Shared State
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 |
| Observer | 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:
Endpoints Diagram
