Architecture: Shared State

BlackMisc::SharedState is a hierarchical library of reusable building blocks for composing subsystems for sharing transient data between objects, potentially in different processes.

Endpoint Layer

The middle layer contains the mutators and observers. Mutators and observers correspond to commands and queries in the CQRS design pattern. Mutators and observers can be passive and/or active, as seen in this 2x2 matrix:

	Passive	Active
Mutator	publishes events	handles requests
Oberserver	subscribes events	submits requests

Events are fire-and-forget signals sent by passive mutators and received by passive observers. Requests are stateful signals that expect replies; requests are sent by active observers and received by active mutators, which send replies back to the observers that sent the requests.

Events and requests are sent across "channels". Each channel can have multiple mutators and multiple observers. Each mutator and each observer is on exactly one channel. For an observer to communicate with a mutator, they must both be on the same channel. Conceptually, a channel is just a unique key string.

These concepts are implemented by concrete QObject subclasses: CPassiveMutator, CPassiveObserver, CActiveMutator, CActiveObserver. Instances of these classes are created by calling their static create() method, which returns a QSharedPointer pointing to the new object. Their channel is determined by the BLACK_SHARED_STATE_CHANNEL macro in the class of their parent object, combined with the name of their parent object.

Transport/Datalink Layer

The middle layer is completely decoupled from the mechanism for transporting events and requests. This mechanism is the responsibility of the bottom layer. IDataLink is an abstract interface that is implemented by concrete subclasses to provide different transport mechanisms. A mutator is connected by passing it to the IDataLink::publish method. An observer is connected by passing it to the IDataLink::subscribe method.

Content/Subject Layer

The middle layer is also completely decoupled from the payloads carried by the events, the requests, and the replies, which are handled by the top layer. A payload is a CVariant, and its contents are opaque to the mutators and observers. It is for the parent objects of the mutators and observers to confer meaning upon the events and requests that they send and receive.

References

• CQRS design pattern:
  • http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/
  • http://cqrs.nu/Faq/command-query-responsibility-segregation
  • http://martinfowler.com/bliki/CQRS.html

Endpoints Diagram

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