Lock an item in DynamoDB for exclusive processing.

dynamodb-lock-item.ts

/*
  Assuming you have a blog Posts table with the following structure:
  {
    // hash key
    "id": 154325,
    "title": "Things you need to know before going bouldering for the first time.",
    "text": "...",
    // represents unix time of a moment this record was locked at
    "lockedAt": 1684067555
  }
*/

import { DynamoDBClient, UpdateItemCommand } from '@aws-sdk/client-dynamodb'

const example = async () => {
  const client = new DynamoDBClient({ region: 'eu-west-1' })

  // Lock the item for 5 minutes, usually you should only lock for your lambda's timeout
  const lockFor = 300_000
  const now = Date.now()

  const postId = '154325'

  // Will lock a record or throw a ConditionalCheckFailedException if it's already locked
  const result = await client.send(new UpdateItemCommand({
    "TableName": "posts",
    "ReturnValues": "ALL_NEW",
    "Key": {
      "id": {
        "N": postId
      }
    },
    "ConditionExpression": "(attribute_exists(#id)) AND ((#lockedAt = :null) OR (#lockedAt <= :lockExpiredAt))",
    "UpdateExpression": "SET #lockedAt = :now",
    "ExpressionAttributeNames": {
      "#id": "id",
      "#lockedAt": "lockedAt"
    },
    "ExpressionAttributeValues": {
      ":null": {
        "NULL": true
      },
      ":lockExpiredAt": {
        "N": (now - lockFor).toString()
      },
      ":now": {
        "N": now.toString()
      }
    }
  }))

  try {
    // Do some processing

  } finally {
    // Unlock the record
    await client.send(new UpdateItemCommand({
      "TableName": "posts",
      "ReturnValues": "ALL_NEW",
      "Key": {
        "id": {
          "N": postId
        }
      },
      "UpdateExpression": "SET #lockedAt = :null",
      "ExpressionAttributeNames": {
        "#lockedAt": "lockedAt"
      },
      "ExpressionAttributeValues": {
        ":null": {
          "NULL": true
        },
      }
    }))
  }
}

example()