Notes and anecdotes

Techstuff rambling

Send AWS CloudWatch Alarms to Slack

I have a few servers on AWS. Some information on those, e.g. high load, would be nice to be notified of. Preferably on Slack. How do I do that?

Part 1: Send alarms to an SNS Topic

  1. Using awscli on a local machine, create a new SNS Topic. What’s an SNS Topic, you say? Think of a Topic as a bucket where your logs are pushed to, and your notification channels (SMS, Email, Slack++) subscribe to. It’s merely a logical grouping of notifications.
    aws sns create-topic 
        --region eu-central-1
        --name my-topic-name

    Note down the returned TopicArn for part 2.

  2. Then, let’s create an Alarm that posts to this Topic when an ELB gets a 500 error.  Either with aws cli:
    aws cloudwatch put-metric-alarm --region eu-central-1 
        --alarm-name "ELB_500" 
        --alarm-description "Sends 500-errors to Slack" 
        --alarm-actions "aYour-SNS-returned-from-last-step" 
        --metric-name "HTTPCode_Backend_5XX" 
        --namespace AWS/ELB --statistic "Sum" 
        --dimensions "Name=LoadBalancerName,Value=your-elb-name" 
        --period 60 
        --evaluation-periods 60 
        --threshold 1 
        --comparison-operator "GreaterThanOrEqualToThreshold"

    (See aws-cli for documentation) or create it using the GUI (It’s actually pretty straight forward)

    There’s several hundred alarm metrics to choose from, so I’ll suggest a few:

    • ELB: HTTP_Backend_400, HTTP_Backend_500, Latency, HealthyHostCount, UnHealthyHostCount
    • EC2: CPUUtilization
    • Logs: IncomingLogEvents

Part 2: Send AWS SNS Topic to Slack

Here we will send SNS messages to AWS Lambda,

  1. Create an incoming webhook for Slack at Note down the Webhook URL, and Channel for step 3.
  2. Next, we’ll create a Lambda function which will be a subscriber from this topic, and send them to Slack.
    • Under blueprint, select cloudwatch-alarm-to-slack
    • Under trigger, select the SNS-topic you created in the previous step.
    • Under trigger, check “Enable trigger”
    • Under function,
      • Specify your own function name and Description
      • Use Runtime Node 4.3
      • Code entry type: Edit code inline and insert this gist. Replace CHANNEL and PATH variable with the Slack channel and hook url from step 1 (Remove
      • Let handler be index.handler
      • For Role: If you have the role lambda_basic_execution available, select that. If not, create it by selecting Create a custom role. It should automatically suggest a role that has a Policy document which allows the actions “logs:CreateLogGroup”, “logs:CreateLogStream”, and “logs:PutLogEvents”.
      • VPC: no VPC

You can test that your SNS posts to Slack by publishing to the SNS topic manually:

aws sns publish
    --topic-arn "arn:aws:sns:eu-central-1:3...:my-topic-name"
    --message "Test message"

Helpful sources:


tomfa • 2016-09-02

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