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_HANDLERS_DIR: optional, if other than ./handlers
• 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'

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. Add handlers by passing a block to MosEisley::Handler.add() for the types below.

:action
:command_response
:command
:event
:menu
:nonslack

:command_response types are Slack command keyword and response pair. The response is sent as-is back to Slack as an immediate response. ME is an alias to MosEisley.

ME::Handler.add(:command_response, '/sample') do |event, myself|
  {
    response_type: "in_channel",
    text: "_Working on `#{event[:command]}`..._",
  }
end

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

If your function receives non-Slack events, you can add handlers for that as well.

ME::Handler.add(:nonslack, 'A CloudWatch event') do |event, myself|
  next unless event['source'] == 'aws.events'
  myself.stop
  channel = 'C123SLCK'
  txt = 'Shceduled event was received.'
  ME::SlackWeb.chat_postmessage(channel: channel, text: txt)
end

Helpers

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

Event Lifecycle

Inbound

To an incoming Slack event, Mos Eisley will quickly respond with a blank HTTP 200. This is to keep Slack's 3-second rule. To do this, handlers are not called yet, but the Slack event is passed on to a recursive asynchronous invoke and then the handlers are called.

The exception is when the incoming Slack event is for a slash command. You can define :command_response handlers for the purpose of generating a simple response message, but nothing more.

sequenceDiagram
  participant S as Slack
  participant L as Lambda MosEisley
  S->>+L: Slack event via API Gateway
  alt Slash command
    L-->>S: Response message
    Note left of L: If a response handler is defined
  else All other events
    L-->>-S: HTTP 200 (blank)
  end
  L->>+L: Slack event
  Note right of L: Handlers are called
  opt
  L-->>-S: E.g., chat.postMessage
  end

Loading

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'