mos-eisley-lambda

“You will never find a more wretched hive of scum and villainy.” – Obi-Wan Kenobi

Episode 2 of the Ruby based Slack app framework, this time for AWS Lambda. Pure Ruby, no external gem/library dependency.

Setup

AWS

1. Create an IAM role for MosEisley Lambda function
2. Create a Lambda function for MosEisley
   • You can install this gem using Lambda Layer or just copy the lib directory to your Lambda code.
3. Create an HTTP API Gateway
   1. Create the appropriate routes (or use the OpenAPI spec)
   2. Create Lambda integration and attach it to all the routes

Configure Lambda environment variable.

• SLACK_CREDENTIALS_SSMPS_PATH: hierarchy path to System Managers Parameter Store; e.g., /slack/credentials/ would reference two parameters:
  • /slack/credetials/signing_secret
  • /slack/credetials/bot_access_token
• MOSEISLEY_LOG_LEVEL: optional, could be DEBUG, INFO, WARN, or ERROR
• SLACK_LOG_CHANNEL_ID: optional, if you want to use ME::SlackWeb.post_log()

Configure Lambda code in your lambda_function.rb file.

require 'mos-eisley-lambda'
# Or, you can just copy the `lib` directory to your Lambda and...
# require_relative './lib/mos-eisley-lambda'

MosEisley::Handler.import
# Or, if you store your handlers in a non-default location, dictate by...
# MosEisley::Handler.import_from_path('./my-handlers')

def lambda_handler(event:, context:)
  MosEisley::lambda_event(event, context)
end

Slack

Create a Slack app and configure the following.

• Interactivity & Shortcuts – Request URL should be set to the /actions endpoint and Options Load URL should be set to the /menus endpoint.
• Slash Commands – Request URL should be set to the /commands endpoint.
• OAuth & Permissions – This is where you get the OAuth Tokens and set Scopes.
• Event Subscriptions – Request URL should be set to the /events endpoint. You'll likely Subscribe to bot events app_mention at a minimum.

Handlers

Create your own Mos Eisley handlers as blocks and register them. By default, store these Ruby files in the handlers directory.

ME::Handler.command_acks holds [Hash<String, Hash>] which are Slack command keyword and response pair. The response is sent as-is back to Slack as an immediate response.

ME::Handler.command_acks.merge!({
  '/command' => {
    response_type: 'in_channel',
    text: '_Working on it…_',
  },
  '/secret' => {
    response_type: 'ephemeral',
    text: '_Just for you…_',
  },
})

Add handlers to process the Slack event.

ME::Handler.add(:command, 'A Slack command') do |event, myself|
  next unless event[:command] == '/command'
  myself.stop
  txt = "Your wish is my command."
  payload = {
    response_type: 'ephemeral',
    text: txt,
    blocks: [ME::S3PO::BlockKit.sec_text(txt)],
  }
  ME::SlackWeb.post_response_url(event[:response_url], payload)
end

Helpers

• ME::S3PO – collection of helpers to analyze/create Slack messages.
• ME::SlackWeb – methods for sending payloads to Slack Web API calls.

Event Lifecycle

Inbound

1. Slack event is sent to Mos Eisley Lambda function via API Gateway
2. Slack event is verified and produces a parsed object
3. If it's a slash command, MosEisley::Handler.command_acks is referenced and immediate response is sent
4. The original Slack event JSON is sent to the function in a recursive fashion (this is to return the inital response ASAP)

Event Processing

1. Lambda function is invoked by itself with the original Slack event
2. Handlers are called and processed according to original endpoint the event was sent to; actions, commands, events, menus
3. Send a Slack message as necessary and the Slack event cycle is complete

Using with Lambda Layers

Used the Makefile to create a zip file which can be uploaded to a Lambda Layer.

make
# Installs the gem to './ruby' then archives it to 'lambda-layers.zip'