Notes from Greg Young's DDD and CQRS course

In this post, I expand on some notes I took during Greg Young's DDD/CQRS course in Krakow, Poland.

Domain-driven design (DDD) gives us some important things to think about. In particular, we distinguish between the following components:

• Aggregate roots
• Entities
• Value objects

An aggregate groups things that belong together. It should have a name that describes the whole.

My notes outline the following steps toward a domain-driven design:

1. Denormalization
2. Transactions — try to avoid them
3. Domain service consistency — try to avoid it
4. Use soft links to learn the approach

When applying these ideas, keep the following points in mind:

• Remove bidirectional relationships
• Define your aggregates
• Make sure operations affect only one aggregate at a time
• Put methods where state is encapsulated and data is changed (objects should change their own state)
• A command may only mutate state and always has a void return value
• A query may only expose state and cannot have a void return value
• Avoid distributed transactions for better scalability

Even without transactions, you can provide consistency in your systems through merging. The following code snippet illustrates this approach. Notice the recursive call in the catch clause.

public class MergingHandler<T> : Consumes<T>
{
    public MergingHandler(Consumes<T> next)
    {
    }
 
    public void Consume(T message)
    {
        try
        {
            var commit = eventStore.GetEventsSinceVersion(message.AggregateId, message.ExpectedVersion);
            next.Handle(message);
 
            foreach(var e in commit)
            {
                foreach (vat attempted in UnitOfWork.Current.PeekAll())
                {
                    if (ConflictsWith(attempted, e)) {
                        throw new RealConcurrencyException();
                    }
                }
            }
            UnitOfWork.Current.Commit();
        }
        catch(ConcurrencyException e) {
            this.Consume(message);
        }
    }
}