Using AWS Lambda to write from S3 to DynamoDB

Creating a lambda function triggered by an S3 event can be done in different ways. Using a blueprint, the console or AWS CDK.

We’ll take a look at creating a Python Lambda function created from Typescript CDK code.

CDK Resources

We create a separate CDK Stack for an S3 bucket

    const bucket = new s3.Bucket(this, 'S3Bucket', {
      bucketName: 'somebucketnamethatsnottaken',
      autoDeleteObjects: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
      encryption: s3.BucketEncryption.S3_MANAGED,
    bucket.grantRead(new AccountRootPrincipal());

a DynamoDB table

    const dynamoTable = new Table(this, 'DynamoDb', {
      partitionKey: {
        name: 'id',
        type: AttributeType.STRING
      tableName: 'sometable',
      removalPolicy: cdk.RemovalPolicy.DESTROY,

and finally a Lambda function.

    const lambdaFunction = new lambda.Function(this, "Lambda", {
      runtime: lambda.Runtime.PYTHON_3_9,
      code: new lambda.AssetCode("./lambda"),
      handler: "lambda_function.lambda_handler",
      vpc: vpc,

    // this grants the Lambda function rights to read from the bucket
    // this grants the Lambda function right to write to DynamoDB

    // this adds a trigger for any time an object is put into the bucket
    const s3PutEventSource = new lambdaEventSources.S3EventSource(bucket, {
      events: [

The typescript files will usually be in the lib folder (easy to get starting using the AWS docs) and are added to the app in bin. The above Lambda function code is expected to be found in a file lambda/ in the same directory as bin and lib.

Python code

This function consists of two simple pieces, reading the CSV file from S3 and writing its contents to the DynamoDB table.

"""Imports CSV from S3 to DynamoDB"""
import uuid
import boto3

TABLE = 'sometable'

s3 = boto3.resource('s3')
db_table = boto3.resource('dynamodb').Table(TABLE)

def write_to_dynamodb(header, arr):
    """Assigns random id to csv line and writes to table"""
    items = dict(zip(header, arr))
    items['id'] = str(uuid.uuid4())
    return db_table.put_item(Item=items)

def lambda_handler(event, context):
    """Gets CSV and splits into lines"""
    bucket = event['Records'][0]['s3']['bucket']['name']
    key = event['Records'][0]['s3']['object']['key']
    tmp_file = '/tmp/' + key

    s3.meta.client.download_file(bucket, key, tmp_file)
    with open(tmp_file, 'r') as f:
        header = next(f).rstrip().split(';')
        for line in f:
            if line != '\n':
                arr = line.rstrip().split(';')
                write_to_dynamodb(header, arr)
    return {'statusCode': 200}


Now all you have to do is create your stack using cdk deploy and testing the function by adding a CCSV file to your S3 bucket.

User; Id; First Name; Age