![]() We start with the scenarios that the Shipping bounded context must handle. These can be useful patterns for when you are implementing a microservice, but they are less relevant when designing the boundaries between microservice. There are a few other DDD patterns not listed here, including factories, repositories, and modules. The article Interservice communication discusses asynchronous messaging in more detail. Because microservices are distributed and don't share data stores, domain events provide a way for microservices to coordinate with each other. Domain events are especially relevant in a microservices architecture. "A delivery was cancelled" is a domain event. For example, "a record was inserted into a table" is not a domain event. As the name suggests, domain events should mean something within the domain. Domain events can be used to notify other parts of the system when something happens. The definition here is not directly related to microservices.ĭomain events. ![]() The term service is overloaded in software development. That's what aggregates are meant to model. Ultimately it's up to the application, not the data layer, to enforce the invariants required for the domain. A single business transaction may span multiple data stores, or may be long running, or may involve third-party services. In a distributed application, however, that's often not feasible. Traditional applications have often used database transactions to enforce consistency. If the application modifies several related objects, how does it guarantee consistency? How do we keep track of invariants and enforce them? Customers create orders, orders contain products, products have suppliers, and so on. Things in the real world have complex webs of relationships. The purpose of an aggregate is to model transactional invariants. Any other entities in the aggregate are children of the root, and are referenced by following pointers from the root. Lookup is done using the root entity's identifier. Exactly one entity in an aggregate is the root. An aggregate defines a consistency boundary around one or more entities. Typical examples of value objects include colors, dates and times, and currency values.Īggregates. Value objects can have methods that encapsulate domain logic, but those methods should have no side-effects on the object's state. To update a value object, you always create a new instance to replace the old one. It is defined only by the values of its attributes. ![]()
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