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The module ships with a built-in Lambda function (functions/notify_slack.py) that handles CloudWatch alarms, GuardDuty findings, Security Hub findings, and more. If you need different formatting, additional logic, or a completely different notification target, you can provide your own Python file.

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How it works

Set lambda_source_path to the path of your Python file, relative to the root module. The module derives the handler name from the filename automatically: 
local.lambda_handler = try(split(".", basename(var.lambda_source_path))[0], "notify_slack")
For example, if lambda_source_path = "functions/mylambda.py", the handler becomes mylambda.lambda_handler. Your Python file must define a function named lambda_handler.

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Example custom function

The following is the functions/mylambda.py example included in the module repository. It sends the raw SNS message body to a Slack channel using urllib3: 
import json
import os

import urllib3

http = urllib3.PoolManager()


def lambda_handler(event, context):
    url = os.environ["SLACK_WEBHOOK_URL"]
    msg = {
        "channel": "#channel-name",
        "username": "Prometheus",
        "text": event["Records"][0]["Sns"]["Message"],
        "icon_emoji": "",
    }

    encoded_msg = json.dumps(msg).encode("utf-8")
    resp = http.request("POST", url, body=encoded_msg)
    print(
        {
            "message": event["Records"][0]["Sns"]["Message"],
            "status_code": resp.status,
            "response": resp.data,
        }
    )
The SLACK_WEBHOOK_URL, SLACK_CHANNEL, SLACK_USERNAME, and SLACK_EMOJI environment variables are always set by the module and available to your function.

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Deploy a custom function

1

Write your custom Lambda handler

Create a Python file with a lambda_handler(event, context) function. The file name (without the .py extension) becomes the handler module name. Place it anywhere accessible from your Terraform root module.
2

Reference the file with lambda_source_path

Pass the path to your file using lambda_source_path. The path is relative to your Terraform root module (where you run terraform apply).
resource "aws_sns_topic" "custom" {
  name = "custom-lambda-topic"
}

module "custom_lambda" {
  source  = "terraform-aws-modules/notify-slack/aws"
  version = "~> 7.0"

  lambda_function_name = "custom_lambda"
  lambda_source_path   = "functions/mylambda.py"

  iam_role_name_prefix = "custom"

  sns_topic_name   = aws_sns_topic.custom.name
  create_sns_topic = false

  slack_webhook_url = "https://hooks.slack.com/services/AAA/BBB/CCC"
  slack_channel     = "aws-notification"
  slack_username    = "reporter"
}
3

Handle multiple functions sharing the same source path

If you deploy multiple module instances from the same source file, the module uses a hash of the source path to name the deployment package. Because both instances share the same path, they would produce the same hash and conflict.Use hash_extra to add a unique string to the hash for each instance:
module "notify_slack_dev" {
  source  = "terraform-aws-modules/notify-slack/aws"
  version = "~> 7.0"

  lambda_function_name = "notify_slack_dev"
  lambda_source_path   = "functions/mylambda.py"
  hash_extra           = "dev"

  sns_topic_name    = "slack-topic-dev"
  slack_webhook_url = "https://hooks.slack.com/services/AAA/BBB/CCC"
  slack_channel     = "dev-alerts"
  slack_username    = "reporter"
}

module "notify_slack_prod" {
  source  = "terraform-aws-modules/notify-slack/aws"
  version = "~> 7.0"

  lambda_function_name = "notify_slack_prod"
  lambda_source_path   = "functions/mylambda.py"
  hash_extra           = "prod"

  sns_topic_name    = "slack-topic-prod"
  slack_webhook_url = "https://hooks.slack.com/services/AAA/BBB/CCC"
  slack_channel     = "prod-alerts"
  slack_username    = "reporter"
}
When lambda_source_path is null (the default), the module uses its own bundled functions/notify_slack.py. You only need to set lambda_source_path when you want to override the default handler.

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