Configure event notifications using S3 buckets notifications

by Rehan Haider, Sat 04 November 2023 Category: AWS Academy

Apart from using EventBridge to gather events from S3 buckets and send them to a target, you can also configure notifications on the S3 bucket itself. This is done using the add_event_notification method of the Bucket construct.

Configure S3 bucket notifications

Bucket notifications can be configured using the add_event_notification method of the Bucket construct. This method takes two parameters:

Import the required modules

We need to import the s3_notifications module to create the S3 bucket notification.

# filename: cdk_app/s3_stack.py

from aws_cdk import (
    Stack,
    aws_s3 as s3,
    aws_s3_notifications as s3n,  # 👈🏽 Import the s3 notifications module
    RemovalPolicy,
    aws_lambda as _lambda,
)

from constructs import Construct

Initialise the stack and create the S3 bucket

We will initialise the stack and create the S3 bucket as we did in the previous examples.

class S3Stack(Stack):
    BUCKET_ID = "MyS3Bucket"

    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        my_bucket = s3.Bucket(
            self,
            id=self.BUCKET_ID,
            # 👇🏽 Block all public access
            removal_policy=RemovalPolicy.DESTROY,
        )

Create the Lambda function

We will create a simple Lambda function that will be triggered when an object is created in the S3 bucket. The Lambda function will simply print File Uploaded message to console.

        # 👇🏽 Create a lambda function
        my_lambda_fn = _lambda.Function(
            self,
            id="MyLambda",
            runtime=_lambda.Runtime.PYTHON_3_7,
            code=_lambda.Code.from_inline("def main(event, context):\n\tprint('File Uploaded')"),
            handler="index.main",
        )

Create the S3 bucket notification

We use the add_event_notifications method of the Bucket construct to create the notification. This method takes three parameters:

event: The type of event that will trigger the notification. We will use s3.EventType.OBJECT_CREATED to trigger the notification when an object is created in the bucket.
destination: The destination of the notification. We will use the Lambda function we created earlier as the destination.
filters: The filters to apply to the notification. We can use this to specify the prefix and suffix of the object key that will trigger the notification. E.g. if all objects are being stored in a folder named images we can use prefix="images/" to trigger the notification.

        # 👇🏽 Create the S3 bucket notification
        my_bucket.add_event_notification(
            s3.EventType.OBJECT_CREATED, # 👈🏽 Trigger the notification when an object is created
            s3n.LambdaDestination(my_lambda_fn), # 👈🏽 Use the Lambda function as the destination
        )

Deploy the stack

After the above changes, the full code will look like this:

# filename: cdk_app/s3_stack.py

from aws_cdk import (
    Stack,
    aws_s3 as s3,
    aws_s3_notifications as s3n,  # 👈🏽 Import the s3 notifications module
    RemovalPolicy,
    aws_lambda as _lambda,
)

from constructs import Construct


class S3Stack(Stack):
    BUCKET_ID = "MyS3Bucket"

    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        my_bucket = s3.Bucket(
            self,
            id=self.BUCKET_ID,
            # 👇🏽 Block all public access
            removal_policy=RemovalPolicy.DESTROY,
        )

        # 👇🏽 Create a lambda function
        my_lambda_fn = _lambda.Function(
            self,
            id="MyLambda",
            runtime=_lambda.Runtime.PYTHON_3_7,
            code=_lambda.Code.from_inline("def main(event, context):\n\tprint('File Uploaded')"),
            handler="index.main",
        )

        # 👇🏽 Configure bucket notifications
        my_bucket.add_event_notification(
            s3.EventType.OBJECT_CREATED,
            s3n.LambdaDestination(my_lambda_fn),
        )

You can use cdk deploy to deploy the app.

The app will look like the below.

# filename: app.py

import aws_cdk as cdk
from cdk_app.s3_stack import S3Stack

app = cdk.App()

s3_stack = S3Stack(app, "S3Stack")

app.synth()

Configuring filters in event notifications

Let's say we wanted to configure a filter wheere if a new object is created in uploads/ folder in S3 bucket and is a .png file, it triggers the notification. We can do this by adding a prefix and suffix to the notification.

        # 👇🏽 Create the S3 bucket notification
        my_bucket.add_event_notification(
            s3.EventType.OBJECT_CREATED, # 👈🏽 Trigger the notification when an object is created
            s3n.LambdaDestination(my_lambda_fn), # 👈🏽 Use the Lambda function as the destination
            # 👇🏽 Add a prefix and suffix to the notification
            prefix="uploads/",
            suffix=".png",
        )

EventBridge vs S3 bucket notifications

While both are acceptable ways to configure notifications for S3 buckets, I personally prefer using EventBridge as it provides more flexibility.

Also, Bucket Notification are configured as a workaround and you would notice in the AWS console that the notification is configured using a Lambda to configure the S3. This is because S3 bucket notifications are not natively supported by CDK. CDK uses a Lambda function to configure the S3 bucket notification.