Building a Slack Block Kit app with Serverless and Go
It was a running joke at Twitter for new hires to post to the company-wide list asking why we couldn’t change the “dogs in the office” policy. At Clever we developed a theory that someone on the executive team had an allergy given the continued resistance to bringing dogs to work. I was stoked to work in a dog-friendly office at LaunchDarkly, but we (and our dogs) have all been WFH since I started in May 2020.
I built Pupster to make it easy for remote teams to meet each other’s dogs. It’s a Slack App that you add to the channel where you share dog photos. You can create “pup profiles” to get to know your team’s pets. I was inspired by a low-code system built at Clever based on Tumblr and Hubot, but I wanted to give it a tech stack upgrade:
- Serverless, which makes it easy to deploy an HTTPS API on AWS
- slack-go, since I’ve been using Go and had worked with this SDK before
- Slack Block Kit, which I had seen in modern Slack apps that support in-message UI controls like buttons, select menus, etc.
In this post, I’d like to share the interesting bits behind setting up a Serverless Slack Block Kit app with Go. This doesn’t go into the process for creating a Slack app, so if you’re following along you’ll want to do that first.
I needed to support four APIs for my Slack app:
POST /interactionscalled by the Slack Block Kit Interactivity API to build the UIPOST /eventscalled by the Slack Events API to respond to Slack events outside interactions with your Block Kit appGET /auth_callbackfor responding to OAuth 2 authorization code grant flows when people add your Slack appPOST /selectfor returning dynamic external data to select menu dropdown-style controls
The following serverless.yml snippet shows the interesting parts of defining these endpoints based on Go binaries. It supports different Serverless stages (e.g. dev , prod ).
The build and deploy logic lives in a Makefile :
The interactions/main.go code needs to handle different user interactions: button clicks, modal submissions, and shortcuts. You can lean on the slack-go models for most of the heavy lifting. Don’t forget to verify each request using the Slack signing token.
The eventhandler/main.go code needs to handle different events: URL verification, “App Home Open”, “App Mention”, etc.
The authcallback/main.go code plays it’s part in the OAuth 2 dance to get an API token:
The final piece is the selectmenu/main.go code which you’ll only need to support external select menus:
That’s all the boilerplate you need to get going. You can see some examples of building Slack apps using Block Kit with slack-go in the following public repos:
You can start building Slack Blocks, responding to events, and eventually deploying to the Slack App Store to delight thousands of happy customers, or in my case, dog owners. Have fun!
