Jef Claes

On software and life

05 Jan 2014

Command and event semantics

Yesterday, I read this blog post by Michael Feathers. In the post he goes over a pain point he has often found himself struggling with while breaking down a large method; conditional statements. 

if (alarmEnabled) {
    var alarm = new Alarm();  

Should we extract the if and the associated block into a new method, or just the content of the block? Is the condition too important to hide in a method? How would we name the extracted method? How do we avoid the code from telling us lies?

To read up on all the nuances that the answers to these questions bring, you should read the full post. What’s important for this post - spoiler alert - is that he ends up with two strategies.

He either gives the method an event-ish name…


…or raises the level of abstraction by giving the method a very general name, avoiding lies too.


While both options solve the original problem, their semantics are very different. When I raise an event, I don’t care who is listening; one, multiple or no things might be listening. When I issue a command (the second strategy), I expect it to go to exactly one destination. Sending commands indicates a rather strong dependency; you expect something to happen because of it. Events are more loosely coupled; you broadcast something happened, and something might or might not happen because of it.

With this in mind, you can still make arguments for both options. The key here is whether notifications are considered to be an essential component of intrusion detection. Is intrusion detection still conceptually whole without notifications? Are notifications just a side effect of an intruder being detected?
Since the alarm is only sounded when it’s enabled, it doesn’t seem to be an indispensable part. Intrusion detection can live on its own, unaware of notifications. This makes a pretty strong case for having the detection component just raise an event - indirectly resulting in the notifications being sent.

In this example, I’d probably pull notifications out, add an event and a bit of infrastructure that dispatches events, making the separate concepts and messages between them explicit.

Events.Raise(new IntruderDetected());

public class NotificationService : IHandle<IntruderDetected>
    public void Handle(IntruderDetected event)    { ... }        

With those extra bits, we could also, instead of listen for the event and only sounding the alarm when it’s enabled, only subscribe to the event when the alarm is enabled.