Monitor your failing Lambdas with CloudWatch and LocalStack

LocalStack now supports CloudWatch metric alarms! In this article, you will learn how to configure and test a simple AWS CloudWatch metric alarm with LocalStack, to get notified on infrastructure failures.

AWS CloudWatch is a service that enables monitoring of your AWS infrastructure by collecting logs and operational metrics of your deployment. An integral part of infrastructure operations is reacting in real-time to anomalous changes in metrics, e.g., spiking disk or CPU usage or failures in your serverless functions. This is where CloudWatch metric alarms and actions come into play, which you can now develop and test using LocalStack!

LocalStack now supports CloudWatch metric alarms with statistic and comparison-operator. This article will discuss how you can use a CloudWatch metric alarm to get notified automatically when your Lambda function invocations fail. We will set up an email notification using the Simple Email Service (SES) in our example.


For this tutorial you will need:

To connect LocalStack with the SMTP server, you need to configure the following SMTP environment variables when starting LocalStack:

  • SMTP_HOST this should contain the hostname and the port of your mock SMTP server
  • SMTP_USER optional, if there is user to connect
  • SMTP_PASS optional

For example, when using smtp4dev, simply run:

docker run --rm -it -p 3000:80 -p 2525:25 rnwood/smtp4dev

and set SMTP_HOST=localhost:2525. Navigating to http://localhost:3000 will open a UI to access the email notifications.

CloudWatch Basics

CloudWatch can help you to get a better understanding of how your AWS infrastructure resources behave over time. While some metrics are collected automatically, e.g., the execution of Lambdas, you can also define custom metrics you would like to monitor. To that end, you can use the CloudWatch API put-metric-data.

Here are some basic commands to get started:

# add new metric data
$ awslocal cloudwatch put-metric-data \
   --namespace test \
   --metric-data '[{"MetricName": "Orders", "Value": 20}]' 

# get information about all collected metrics
$ awslocal cloudwatch list-metrics
    "Metrics": [
            "Namespace": "test",
            "MetricName": "Orders",
            "Dimensions": []

# query metric data
$ awslocal cloudwatch get-metric-data \
   --metric-data-queries '[{"Id": "my-id","MetricStat": {"Metric": {"Namespace": "test","MetricName": "Orders"},"Period": 3600, "Stat": "Sum" }}]' \
   --start-time 2022-05-04T08:00:00Z \
   --end-time  2022-05-04T19:00:00Z
    "MetricDataResults": [
            "Id": "my-id",
            "Label": "Orders Sum",
            "Timestamps": [
            "Values": [
            "StatusCode": "Complete"

You can also use our LocalStack WebApp and navigate to Here you can add metrics (and alarms) as well, supported by a nice UI, which already gives you configurable parameters.

In order to create an alarm, you need to define what kind of metrics the alarm should evaluate. LocalStack supports metric alarms with simple statics. This typically includes:

  • metric-name
  • namespace
  • threshold - a double value
  • comparison-operator - e.g. GreaterThanThreshold, LessThanThreshold, GreaterThanOrEqualToThreshold, LessThanOrEqualToThreshold
  • statistic - e.g. SampleCount, Average, Sum, Minimum, Maximum
  • period - the interval in seconds used to calculate the metric
  • evaluation-periods - defines the number of periods that will be evaluated to check the threshold
  • treat-missing-data - defines how missing data points should be evaluated: missing, ignore, notBreaching, breaching

For a deeper understanding of alarm evaluation, we advice to consult the official AWS docs.

Even though LocalStack’s implementation of CloudWatch is not yet feature complete, you can already cover a range of use-cases. Next, we look into how we can make use of CloudWatch alarms with Lambdas.

Setup a Lambda Function

For simplicity, we will create a Lambda that will always fail. This will make it easier to demonstrate the alarm’s functionality. Suppose you are interested in further learning about Lambdas. In that case, you can check our previous blog post on Hot Swapping Python Lambda Functions using LocalStack, which also gives a solid introduction to Lambdas.

import json

def lambda_handler(event, context): 
    raise Exception('fail on purpose')

Next we zip and create our lambda-function:

$ zip

$ awslocal lambda create-function \
    --function-name my-failing-lambda \
    --zip-file fileb:// \
    --handler failing-lambda.lambda_handler \
    --runtime python3.8 \
    --role my-role

When this lambda is invoked, it will fail immediately. Also the metrics are collected automatically, so we can now define a metric alarm that will notify us, once this happens.

