Open Host Service

The open host service relationship is a type of relationship between bounded contexts that involves one bounded context providing a service that another bounded context can access. In this relationship, one bounded context is considered the “host” and the other bounded context is considered the “client” or “consumer” of the service. The host provides an interface or API for the client to use, and the client is responsible for implementing the necessary code to interact with the host and consume the service.

This relationship is often used when one bounded context has a particular expertise or capability that is useful to other bounded contexts in the system. By providing an open host service, the bounded context with the expertise can share its capabilities with other bounded contexts in a controlled and standardized way.

The open host service relationship can help to reduce duplication of functionality across bounded contexts and promote modularity and reusability. However, it also introduces some coupling between the bounded contexts, as changes to the service provided by the host may require changes to the clients that depend on it. As with all relationships between bounded contexts, the tradeoffs between coupling and cohesion must be carefully considered when deciding whether to use an open host service relationship.

Implications

The open host service relationship is a type of relationship between bounded contexts in Domain-Driven Design. In this relationship, one bounded context exposes its functionality as a service to another bounded context. Here are some pros and cons of this relationship:

Pros

Allows for clear separation of concerns between bounded contexts
Encourages a service-oriented architecture and promotes loose coupling between systems
Enables bounded contexts to be developed and deployed independently
Can facilitate scalability by allowing services to be distributed across multiple systems

Cons

Can introduce additional complexity to the system, particularly in terms of communication between services
Requires a well-defined service contract to ensure interoperability between systems
May result in performance issues if there is a lot of inter-service communication
Can make it more difficult to maintain consistency between systems, particularly in the event of failure or partial failure

The open host service relationship can be a powerful tool for promoting separation of concerns and enabling independent development and deployment of bounded contexts. However, it requires careful planning and design to ensure that the benefits outweigh the drawbacks.

Example

Here’s an example of an open host service relationship between bounded contexts:

Let’s say we have two bounded contexts: “Sales” and “Inventory”. The “Sales” bounded context is responsible for managing orders and the “Inventory” bounded context is responsible for managing the stock levels of items.

The “Sales” bounded context needs to know the stock levels of items when processing orders. Instead of tightly coupling the two bounded contexts, they can establish an open host service relationship. The “Inventory” bounded context exposes an API for the “Sales” bounded context to call, providing access to the current stock levels of items.

The pros of this approach are that it allows for loose coupling between the bounded contexts, enabling them to evolve independently. The cons are that it can introduce additional complexity and potential performance issues if the open host service is not well-designed.

In summary, the open host service relationship enables one bounded context to provide a service that can be used by another bounded context, without being tightly coupled.

When does it work?

The open host service relationship works effectively in the following situations:

When one bounded context needs to consume the functionality provided by another bounded context.
When there is a need to expose a domain capability or service to external systems or users.
When there is a requirement for a standard way of integrating with other systems or services.

In these situations, the open host service relationship can provide a standardized way of accessing the functionality of another bounded context or service. This can help to reduce coupling between systems, while still allowing them to interact in a meaningful way.

When does it not work?

The open host service relationship between bounded contexts may not work effectively in the following scenarios:

Security concerns: Since this relationship involves exposing the internal workings of one bounded context to another, security concerns such as data breaches and unauthorized access may arise.
Tight coupling: The open host service relationship can lead to tight coupling between the two bounded contexts, which can make changes to one bounded context difficult without affecting the other.
Communication overhead: This relationship requires communication between the two bounded contexts, which can lead to additional overhead and latency.
Maintenance challenges: The open host service relationship requires maintaining backward compatibility for the exposed service, which can become challenging as the service evolves over time.

Domain-driven Design: A Practitioner's Guide