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In this tutorial, we will build a complete, production-grade Bank Account aggregate from scratch using Event Sourcing. A Bank Account aggregate needs to support:
  1. Opening a new account with an owner’s name.
  2. Depositing money to increase the account’s balance.
  3. Withdrawing money to decrease the account’s balance (ensuring the account does not underflow).

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Defining Commands

A Command represents a user’s intent or instruction to change state. Because commands represent an action that should happen, they are always named using the present tense (e.g., OpenAccount, DepositMoney). In Rust, we represent commands as a simple enum. This enum encapsulates all possible actions that can be executed on our bank account, along with any parameters required for those actions. Create your command enum like so: 
// =========================================================================
// Define Commands (The Intent)
// =========================================================================
// Commands represent user intents or instructions. They can be rejected
// if they violate domain business rules.
pub enum BankAccountCommand {
    /// Open a brand-new bank account for an owner.
    OpenAccount {
        account_id: String,
        owner: String,
    },
    /// Deposit a positive sum of money into the account.
    DepositMoney {
        amount: u64,
    },
    /// Withdraw a positive sum of money from the account.
    WithdrawMoney {
        amount: u64,
    },
}

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Command Design Guidelines

When designing commands in your own applications, keep these best practices in mind:
  1. Be Specific: Make commands highly specific to the business operation. Prefer DepositMoney over UpdateBalance { delta: i64 }.
  2. Include Relevant Payload: Provide only the data required to validate and execute the operation.
  3. No Aggregate ID in Payload (Optional): In our framework, the aggregate ID is loaded and specified at the routing level (e.g., when calling repo.execute(&id, command, ...)). While some commands (like OpenAccount) might include the initial ID in their payload for initialization, subsequent commands (like DepositMoney) only need the operational parameters like amount.