking systems. The combination of these approaches allows banks to modernize their systems, making them more flexible, scalable, and adaptable to rapidly changing business requirements.

However, this transformation is not a trivial task. The banking domain is characterized by high complexity, strict regulations, and high demands for transaction accuracy and consistency. Here, developers face a unique challenge: how to model complex business scenarios in a microservice architecture while maintaining data consistency and business rules. This is where the concept of DDD aggregates comes into play.

In this article, we will consider a real-world scenario from the banking industry and explain step by step how aggregates can be used to solve this problem.

Scenario: Integrated Payment System Let's imagine a modern bank that plans to upgrade its payment system and implement a microservice architecture. The new system should cover various types of payments, such as direct transfers, remittances, payments by credit and debit cards, payment schedules, and recurring payments. In addition, the system should provide real-time balance updates and transaction status tracking.

Step 1: Domain Modeling and Aggregate Identification The first step will be to deeply study the payment domain and identify its key concepts, such as Account, Transaction, Payment, etc. As a result of this analysis, we can distinguish the following aggregates:

1. Account Aggregate: Contains account details, balance, and balance operations.
2. Payment Aggregate: Represents a payment instruction, including payer, recipient, amount, status, and other metadata.
3. Transaction Aggregate: Reflects the actual movement of money between accounts or between an account and the external world.

Step 2: Microservice Design Next, we need to group these aggregates into logical microservices. One possible division could be:

1. Account Service: Responsible for the Account Aggregate.
2. Payment Service: Manages the Payment Aggregate and initiates payments.
3. Transaction Service: Executes transactions between Account Aggregates.

It's important to remember that each microservice should be independent and autonomous. They interact through synchronous and asynchronous communication protocols (e.g., REST API, message queue).

Step 3: Transaction Management Using Aggregates Now let's consider how we can use aggregates for transaction management. Imagine the following scenario: a user makes a payment from their account to another account.

1. The Payment Service creates a new Payment Aggregate, which contains information about the payer's account, the recipient's account, the amount, and other details.

2. The Payment Service calls the Account Service to reserve the amount on the payer's account. This is done by acting on the Account Aggregate. If there are insufficient funds in the account, the operation will fail.

3. If the amount reservation is successful, the Payment Service sends a message to the Transaction Service (e.g., via a message queue), which initiates a new transaction.

4. The Transaction Service should create a new Transaction Aggregate, which performs the actual transfer of money between accounts. This involves several operations on Account Aggregates in an atomic manner.

5. After the transaction execution (successfully or unsuccessfully), the Transaction Service should send a "feedback" message to the Payment Service about the transaction status.

6. Based on the received status, the Payment Service should update the status of the corresponding Payment Aggregate and either releases the reserved amount on the Account Aggregate (in case of a failed transaction) or deduct it permanently (in case of a successful transaction).

This is a fairly complex process that involves several aggregates and microservices. The main idea here is that each change to an aggregate is an atomic operation that maintains the aggregate's invariants and business rules.

Conclusion

Implementing microservices and aggregates in the banking industry is possible but requires significant effort and careful planning. We must consider the complexity of the domain, the importance of transaction consistency, and existing business rules. Aggregates give us a powerful tool to manage this complexity. They help us express domain concepts, adhere to business rules, and ensure transaction consistency.

However, this is not a silver bullet. Developing such an architecture requires a very good knowledge of the domain, technical experience, and team coordination. This effort needs to be carefully executed constantly reviewed and evolved.

In this article, we touched on only a few challenges and possible solutions. In real life, there are many other factors to consider, such as performance, scalability, resilience, monitoring, etc. However, we believe that understanding and correctly applying the concept of aggregates is an important step for any developer who wants to create a modern, scalable, and flexible banking system.