DynamoDBとPythonでユニークな連番を採番する

import boto3
import time
import asyncio

dynamodb = boto3.resource('dynamodb')
table_name = 'sequence'

# テーブル作成
try:
    res = dynamodb.create_table(
        AttributeDefinitions = [
            {
                'AttributeName': 'sequence_key',
                'AttributeType': 'S'
            },
        ],
        TableName = table_name,
        KeySchema = [
            {
                'AttributeName': 'sequence_key',
                'KeyType': 'HASH'
            },
        ],
        ProvisionedThroughput = {
            'ReadCapacityUnits': 3,
            'WriteCapacityUnits': 3
        }
    )

    # テーブルができるまでスリープ
    time.sleep(10)
except Exception as e:
    pass

table = dynamodb.Table(table_name)

# sequenceをカウントアップする関数
async def countup(task_name, wait, sequence_key):
    for i in range(5):
        res = table.update_item(
            Key= {
                'sequence_key': sequence_key
                },
            UpdateExpression="ADD #name :increment",
            ExpressionAttributeNames={
                '#name':'sequence'
                },
            ExpressionAttributeValues={
                ":increment": int(1)
                },
            ReturnValues="UPDATED_NEW"
        )

        print('Task:' + task_name + ',sequence_key:' + sequence_key + ',sequence:' + str(res['Attributes']['sequence']))
        await asyncio.sleep(wait)

# カウントアップ処理を並列実行
loop = asyncio.get_event_loop()
result = loop.run_until_complete(asyncio.gather(
    countup("A", 0.5, 'Key1'),
    countup("B", 0.5, 'Key2'),
    countup("C", 1, 'Key1'),
    countup("D", 1, 'Key2'),
    countup("E", 1.5, 'Key1'),
    countup("F", 1.5, 'Key2'),
))