// backend/src/repositories/dynamoDbRepo.ts
import { DynamoDBDocumentClient, PutCommand } from '@aws-sdk/lib-dynamodb';

export class DynamoDbRepository implements IRepository {
  constructor(
    private docClient: DynamoDBDocumentClient,
    private tableName: string
  ) {}

  async createTransaction(tx: TransactionRecord): Promise<void> {
    const command = new PutCommand({
      TableName: this.tableName,
      Item: tx,
      // CRITICAL IDEMPOTENCY GUARD:
      ConditionExpression: 'attribute_not_exists(eventId)',
    });

    try {
      await this.docClient.send(command);
    } catch (err: any) {
      if (err.name === 'ConditionalCheckFailedException') {
        throw new Error(`Duplicate transaction detected for eventId: ${tx.eventId}. Dispense rejected.`);
      }
      throw err;
    }
  }
}

---## 3. The Core Challenge: Absolute Idempotency in Reverse VendingIn traditional web applications, duplicate requests might create a duplicate cart or harmless log entry. In an IoT reverse vending machine dispensing physical food portions, **duplicate requests cause physical motor actuation, hopper depletion, and potential over-feeding hazards.**When an edge kiosk connects over cellular (4G/LTE) or public campus Wi-Fi, temporary connection drops are frequent:1. The kiosk submits a transaction event (`eventId: evt-173892-x89f2`).2. The Lambda function processes the event and decrements inventory.3. The network drops before the kiosk receives the 200 OK HTTP response.4. The kiosk retry-client re-transmits the identical payload.### The Solution: DynamoDB Atomic Conditional WritesRather than implementing brittle distributed locks in Redis or relying on memory caches, we leverage Amazon DynamoDB's native conditional expressions:```typescript// backend/src/repositories/dynamoDbRepo.tsimport { DynamoDBDocumentClient, PutCommand } from '@aws-sdk/lib-dynamodb';export class DynamoDbRepository implements IRepository {  constructor(    private docClient: DynamoDBDocumentClient,    private tableName: string  ) {}  async createTransaction(tx: TransactionRecord): Promise<void> {    const command = new PutCommand({      TableName: this.tableName,      Item: tx,      // CRITICAL IDEMPOTENCY GUARD:      ConditionExpression: 'attribute_not_exists(eventId)',    });    try {      await this.docClient.send(command);    } catch (err: any) {      if (err.name === 'ConditionalCheckFailedException') {        throw new Error(`Duplicate transaction detected for eventId: ${tx.eventId}. Dispense rejected.`);      }      throw err;    }  }}