One of the powerful traits of function as a service is you can execute your function using different triggers, in this article we will go through the different triggers' chalice support.

# Scheduled event

The cloudwatch scheduled events is one of the powerful triggers for a lambda, it is attractive for moving crons to the serverless realm, and for scheduled ETL jobs as well. Here is 4 different forms you can schedule a lambda to run using chalice.


app = chalice.Chalice(app_name='chalice-scheduled-event-demo')
@app.schedule('cron(15 10 ? * 6L 2002-2005)')
def cron_handler(event):
    pass
@app.schedule('rate(5 minutes)')
def rate_handler(event):
    pass
@app.schedule(Rate(5, unit=Rate.MINUTES))
def rate_obj_handler(event):
    pass
@app.schedule(Cron(15, 10, '?', '*', '6L', '2002-2005'))
def cron_obj_handler(event):
    pass

The Chalice.schedule() method accepts either a string or an instance of Rate or Cron. You can use the schedule() decorator multiple times in your chalice app, each one will result in a different lambda function.

# CloudWatch event

Cloudwatch events allows you to extend your lambda triggers to many different events. for example you can trigger your lambda to run upon code commit.

app = chalice.Chalice(app_name='chalice-cloudwatch-event-demo')
@app.on_cw_event({"source": ["aws.codecommit"]})
def on_code_commit_changes(event):
    print(event.to_dict())

Check the CloudWatch Event pattern docs for additional syntax and examples https://amzn.to/2OlqZso

# S3 event

S3 buckets are used for file storage, your setup might require you to run certain processes after a file has been uploaded, for example scan it. or resize it for example. these processes can sit in a single or multiple lambda functions. You can use s3 chalice to write a lambda function that will be triggered when a certain file is uploaded to the s3 bucket.

app = chalice.Chalice(app_name='chalice-s3-event-demo')
app.debug = True
@app.on_s3_event(bucket='mybucket-name',
                 events=['s3:ObjectCreated:*'])
def handle_s3_event(event):
    app.log.debug("Received event for bucket: %s, key: %s",
                  event.bucket, event.key)

# SNS event

You can use chalice to setup your lambda as a subscriber for an SNS topic push notifications, just by defining the name of the topic in the decorator.

app = chalice.Chalice(app_name='chalice-sns-event-demo')
app.debug = True
@app.on_sns_message(topic='my-demo-topic')
def handle_sns_message(event):
    app.log.debug("Received message with subject: %s, message: %s",
                  event.subject, event.message)

# SQS event

You can use chalice to setup your lambda as a consumer for an SQS queue messages, just by defining the name of the SQS in the decorator, and the number of messages you can consume as the batch_size.

app = chalice.Chalice(app_name='chalice-sqs-event-demo')
app.debug = True
@app.on_sqs_message(queue='my-queue', batch_size=1)
def handle_sqs_message(event):
    for record in event:
        app.log.debug("Received message with contents: %s", record.body)

# Kinesis and DynamoDB stream events

Right now Chalice doesn't support both Kinesis and DynamoDB stream yet, but the proposal here https://github.com/aws/chalice/issues/987 mentions it is planned for the next release. which is a good feature for the data teams to build ETL jobs based on these events.