Event Sources Tutorial

In the Quickstart guide, we looked at how to create a REST API using the @app.route() decorator. Chalice also has additional decorators that connects your code to specific event sources. This results in your code being invoked when a specific event occurs.

In this tutorial we’ll look at a few examples.

Installation and Configuration

If you haven’t already setup and configured Chalice, see the Quickstart for a step by step guide. In a nutshell, you can get a basic Chalice app created with:

$ python3 --version
Python 3.7.3
$ python3 -m venv venv37
$ . venv37/bin/activate
$ python3 -m pip install chalice
$ chalice new-project chalice-sns-demo
$ cd chalice-sns-demo

We’ll also be using the AWS CLI in this tutorial. You can follow these instructions for installing the AWS CLI v2.

Amazon SNS Topics

In this first example, we’ll create a Chalice application that will call our Lambda function whenever a message is published to an SNS Topic.

First, we’ll create an SNS topic. This is what we’ll connect to our Lambda function:

$ aws sns create-topic --name MyDemoTopic
    "TopicArn": "arn:aws:sns:us-west-2:12345:MyDemoTopic"

Be sure to save the TopicArn value for later. In this example that would be arn:aws:sns:us-west-2:12345:MyDemoTopic.

Next, we’ll update the app.py to create a lambda function that connects to an SNS topic:

from chalice import Chalice

app = Chalice(app_name='chalice-sns-demo', debug=True)

def handle_sns_message(event):
    app.log.debug("Received message with subject: %s, message: %s",
                  event.subject, event.message)

In the code above, we’re using the @app.on_sns_message() decorator to connect the SNS topic named MyDemoTopic with the handle_sns_message function. Note that we’re using the name of the topic and not the TopicArn.

Now we can deploy our chalice app:

$ chalice deploy
Creating deployment package.
Creating IAM role: chalice-demo-sns-dev
Creating lambda function: chalice-demo-sns-dev-handle_sns_message
Subscribing chalice-demo-sns-dev-handle_sns_message to SNS topic my-demo-topic
Resources deployed:
  - Lambda ARN: arn:aws:lambda:us-west-2:123:function:...

Now we can test our app by publishing a few SNS messages to our topic.

$ aws sns publish --topic-arn arn:aws:sns:us-west-2:12345:MyDemoTopic \
    --subject TestSubject --message TestMessage
    "MessageId": "abcdefgh-3e56-54bd-a471-72477b5388af"
$ aws sns publish --topic-arn arn:aws:sns:us-west-2:12345:MyDemoTopic \
    --subject TestSubject2 --message TestMessage2
    "MessageId": "abcdefgh-3e56-54bd-a471-72477b5388ag"

We should now see log messages showing that our Lambda function was invoked. We can wait for the messages using the chalice logs command.

$ chalice logs --follow -n handle_sns_message
... 217378 chalice-sns-demo - DEBUG - Received message with subject: TestSubject, message: TestMessage
... 217378 chalice-sns-demo - DEBUG - Received message with subject: TestSubject2, message: TestMessage2

Next Steps

In addition to SNS, chalice supports other event sources including Amazon S3, Amazon SQS, as well as scheduled events. You can check out the topic guide on Lambda Event Sources for more details.

Cleaning Up

Once you’re done experimenting you can clean up by deleting the Chalice app and deleting the SNS topic:

$ chalice delete
Deleting function: arn:aws:lambda:us-west-2:21345:function:chalice-sns-demo...
Deleting IAM role: chalice-sns-demo-dev
$ aws sns delete-topic --topic-arn arn:aws:sns:us-west-2:12345:MyDemoTopic
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