Metric alarms

Now let’s think about when we want to trigger the alarm. We need to define which metrics we are interested in, and how thresholds should be calculated. If you invoke the lambda once, and run list-metrics you will see that we already have some metrics here:

$ awslocal cloudwatch list-metrics
    "Metrics": [
            "Namespace": "AWS/Lambda",
            "MetricName": "Invocations",
            "Dimensions": [
                    "Name": "FunctionName",
                    "Value": "my-failing-lambda"
            "Namespace": "AWS/Lambda",
            "MetricName": "Errors",
            "Dimensions": [
                    "Name": "FunctionName",
                    "Value": "my-failing-lambda"

Obviously, there are metrics reported for every invocation of a Lambda - we can see the metric name “Invocations”.

Additionally, there is a metric for “Errors”. This is the one we want to track. For the alarm, we will need the Namespace, MetricName, and the Dimensions. The latter one will make sure we only consider alarms for the lambda my-failing-lambda.

Now, we just need to think about reasonable evaluation criteria - what is reasonable will depend a lot on your specific use case.

Let’s say we want to get notified if the lambda fails at least once within the past minute. Then the evaluation-periods would be 1, and period would be 60.

For the statistic we use Sum (failing invocation within the last minute will be summed up), and our threshold is 1, with the comparison-operator GreaterThanOrEqualToThreshold.

In case the lambda was not invoke, we assume everything is fine. So we set treat-missing-data to notBreaching.

At this time we know how we want to evaluate the metrics. So the only thing left is configuring the action that should be executed, once the alarm changes its state to ALARM.

Alarm actions

Actions are optional parameters that we can configure for alarms. There is one configuration for every state: alarm-actions, ok-actions, and insufficient-data-actions. We will execute the action once the alarm state changes.

LocalStack supports SNS Topics as actions. We must create the topic beforehand, and we need to know the topic’s ARN for the metric-alarm configuration.

Before we can use the email feature, we have to do some preparations: We need to verify the email address where you want to send the messages to:

$ awslocal ses verify-email-identity --email-address

Next, we can create the SNS topic. Remember: we need this ARN for the alarm configuration. We also need to create a subscription to this topic, so the that the email will be sent to our address.

$ awslocal sns create-topic --name my-topic-alarm
    "TopicArn": "arn:aws:sns:us-east-1:000000000000:my-topic-alarm"

$ awslocal sns subscribe --topic-arn arn:aws:sns:us-east-1:000000000000:my-topic-alarm --protocol email --notification-endpoint

Finally, we have everything we need to create the metric-alarm:

$ awslocal cloudwatch put-metric-alarm \
  --alarm-name my-lambda-alarm \
  --metric-name Errors \
  --namespace AWS/Lambda \
  --dimensions "Name=FunctionName,Value=my-failing-lambda" \
  --threshold 1 \
  --comparison-operator GreaterThanOrEqualToThreshold \
  --evaluation-periods 1 \
  --period 60 \
  --statistic Sum \
  --treat-missing notBreaching \
  --alarm-actions arn:aws:sns:us-east-1:000000000000:my-topic-alarm 

If we invoke the Lambda function now, you should get an email notification after some time:

$ awslocal lambda invoke --function-name my-failing-lambda out.txt

The email will contain plain text, and gives you information about the alarm that was triggered:

Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: SNS-Subscriber-Endpoint
From: LocalStack <>

{"OldStateValue": "OK", "AlarmName": "my-lambda-alarm", "AlarmDescription": "", "AlarmConfigurationUpdatedTimestamp": "2022-05-04T13:58:57.297769000Z", "NewStateValue": "ALARM", "NewStateReason": "Threshold crossed", "StateChangeTime": "2022-05-04T13:59:20.438156000Z", "Region": "us-east-1", "AlarmArn": "arn:aws:cloudwatch:us-east-1:000000000000:alarm:my-lambda-alarm", "OKActions": "", "AlarmActions": ["arn:aws:sns:us-east-1:000000000000:my-topic-alarm"], "InsufficientDataActions": "", "Trigger": {"MetricName": "Errors", "Namespace": "AWS/Lambda", "Unit": "", "Period": 60, "EvaluationPeriods": 1, "ComparisonOperator": "GreaterThanOrEqualToThreshold", "Threshold": 1.0, "TreatMissingData": "notBreaching", "EvaluateLowSampleCountPercentile": "", "Dimensions": [{"value": "my-failing-lambda", "name": "FunctionName"}], "StatisticType": "Statistic", "Statistic": "SUM"}}


CloudWatch is an integral part of infrastructure management to monitor and react to operational metrics. Metric alarms are periodically evaluated and make you aware if thresholds of metrics are breached.

The configured actions automatically trigger once the alarm state changes and can help respond quickly to any anomalies. In our example, we showed how to use email notifications, but this is just one way. You could also use webhooks, SQS queues, or other SNS topic subscribers. Check out the code over our samples repository and create an issue on GitHub or connect with us on LocalStack Slack to get help.

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