Aplicación y recomendaciones

Individual feedback is intended for a dedicated participant. It contains state updates from the MOM about expectations of a subscriber or adaptation advices for capabilities of a publisher.

Expectation State Updates to Subscribers

Subscribers get informed if the state of their active expectations has changed. This individ- ual feedback is provided by the MOM and can be used by subscribers to assess the currently consumed notifications regarding their QoI. It may trigger changes to expectations and start a renegotiation. From the perspective of a subscriber, an active expectation can be either satisfied,

unsatisfied or pending to be satisfied as shown in Figure 3.24.

Satisfied

Pending

Unsatisfied

Figure 3.24.: States and state transitions of an expectation from the subscriber’s perspective. Receiving feedback that an expectation is currently satisfied informs the subscriber that from now on notifications being forwarded from the MOM correspond to the requirements formalized in the expectation to the degree pre-defined by the MOM. For example, 90% or 95% of notifications satisfy the mentioned expectation.

Sometimes, the system needs to adapt itself first before being able to satisfy an expectation. In this case, the subscriber is informed about the expectation being pending to be satisfied. As soon as the system has finished its adaptation and the expectation becomes satisfied, the subscribers is informed about this change of state as described above.

If an expectation’s state changes to unsatisfied, subscribers are informed about the reason an expectation cannot be satisfied at the moment. For example, the context of a publisher has changed so that certain capabilities are not available anymore to the degree necessary to satisfy the expectation about the corresponding property; or a certain property is currently not sup- ported by the system at all (e.g., as there are no active publishers with suitable capabilities).

Definition 6 (Reason). The MOM expresses reasons for not being able to satisfy an expectation Xe

i as tuples (X e

i,px,α) with α describing the value currently provided by the system for the generic propertyp_x that has an unsatisfied requirement defined as part of expectationXe

i.

Including the currently available value for a specific generic property into a reason provides the subscriber with a reference value to check its current requirements against. Knowing the current state of the system for its individual bundle of requirements, a subscriber becomes empowered to revise its requirement. It might be that the currently available value for this generic property turns out to be sufficient for subscriber in its current situation or context even if this had not been foreseen and expressed by a matching expectation beforehand. Without any reference value, however, the subscriber would just receive the information that its requirements are not satisfied but have no indication about the orders of magnitude it would have to adjust its requirements in order to get them satisfied.

As soon as the expectation can be satisfied or is pending to be satisfied, the subscriber is notified about the new state.

Adaptation Advices to Publishers and MOM

Publishers and neighboring message brokers in a distributed MOM receive explicit adaptation

advices if the support for a generic property has to be adjusted. Reacting to context changes that

affect publishers, subscribers or the MOM, optimizing the usage of system resources, or becoming able to satisfy expectations can be reasons for having to adjust support for properties. Using adaptation advices, we extend the scope of support for generic properties to those manipulated by publishers.

Referring to the notion of actions introduced in Section 3.2.2, an adaptation advice can be the result of the MOM choosing an action to increment or decrement the current state of a property as shown in Figure 3.25. For example, as shown in Equation (3.1), the action adaptPublisher defined in the MOM for p_{sampl ingRat e}, indicates that the current state of the generic property

sampling rate can be decreased by adapting a publisher with a registered capability for sampling rate. This would result in an adaptation advice given from the MOM to the respective publisher.

Message-oriented Middleware Action Publisher Adaptation Advice Capability Generic Property

Register new / Update existing

Triggers Defined for

Advises new current value Adjust

Figure 3.25.: Relationship adaptation advice, capability, generic property and actions.

Definition 7 (Adaptation Advice). An adaptation advice for a specific capability profile includes the list of capabilities to adapt. This information is provided as tuples (CPe

j,C e

k,β) where C e

k denotes

the capability to adapt as part of capability profileCPe

j, andβ defines the required target value for

Ce_k.C V .

Please note that an adaptation advice does not state how a participant should adapt but rather de- fines the desired outcome of the adaptation for a certain generic property. Thus, we abstract from the implementation of the publisher and enable for triggering self-adaptation in heterogeneous populations of publishers. Participants can decide locally about the most suitable mechanism to apply in order to meet the new requirements set by the MOM.

In distributed networks of brokers, adaptation advices to publishers are forwarded to the edge broker being directly connected to the respective publisher.

For complex properties or those that require a sequence of actions to be executed, adaptation ad- vices received from a neighboring broker are broken down into several other adaptation advices or parameters for locally applied actions.

After a successful adaptation, the publisher or broker would update the current valueCe k.C V of

the capabilityCe

kthat has changed.