Single Purpose Events

Over the last few weeks Jeff Fritz and I have been slowly rolling out (darkly) support in KlipTok for tracking the live status of tracked channels. This feature leverages Twitch’s EventSub system that enables a sort of event driven programming for client applications. The integration of EventSub into KlipTok will enable the site to take the next step in terms of functionality and offerings.

Tracking stream status involves receiving callbacks for two different events which can occur: stream.online and stream.offline. Following this, I implemented support for this using Azure Service Bus with SQLFilters. The filters were organized as such:

EventType = ‘stream.online’ and EventVersion = ‘1’ => Stream is online

EventType = ‘stream.offline’ and EventVersion = ‘1’ => Stream is offline

Initial testing of this logic showed it worked as the events were received. Following the success small tests I worked with Jeff to subscribe all channels that had opted into tracking. Once successful, I began monitoring the channels during control times (when @csharpfritz stream was active) as well as random points throughout the day. For the most part I had confirmed the functionality working properly.

But then I started seeing some irregularities. @wintergaming is a popular StarCraft 2 Twitch channel but I noticed that the entry never seemed to go offline. Digging deeper into Twitch’s documentation I realized that, in fact, there is a difference between a channel being online and a channel being live; @wintergaming for example is NEVER offline rather, he plays reruns when he is not live. According to the documentation, this is still a valid online status, a status which i was not accounting for. As our requirement for KlipTok was to note LIVE channels, a change needed to be made.

Many Options

As I sat down to address this I was faced with a few ways to go about it. A couple approaches which I considered were:

  • I could update the StreamOnlineUpdate subscription such that, if the type was NOT ‘live’ I should treat it as an offline event and delete the entry from the LiveChannel tracking table
  • I could update the StreamOnlineUpdate subscription such that, if the not ‘live’ type was detected the event would be redirected to the Offline subscription

Both of these were, in my mind, bad choices. For starters, taking Option 1 would create duplicative code between the Online and Offline subscriptions. Further, it would obscure the purpose of the OnlineUpdate whose intent is to handle the case of the channel being ‘online’. I decided to not pursue this.

Option 2 is a bit better since it avoids duplicating logic but, when creating event handlers, the intent should be as straightforward and clear as possible. Event redirection like this is foolish and only adds extra processing logic to the handler. It would be different if I was creating a new event to “advance the flow”. But, in this case, I am effectively using the subscriptions as a mechanism of logic checking.

So, I thought about it more deeply and I realized that I was restricting myself based on the naming I had chosen. Recall what I said earlier “there is a difference between a channel being online and a channel being live”. The solution lie in honoring this distinction Twitch was making in our system as well.

Thus, the solution I arrived at is to alter the names of the subscriptions as such:

  • StreamOnlineUpdate => StreamLiveUpdate
  • StreamOfflineUpdate => StreamNotLiveUpdate

By creating this distinction, it meant that I could not adjust the Service Bus SQL Filter to only send the message to StreamLiveUpdate if the channel is, in fact, live. In other cases, the channel is NOT live and thus we should send to the StreamNotLiveUpdate.

In effect, this enables the sort of Single Purpose Events which are ideal in complex system which depend on low amounts of coupling to ensure the sanity of the maintainers.

Making it Work

The SQL Filter syntax of Service Bus works quite well (though I am still partial to what is offered through EventGrid) and enables clear definition of criteria. Unlike EventGrid however, the message itself cannot be analyzed (or if it can I have not found out how). Thus, we rely on the use of the Message class (from Microsoft.Azure.ServiceBus NuGet package) to apply custom UserProperties that we can use for filtering.

We end up defining the following for StreamLiveUpdate

EventVersion = ‘1’ and EventType = ‘stream.online’ and StreamType = ‘live’

EventVersion comes from Twitch so we can distinguish between different formats of the Event (this approach is also highly advised for testing and development to ensure discreteness with events already in play).

The EventType is the Twitch event being received. Our code also, upon knowing its receiving a stream type event offers the StreamType value as well, which will contain live, rerun, and other values indicating what the online stream type corresponds to.

For StreamNotLiveUpdate we define the following SQL Filter:

EventVersion = ‘1’ and (EventType = ‘stream.offline’ or (EventType = ‘stream.online’ and StreamType <> ‘live’))

You can see this combines our criteria for the normal case (EventType = ‘stream.offline’) and the exceptional case around an online event that is NOT of type live.

Conclusion

Through this approach we ensure our event handlers have but a single purpose, an application of the Single Responsibility Principle from SOLID design. The only time we should have to modify the Live event handler is if the meaning of a live channel changes. We are not redirecting or overloading this handler and obscuring its meaning. Instead we adjusted our understanding to better match the reality of the data we would be receiving. Thus, we are able to maintain a single purpose event pattern and control the complexity of the feature.

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