Command Messages - classification design question

Question

I'm trying to design one service using event sourcing. One use case is - when command arrives, I have to execute some logic and transform object from one MODE to other one. Now, the "problem" I'm facing is to decide between these two options for command messages:

Option 1: (put mode directly into name of the command message)

SetToManualModeCommand{
  int val = 22;
}
SetToAutoModeCommand{
  int val = 22;
}
SetToSemiAutoModeCommand{
  int val = 22;
}
SetTo...ModeCommand{
  int val = 22;
}

Option 2: (put mode as an enum inside single command message)

enum Mode{
  AUTO, 
  SEMI_AUTO, 
  MANUAL;
}

SetToModeCommand{
  Mode mode;
  int val = 22;
}

• modes are not changing that often - but they can change. If they change - in option 1 I have to make new class. In option 2 I have to add one additional enum value.

Do you see any drawbacks/benefits of any of these 2 options, or are they more less the same?

Answer 1

From the context you’ve given I don’t see a convincing argument either way. I know you’re talking about commands, not events, but try thinking about it from the event subscribers point of view. Is the significant event that the mode changed in some way, or that it changed to a specific value? In other words, would subscribers want a single ModeChanged event with details inside the event, or would some subscribers want just ModeChangedToManual and others just want ModeChangedToAuto, etc. You may consider the event storage system you’re using and how easy it is to create a filtered subscription.

It’s convenient (not required) that each command creates a single event. If subscribers would prefer a single event and you have 4 commands issuing that event it makes the system just a tiny bit more complicated, and those tiny bits tend to add up. If it’s better for subscribers that you have 4 separate events, then have 4 separate commands.