Aggregates and Aggregate Roots

Background

An Aggregate is an object or collection of objects (Entities and Value Objects) that must be treated as a single unit for the purposes of a business transaction. The Aggregate will maintain all invariants for all objects that comprise the Aggregate, enforcing all business rules internal to the Aggregate.

Aggregate Root

All Aggregates have a single point of entry that is know as the root. The Aggregate Root is the interface to the external world. All interaction with an Aggregate is via the Aggregate Root. As such, an Aggregate Root MUST have a globally unique identifier within the system. Other Entites that are present in the Aggregate but are not Aggregate Roots require only a locally unique identifier, that is, an Id that is unique within the Aggregate.

For example: an Order will contain Order Lines. The Order is the Aggregate Root and must have a unique Order Id. The Order Lines cannot be accessed directly. Any updates to Order Lines must be via the Order. That is, you ask the Order to make the change to ITS Order Line. An Order Line is only required to be identifiable with the Order therefore it is permissible to have Order Line Ids such as 1, 2, 3 etc. on each order. It is, of course, valid to use a globally unique identifier on each Order Line for persistence purposes, while having a locally unique candidate key for presentation, but it is not required.

It is not permissible to have references to entities or value objects from an Aggregate Root other than the Aggregate Root itself. In a CQRS system, it is permissible, indeed required, to have read models that have data from one or more Aggregate Roots.

Modelling

The modelling of an Aggregate Root is based on the invariants that must be maintained when updating. In our example above, an Order will perform aggregate functions on the order lines (e.g. summing the totals, assigning ids, maintaining ordering, etc.) and therefore will potentially update all order lines at the same time. An Aggregate is essentially a business transaction and should include everything that has to be kept consistent.

System Invariants

The Aggregate maintains only its internal invariants. It is not responsible for maintaining invariants between Aggregates.

For example, it is not the responsiblity of the individual Aggregate to ensure that a Username is unique. It is the responsibility of the System as a whole to maintain these system invariants. The exact mechanism it uses will depend on the invariant and the consequences of breaking them.

What you must learn is that these rules are no different that the rules of a computer system. Some of them can be bent. Others can be broken. Morpheus, The Matrix

All rules are not created equal. Business Rules are essentially business problems, not primarily technical problems. It may be simpler and cheaper to break a rule and then clean it up later in a compensating transaction, than to expend a lot of time, energy and money ensuring the rule is not broken.

Aggregate Roots in Dolittle

Dolittle provides a base class AggregateRoot that implements functionality that makes it easier to rehydrate Aggregate Roots, generate events and persist these to the Event Store. The AggregateRoot is an implementation of an Event Source. It is RECOMMENDED that all your Aggregate Root classes inherit from this class.

State

In Dolittle, an Aggregate is a purely WRITE concern. The aggregate plays no role in reading or presentation. As such, an Aggregate MUST NOT expose any public state, via variables or properties. Neither should the Aggregate return any of its internal entities or values objects, even in a transient state. The Aggregate is only required to keep such internal state that it requires to maintain its invariants. Since no public state is exposed by the Aggregate it does not need to keep in state values that are not part of its internal structure.

This is the natural conclusion of all actions on the Aggregate springing from the Aggregate Root. If the root were to allow access to its internal entities or value-objects they could be stored past the lifetime of the Aggregate Root or even changed without running through the gatekeeper that is the root.

Identity

All AggregateRoot classes must have a Guid key. It is permissible to include other candidate keys, though these should have a map to the Guid.

Rehydration

Aggregate Roots are Event Sources and their internal state is rehydrated from the persistance store by replaying the events associated with this instance. Dolittle distinguishes between Committed Event Streams, which are persisted to the Event Store and *Uncommitted Event Stream*s which are not persisted. A Committed Event Stream can be replayed against an Aggregate Root and each Event re-applied. Since the Event is the necessary and sufficient data needed for setting an internal state, the Aggregate Root will be returned to its actual state when all Events are re-applied. The Committed Event Stream is a perfect audit record of all changes of the Aggregate Root.