docs/catalog-of-terms/Command/README.md
A command is a request made to do something. A command represents the intention of a system’s user regarding what the system will do to change its state.
Command characteristics:
The result of a command can be either success or failure; the result is an Event(s)
In case of success, state change(s) must have occurred somewhere (otherwise nothing happened)
Commands should be named with a verb, in the present tense or infinitive, and a nominal group coming from the domain (entity of aggregate type)
Source: Open Agile Architecture
Meeting class in Domain Model:
public void Cancel(MemberId cancelMemberId)
{
this.CheckRule(new MeetingCannotBeChangedAfterStartRule(_term));
if (!_isCanceled)
{
_isCanceled = true;
_cancelDate = SystemClock.Now;
_cancelMemberId = cancelMemberId;
this.AddDomainEvent(new MeetingCanceledDomainEvent(this.Id, _cancelMemberId, _cancelDate.Value));
}
}
CancelMeetingCommand class in Application Layer
public class CancelMeetingCommand : CommandBase
{
public CancelMeetingCommand(Guid meetingId)
{
MeetingId = meetingId;
}
public Guid MeetingId { get; }
}
internal class CancelMeetingCommandHandler : ICommandHandler<CancelMeetingCommand>
{
private readonly IMeetingRepository _meetingRepository;
private readonly IMemberContext _memberContext;
internal CancelMeetingCommandHandler(IMeetingRepository meetingRepository, IMemberContext memberContext)
{
_meetingRepository = meetingRepository;
_memberContext = memberContext;
}
public async Task<Unit> Handle(CancelMeetingCommand request, CancellationToken cancellationToken)
{
var meeting = await _meetingRepository.GetByIdAsync(new MeetingId(request.MeetingId));
meeting.Cancel(_memberContext.MemberId);
return Unit.Value;
}
}
In the example above, we can distinguish between 2 types of Commands:
The most important thing is that the Command can be rejected until the state changes. In both the CommandHandler (invalid MeetingId) and the Cancel method (business rule broken), an exception can be thrown and the Command is then rejected and all uncomitted changes are rolled back (state does not change).