In summary, the most important issues here are:
- The Aggregate’s main task is to protect invariants (business rules, the boundary of immediate consistency)
- In a multi-threaded environment, when multiple threads are running simultaneously on the same Aggregate, a business rule may be broken
- A way to solve concurrency conflicts is to use Pessimistic or Optimistic concurrency techniques
- Pessimistic Concurrency involves the use of a database transaction and a locking mechanism. In this way, requests are processed one after the other, so basically concurrency is lost and it can lead to deadlocks.
- Optimistic Concurrency technique is based on versioning database records and checking whether the previously loaded version has not been changed by another thread.
- Entity Framework Core supports Optimistic Concurrency. Pessimistic Concurrency is not supported
- The Aggregate must always be treated and versioned as a single unit
- Domain events are an indicator, that state was changed so Aggregate version should be changed as well
public class AggregateRootBase : Entity, IAggregateRoot
{
private int _versionId;
public void IncreaseVersion()
{
_versionId++;
}
} internal sealed class OrderEntityTypeConfiguration : IEntityTypeConfiguration<Order>
{
public void Configure(EntityTypeBuilder<Order> builder)
{
builder.Property("_versionId").HasColumnName("VersionId").IsConcurrencyToken();
//...
}
} var order = await _ordersContext.Orders.FindAsync(orderId);
order.AddOrderLine(request.ProductCode);
var domainEvents = DomainEventsHelper.GetAllDomainEvents(order);
if (domainEvents.Any())
{
order.IncreaseVersion();
}
await _ordersContext.SaveChangesAsync();
