Applying Domain‑Driven Design to Build Maintainable and Scalable Backend Architecture

As business requirements become more complex, traditional monolithic code structures suffer from problems such as chaotic organization, patch‑style development, and high iteration cost, which severely impact development efficiency.

The author introduces Domain‑Driven Design (DDD) as a solution, using strategic design (bounded contexts, ubiquitous language) and tactical design (entities, value objects, aggregates, repositories, domain services) to decouple business logic from technical implementation.

Key Problems Identified

Code organization is chaotic, data access is arbitrary, and business logic is tangled with data logic.

Business logic is directly passed through to the database layer.

Implicit code logic spreads across the codebase, leading to hidden dependencies.

High iteration cost due to increasing code complexity.

Strategic Design

Bounded Context (限界上下文) defines clear domain boundaries, preventing ambiguity when the same term has different meanings in different sub‑domains. Context Mapping visualises relationships such as Anti‑corruption Layer, Conformist, Customer/Supplier, and Shared Kernel.

Tactical Design

Entity : Objects with unique identity and continuity; business rules are encapsulated within the entity.

Value Object : Immutable objects that describe attributes; they have no identity and can be shared across entities.

Aggregate : A cluster of related entities and value objects with a single aggregate root that enforces invariants.

Repository : An abstraction for persisting and retrieving aggregates, placed in the domain layer as an interface and implemented in the infrastructure layer.

Domain Service : Encapsulates domain logic that does not naturally belong to a single entity or value object.

Application Service : Orchestrates use‑cases, coordinates multiple aggregates, handles cross‑cutting concerns (transactions, security, logging) and returns DTOs.

Layered Architecture

Domain Layer : Core business logic, aggregates, entities, value objects, domain services.

Application Layer : Use‑case orchestration, receives CQE (Command‑Query‑Event) objects, produces DTOs.

Interface (API) Layer : Handles external communication, request validation, authentication, rate limiting, and error handling.

Infrastructure Layer : Provides technical services such as databases, message queues, caching, and implements repository interfaces.

CQE and DTO

Command‑Query‑Event (CQE) objects represent intent for write operations, while DTOs are simple data carriers used for output, keeping the domain model isolated from external contracts.

type AdWalletDTO struct {
    AdAccountID          string          `json:"ad_account_id" orm:"ad_account_id"` // 子账户ID
    WalletStatus         uint            `json:"wallet_status" orm:"wallet_status"` // 钱包状态：1-正常 2-欠费状态 3-关闭
    DepositBalance       decimal.Decimal `json:"deposit_balance" orm:"deposit_balance"` // 存款余额
    FrozenDepositBalance decimal.Decimal `json:"frozen_deposit_balance" orm:"frozen_deposit_balance"` // 冻结存款余额
    CreditBalance        decimal.Decimal `json:"credit_balance" orm:"credit_balance"` // 信用余额
    FrozenCreditBalance  decimal.Decimal `json:"frozen_credit_balance" orm:"frozen_credit_balance"` // 冻结信用余额
    CouponBalance        decimal.Decimal `json:"coupon_balance"` // 代金券余额
    Title                string          `json:"title"` // 名字
    Currency             string          `json:"currency"` // 货币
}

Comparison with Traditional MVC

DDD separates strategic concerns (domain modeling) from tactical concerns (implementation details), reducing coupling between business logic and technical infrastructure, improving maintainability, scalability, and team collaboration compared to the classic three‑layer MVC architecture.

Conclusion

While DDD is not a silver bullet, it provides a structured approach to handling complex business domains, enabling cleaner codebases, easier evolution, and better alignment between business and technical teams.