How to do lock item in DynamoDB during update? - php

Question

I'm testing DynamoDB with a lot of concurrent update requests, but I have some doubts.
For example:
In a financial transaction system where I have a table called Account and a table called Transactions.
Accounts:
id: string
balance: Number
Transactions
id: Number
amount: Number
balanceAfter: Number
accountId: string
So when performing a debit transaction I update the balance in the Account table and create a transaction with the account balance after the transaction.
If the account has a balance of 100 and I execute two transactions of 50 at the same time, the account balance would be 50 and not 0 and with two transactions in the database with balanceAfter: 50.
How to lock DynamoDB item for UPDATE with concurrency to avoid double spending? (Similar to TRANSACTION in relational database)
What is the safest way to get the updated item from DynamoDB after running UPDATE?
The code:
<?php
require './vendor/autoload.php';
use Aws\DynamoDb\DynamoDbClient;
use Aws\Credentials\CredentialProvider;
function executeDebitTransaction($accountId, $transactionAmount)
{
$provider = CredentialProvider::defaultProvider();
$client = DynamoDbClient::factory(array(
'version' => '2012-08-10',
'credentials' => $provider,
'region' => 'sa-east-1'
));
$response = $client->getItem(array(
'TableName' => 'Accounts',
'Key' => array(
'id' => array( 'S' => $accountId ))
)
);
$currentBalance = $response['Item']['balance']['N'];
$newbalance = (string)((int)$currentBalance - (int)$transactionAmount);
$response = $client->updateItem(array(
'TableName' => 'accounts',
'Key' => array(
'id' => array( 'S' => $accountId )
),
'ExpressionAttributeValues' => array (
':amount' => array('N' => $transactionAmount),
),
'UpdateExpression' => 'SET balance = balance - :amount'
));
// Generate random ID
$id = (string)(random_int(1, 1000000000));
$client->putItem(array(
'TableName' => 'Transactions',
'Item' => array(
'id' => array('N' => $id),
'amount' => array('N' => $transactionAmount),
'balanceAter' => array('N' => $newbalance),
'accountId' => $transactionAmount
)
));
}
$accountId = 'A1469CCD-10B8-4D31-83A2-86B71BF39EA8';
$debitAmount = '50';
executeDebitTransaction($accountId, $debitAmount);
Running this script with few concurrency works perfectly, but when I increase the parallelism I start to have problems.

Answer 1

Did some tests using optimistic lock and I had good results.
Repo: https://github.com/thalyswolf/dynamo-lock/blob/main/src/Repository/AccountRepositoryDynamoWithOptimisticLock.php