[commandbus] Add formal mechanism for validating commands about eventhorizon

[apterf]

Hi Max,

It will be great to have this implemented and hope to help with it.

As you suggested, I'm planning to add a middleware validator
as middleware/commandhandler/validator/commandhandler.go.

1. Add a command interface with method Validate()

type Command interface {
	eh.Command

	// Validate returns the error when validating the command.
	Validate() error
}

2. The middleware will simply call the validation, if failed, return with the error

func NewMiddleware() eh.CommandHandlerMiddleware {
	return eh.CommandHandlerMiddleware(func(h eh.CommandHandler) eh.CommandHandler {
		return eh.CommandHandlerFunc(func(ctx context.Context, cmd eh.Command) error {
			if c, ok := cmd.(Command); ok {
				err := c.Validate()
				if err != nil {
					return err
				}
			}

			// Immediate command execution.
			return h.HandleCommand(ctx, cmd)
		})
	})
}

How do you feel about that? Any other idea?

[maxekman]

A flexible way to do it would be to create a separate CommandHandler middleware that does only validation.

[apterf]

And also, we found a problem (maybe a separate issue) when a struct with private fields was used in a command.

https://github.com/looplab/eventhorizon/blob/master/command.go#L155-L167

According to existing logic, private fields will be skipped and the struct will also be treated as zero value.

https://github.com/looplab/eventhorizon/blob/master/commandhandler/aggregate/commandhandler.go#L57-L60

e.g. a custom type from time.Time

type customTime time.Time

If validator can be added to each different command, can we make it an optional check?
It will be quite difficult to make it generic and always suitable for all cases.

[maxekman]

The approach for the validator looks good.

As for the other issue, If you want to open a new issue for that it would also be welcome.

[apterf]

Great I will go for this then.