Building an updatable Slack message

Slack is fully awesome. At wehkamp we use it for our internal communication and as a tool for our DevOps. The Slack API allows us to build even more advanced integrations. In this blog I’ll explore how to use the API to create stuff like a powerful progress indicator, just by updating a Slack message:

How cool is this effect? 
Imagine what you could build with this.

We use Axios as a promise-based HTTP client to connect to Slack. Each call needs an authentication-token and a channel ID (even for private messages). There are two end-points we are going to be using:

  • chat.postMessage — this will send a new message to the channel and give us a message identifier ts. We will store and reuse it to update the message.
  • chat.update — this will update the message that corresponds to the ts.

Let’s create a function that uses both end-points to send a message:

The function returns the message identifier in the form of a promise. The only thing we need to do is save the message identifier and reuse it when calling the function.

Asynchronous problems

The first problem I ran into had to do with messages not arriving in the same order that I expected. Sometimes I ended up with an earlier message as end-result. And sometimes I wanted to send an update for a message that had not even been created yet. If we want to work asynchronously we need to synchronize the way we send messages.

Remember: only send 1 message at the time!

We need to build a class that will handle the process of sending messages. It will store the message identifier so we can update the message. The class should also make sure that only a single message is being send at the same time.

If a message is being send and a new message comes in, the class should store that new message and send it after the first message has been sent.

But what if a 3rd message comes in? We are only interested in sending the latest message, so we should only store the last message. A pattern like this prevents useless updates / excessive use of the Slack API. Be aware: rate limiting applies to the Slack API, so you might want to consider only updating the message once a second.

Class me!

So let’s look at the implementation:

By implementing a simple Boolean that checks if a message is being send, we solve most — if not all — of our asynchronous problems.

This class is used in the bot-zero project to show an example of a progress indicator. Go check it out and let us know what you think.

Async problems feel like…

Want to know more about the use and the power of truthy / falsy values in JavaScript, check out this blog. More info about Promises can be found here. More on classes and class expressions here.


Originally published at keestalkstech.com on October 11, 2018.