Coding Challenge: CQRS and Event Sourcing in .NET 9 - Bank API

Objective

Build a simple bank API using CQRS and Event Sourcing principles in .NET 9. The API will support:

• Transferring money between accounts.
• Fetching account balances.
• Depositing and withdrawing money via an ATM.
• Webhooks to notify other systems of balance changes.
• Preventing accounts from going below zero.

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Requirements

1. Domain

   • Accounts have unique identifiers.
   • An account's balance cannot be negative.
   • A transfer operation between accounts is atomic.

2. Commands

   • CreateAccount: Create a new account with an initial balance.
   • TransferMoney: Transfer money from one account to another.
   • DepositMoney: Deposit money into an account.
   • WithdrawMoney: Withdraw money from an account via an ATM.

3. Queries

   • GetBalance: Retrieve the balance of an account.
   • GetTransactionHistory: Fetch the event history for an account.

4. Event Sourcing

   • Store all events (e.g., account creation, deposits, withdrawals, transfers).
   • Reconstruct account states from the event log.

5. CQRS

   • Separate write operations (commands) and read operations (queries).
   • Use separate models for reading and writing.

6. Webhook

   • Notify a configurable URL whenever an account's balance changes.

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Challenge Steps

1. Set Up the Project

   • Fork this project
   • Create a .NET 9 Web API project.
   • Add required NuGet packages for CQRS and Event Sourcing (e.g., EventFlow).

2. Implement the Write Model (Command Side)

   • Define commands: CreateAccount, TransferMoney, DepositMoney, WithdrawMoney.
   • Use a service or handler to process commands.
   • Store events in an in-memory event store or a database (like SQL).

3. Implement the Read Model (Query Side)

   • Maintain a read-optimized projection of account balances.
   • Define queries: GetBalance, GetTransactionHistory.

4. Event Sourcing

   • Store events such as:
     • AccountCreated
     • MoneyDeposited
     • MoneyWithdrawn
     • MoneyTransferred
     • BalanceUpdated
   • Replay events to reconstruct the current state of an account.

5. Business Rules

   • Prevent transferring or withdrawing money if the account has insufficient funds.

6. Webhook Integration

   • Configure a webhook URL.
   • Send a POST request with account details whenever the balance changes.

7. Endpoints

   • POST /accounts: Create a new account.
   • POST /accounts/transfer: Transfer money between accounts.
   • POST /accounts/deposit: Deposit money into an account.
   • POST /accounts/withdraw: Withdraw money from an account.
   • GET /accounts/{id}/balance: Get an account's balance.
   • GET /accounts/{id}/history: Get an account's event history.

8. Testing

   • Write unit tests for command and query handlers.
   • Simulate webhook notifications during balance updates.

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Bonus Challenges

1. Concurrency Handling

   • Ensure that events are processed in the correct order to avoid race conditions.

2. Idempotency

   • Prevent duplicate processing of events.

3. Resilience

   • Retry failed webhook notifications with exponential backoff.

4. Advanced Projections

   • Create projections for monthly account statements.

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Evaluation Criteria

• Clean code and adherence to CQRS and Event Sourcing principles.
• Proper separation of concerns between command and query models.
• Handling edge cases, such as insufficient funds or invalid account IDs.
• Unit tests and documentation.