Create SlackBot using AWS API Gateway and Lambda

By Maimit Patel, Software Engineer at ユニファ

In this blog I am going to create SlackBot using AWS API Gateway, AWS Lambda and Slack Event API.

What is SlackBot?

A SlackBot is a regular app that is designed to interact with the user via conversation. When user mention the SlackBot app from anywhere in the slack then it will access the APIs and do all of the magical things that Slack App can do.

Let's get started

1. Setup Slack App

Go to the Slack apps home page and click Create New App button.
Create Slack App
Add scopes under the OAuth & Permissions menu
Bot scopes
Installing App to Workspace
Install App to Workspace
This will create Bot User, check it on App Home page
Bot User on the App Home

2. Configure Docker Image to run the Lambda function in the local environment

Pull the lambci/lambda:ruby2.7 docker image

# Open command prompt in your local machine and 
# hit this command to pull the docker image of Lambda with runtime Ruby-2.7 
$ docker pull lambci/lambda:ruby2.7

3. Create a Lambda function that responds to the Slack

Setup application directory to store the Lambda ruby function

# Creating directory
$ mkdir greeting-bot 

# Move to directory
$ cd greeting-bot

Create file greeting_bot.rb under greeting-bot directory

# Creating ruby file under greeting-bot directory
$ touch greeting_bot.rb

Set environment variables
- Find Bot User OAuth Access Token under the OAuth & Permissions menu from Slack App
  
  $ export BOT_OAUTH_TOKEN='bot_oauth_token'
- Find Verification Token under Basic Information menu from Slack App
  
  $ export VERIFICATION_TOKEN='verification_token'
Update the file greeting_bot.rb

class GreetingBot
  def self.main(event:, context:)
    new.run(event)
  end

  def run(event)
    case event['type']
    when 'url_verification'
      verify(event['token'], event['challenge'])
    when 'event_callback'
      if event['event']['type'] == 'app_mention'
        process(event['event']['text'], event['event']['channel'])
      end
    end
  end

  private

  # Verify request from the slack
  def verify(token, challenge)
    if token == ENV['VERIFICATION_TOKEN']
      { body: { challenge: challenge } }
    else
      { body: 'Invalid token' }
    end
  end

  def process(text, channel)
    body = if text.strip.downcase.include?('hello')
             'Hi, How are you?'
           else
             'How may I help you?'
           end
    send_message(body, channel)
  end

  # Slack API response to the mentioned channel
  def send_message(text, channel)
    uri = URI('https://slack.com/api/chat.postMessage')
    params = {
      token: ENV['BOT_OAUTH_TOKEN'],
      text: text,
      channel: channel
    }
    uri.query = URI.encode_www_form(params)

    Net::HTTP.get_response(uri)
  end
end

Run this Lambda function in the local environment and response back to the mentioned channel from request

# Make sure the mentioned channel(i.e greeting-channel) has already invited the greeting-bot that we have created in step 1

 docker run \
  -e BOT_OAUTH_TOKEN=$BOT_OAUTH_TOKEN \
  --rm -v "$PWD":/var/task lambci/lambda:ruby2.7 \
  greeting_bot.GreetingBot.main \
  '{"type": "event_callback","event":{"type":"app_mention","text":"<@U>hello!","channel":"greeting-channel"}}'

Result in local

4. Upload Lambda function to the AWS Lambda

Make sure you have created a Lambda function in AWS.
- How to create a Lambda function? read more
Upload Lambda function from the local machine using AWS CLI

# Creating zip file
zip function.zip greeting_bot.rb

# Upload zip file to the Lambda
$ aws lambda update-function-code --function-name slack-greeting-bot --zip-file fileb://function.zip
# add `--profile profile_name` if you got the AccessDeniedException

Set environment variables BOT_OAUTH_TOKEN and VERIFICATION_TOKEN in Lambda
After upload to Lambda, it looks like this
Lambda Function

5. Create REST API end-point using API Gateway that calls the lambda function

I have created API Gateway
- How to create REST API using API Gateway? read more
Add trigger to Lambda function
- Click the Add trigger button from the designer pane of the Lambda function
- Select the trigger(i.e API Gateway) from options
- Select the REST API created in the step 5

f:id:unifa_tech:20200520100910p:plain — Added API Gateway Trigger to Lambda function

6. Set the API Gateway end-point to Slack Event Request URL

Go to Event Subscriptions menu from the Slack App and enable the event subscription
Enter and Verify the Request URL on same page
- Request URL is API Gateway end-point that we have done in step 5
  Enable event subscription & Verify RESR API end-point