CDK EventBridge Slack

A simple CDK app which creates an AWS EventBridge bus and API destination for sending messages to Slack.

The motivation behind this project is to be a simple, generic solution for sending messages to Slack from AWS. Applications within AWS can simply send messages to the custom event bus with a payload targeting a specific Slack channel.

Install

This project requires Node v18+.

npm install

Setup

This typically needs to be done once per Slack workspace.

1. Create an app in Slack

First create an app in Slack.

The app must be configured with a Bot user with the chat:write scope. This can be found under the OAuth & Permissions menu in your app's settings.

2. Store bot token in AWS Secrets Manager

From the OAuth & Permissions page in your app's settings copy the Bot User OAuth Token. The value must then be stored as plain text in AWS Secrets Manager prefixed with the word Bearer e.g.

Bearer xoxb-9999999999999-4687200055635-HX6YDjXm0hz2YL3EsCDaIYDK

The arn of this secret must be supplied as the botOauthTokenSecretArn property passed to the SlackStack constructor.

Configuration

Simply instantiate and configure your SlackStack in bin/cdk-eventbridge-slack.ts.

new SlackStack(app, "SlackStack", {
  workspaceName: "My Workspace",
  botOauthTokenSecretArn:
    "arn:aws:secretsmanager:ap-southeast-2:999999999999:secret:slack-bot-token-QmBx39",
  organisationId: 'o-abcde12345' // optional
});

Note workspaceName is used to derive the name of the event bus. The workspace name is converted to kebab-case with '-slack' appended. For example, the above stack instance will create an event bus named my-workspace-slack.

To allow publishing to the event bus cross-account within the same AWS Organisation, set organisationId. This field is otherwise optional. For more information see Sending and receiving Amazon EventBridge events between AWS accounts.

Enabling channels (IMPORTANT!)

In order to post messages Slack app must be added to the receiving Slack channel. This can be achieved by typing /add in the respective channel.

Deployment

npm run cdk -- deploy SlackStack

Usage

Events sent to the EventBus created by SlackStack MUST contain a message object inside the Detail field. The contents of the message object must be compatible with Slack's postMessage API. At a minimum this requires specifing a channel id in the channel field. A channel's id can be found in Slack by right-clicking on a channel and selecting View channel details. You will find the channel's id at the bottom of channel details dialog box.

The event MUST also have DetailType set to slack-message in order for the event to match the EventBridge rule created by SlackStack.

{
  "message": {
      "channel": "C04KSNT9ABC",
      "text": "Hello world!"
  }
}

Sending event via AWS CLI

[
  {
    "EventBusName": "cdk-eventbridge-slack",
    "DetailType": "slack-message",
    "Detail": "{\"message\":{\"channel\":\"C04KSNT9ABC\",\"text\":\"Hello world!\"}}"
  }
]

aws events put-events --entries file://events.json

Known issues

If you need to update the Slack token, it's best to redeploy your SlackStack instances. EventBridge will copy the token value from the secret you provide to a secret it creates itself. Updating the token secret and redeploying SlackStack does not cause the EventBridge secret to be updated.

References

https://serverlessland.com/patterns/eventbridge-api-destinations-slack

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