Converting Wheatley (Slack bot) to Azure Functions!

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I’ve first created the basis of Wheatley (my Slack bot) a few years ago during a hackathon. Now, I’ve used it at multiple companies helping me with various ops processes as well as adding in any little functionality that will help my co-workers. Normally I run it in a VM (both Windows and Linux, being that it’s Go based I can compile for most things), but have recently decided to add an option of running it as an Azure Function.

I was impressed how trivially easy this was. Kudos to both Microsoft for finally making it easy to make a Go based Azure Function, but also to Slack for allowing various alternate APIs to integrate against.

For details about making an Azure Function in Go, please see my previous post. What I’d like to highlight here is the Slack specifics I had to do to get AFs working properly. The initial hurdle was that Wheatley only used the RTM (real time communications) protocol, which is basically a fancy was of saying websockets. Now, my main aim for using Azure Functions is that I only wanted to have it running when I needed it and not to have hosted compute always running and always having connections to the various clients. Fortunately, Slack has an alternate option called Event API. Events API is basically just falling back to the good old REST protocol…. given how infrequent the messages really are in Slack (in the big scheme of things) REST works for me nicely.

Jumping across to my Slack library of choice, Slack-go also provides Event API functionality as well as the already used RTM functions. Cool… no switching libraries!

Basically the way Wheatley is designed, very little of it is Slack specific. Sure, receiving and sending messages are obviously Slack specific but those are just some very small touch points. Most of the code is integrating with other systems which has absolutely nothing to do with Slack. So, let’s look at the older RTM based code.

api := slack.New(slackKey)
rtm := api.NewRTM()
go rtm.ManageConnection()

for msg := range rtm.IncomingEvents {
  switch ev := msg.Data.(type) {
  case *slack.MessageEvent:
    originalMessage := ev.Text
    sender := ev.User
    
    // lets just echo back to the user.    
    rtm.SendMessage(rtm.NewOutgoingMessage(originalMessage, ev.Channel)
  default:
  }
}

So, we have some pre-defined slackKey (no, not going to tell you mine), we establish a new RTM connection and basically just sit in a loop getting the latest message and replying. Obviously Wheatley does a lot more, see github for the exact details.

So effectively we just need something similar without the websockets shenanigans.

There’s a bit more handshake ceremony going on, but really not much. Instead of 1 token (above called slackKey) there are 2. The one already mentioned and another called the verification token. This token is used to confirm that the messages you’re receiving are actually for this particular instance of the bot.

Fortunately our HTTP handle func is the same type we’re all used to in Go. The highlights of the function are follows:

var slackApi = slack.New(token)   // same token as RTM... 
func slackHttp( w http.ResponseWriter, r *http.Request) {

  // read the request details.
  buf := new(bytes.Buffer)
  buf.ReadFrom(r.Body)
  body := buf.String()
    
  // ug, hate the wordpress formatting, but basically we're using the 
  // Slack-go API to parse the 
  // event we've received. In this part we also confirm that the 
  // VerificationToken we received matches
  // the one we already have from the Slack portal (variable 
  // verificationToken)
  eventsAPIEvent, e := slackevents.ParseEvent(json.RawMessage(body), 
    slackevents.OptionVerifyToken( 			
    &slackevents.TokenComparator{VerificationToken: 
    verificationToken}))
  if e != nil {
        
    // verification token not matching... bugger off.
    w.WriteHeader(http.StatusUnauthorized)
    return
  }
    
  // Check that we're the bot for this acct.
  // Taken directly from the slack-go event api example :)
  if eventsAPIEvent.Type == slackevents.URLVerification {
    var r *slackevents.ChallengeResponse
    err := json.Unmarshal([]byte(body), &r)
    if err != nil {
      w.WriteHeader(http.StatusInternalServerError)
    }
    w.Header().Set("Content-Type", "text")
    w.Write([]byte(r.Challenge))
  }
    
  // Dealing with the message itself.
  if eventsAPIEvent.Type == slackevents.CallbackEvent {
    innerEvent := eventsAPIEvent.InnerEvent
    switch ev := innerEvent.Data.(type) {
    case *slackevents.MessageEvent:

      // return 200 immediately... according to https://api.slack.com
           /events-api#prepare
      // otherwise if we dont return in 3seconds the delivery is 
      // considered to have failed and we'll get another
      // message. So can return 200 immediately but then the code that 
      // processes the messages can
      // return their results later on
      w.WriteHeader(http.StatusOK)
            
      originalMessage := ev.Text
      sender := ev.User
            
      // again, we'll just echo it back.
      slackApi.PostMessage(ev.channelID, slack.MsgOptionText( 
        originalMessage, false))
    }
  }
}

If you’re interested in the real Wheatley version (that’s wrapped in the Azure Function finery) then check on github.

The most awkward part is getting the bot permissions correct in the Slack Portal. So far for the basic messaging I’m needing the permissions of: users:read, app_mentions:read, channels:history, chat:write, im:history, mpim:history, mpim:read are useful. These are set in both the Event API part of the portal and the OAuth section.

After a few more days of testing this out on my private Slack group I think Slack + Wheatley + Azure Functions are ready to be unleashed on my co-workers 🙂

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