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DDD Notes in notes

2022-03-02

(in progress)

Bounded Context

Its a subdomain. Ubiquitous language are within bounded context.

There are Polysemes, same word could can mean different things between contexts.

And shared language between contexts. For example: "Product".

/bounded-context.png

Aggregate and Aggregate root

Aggregates are cluster of entities and values objects and define boundaries around each. Goal of aggregate is to protect the invariants and guarantee consistency.

The size of the aggregate has an impact. Big aggregate will have issues guaranteeing consistency and forcing transactions on the whole aggregate may cause scalibility issues.

Aggregate roots is the entry point for the aggregate. The root entity enforces invariants true and keeps the aggregate consistent, however, root can delegate other objects within the aggregate to ensure consistency.

If we find us trying to guarantee transactions between aggregates, then it is a sign that some invariants are not thought out carefully.

Aggregate should be cohesive and not split between nodes.

Domain Services

Operations which are external to aggregate and represents a domain concept could be encapsulated into domain service. Domain services are part of domain model and stateless.

fn GetCurrentISOWeek() { ... }

fn CalculateTax(price Money) { ... }

In the preceding example, service involving clock could become a domain service. This is useful for testing, by patching (monkey patching/mocking) using the programming language tools.