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When a user issues a command, it is not guaranteed to succeed. For example:
  • A customer cannot withdraw $150 if their account only has $100.
  • A customer cannot deposit a negative amount or $0.
  • A customer cannot deposit money into an account that has not been opened yet.
To protect these business rules (also called domain invariants), we define explicit, domain-specific Domain Errors.

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Defining Domain Errors

Rather than using generic error strings or general database codes, we define our domain errors as a simple Rust enum. This makes errors highly typed, explicit, and easy to handle or translate in our application edge (such as web routers or GraphQL layers). 
// =========================================================================
// Define Domain Errors
// =========================================================================
// Specific errors represent exactly why a business rule validation failed.
#[derive(Debug, PartialEq, Eq)]
pub enum BankAccountError {
    /// Attempted to open an account that is already active.
    AccountAlreadyOpen,
    /// Attempted to operate on an account that has not been initialized yet.
    AccountNotYetOpen,
    /// Attempted to withdraw more money than the current available balance.
    InsufficientFunds {
        available: u64,
        requested: u64,
    },
    /// Attempted to deposit zero or an invalid negative balance amount.
    InvalidDepositAmount,
}

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What About Domain Services?

In some applications, validating a command requires interacting with an external service or performing a query that spans multiple aggregates.
  • In Domain-Driven Design, when an operation doesn’t naturally belong to a single Aggregate Root, we encapsulate that logic inside a Domain Service.
  • In our framework, keeping command handling pure and synchronous is a top priority to maintain local reasoning and easy testability.
  • If a validation requires external data, we recommend querying that data in your application layer first, and then passing the results directly into the command payload or as a parameter to the command execution, keeping the aggregate’s handle method completely free of network or database connections.