Attribute Access

This functionality enables proxies to read (get) and receive attribute notifications. Attributes (also called “fields”) behave like both methods (getter/setter operations) and events (notification delivery). The skeleton stores the attribute value and can notify client proxies when the application emits an attribute notification.

Static View

The following software units are involved in attribute access:

Transceiver Unit (Proxy side): Manages attribute get requests and receive-handler registration (see Proxy/Skeleton Base for its class diagram)
Transceiver Unit (Skeleton side): Stores attribute values and emits attribute notifications (see Proxy/Skeleton Base)
Request/Response Unit: Handles asynchronous getter responses (see Request/Response for its class diagram)
Cluster Connection Unit: Routes attribute requests and notifications between clusters
Message Queue Unit: Delivers attribute messages between participating clusters

' *******************************************************************************
' Copyright (c) 2026 BMW AG
'
' This program and the accompanying materials are made available under the
' terms of the Apache License Version 2.0 which is available at
' https://www.apache.org/licenses/LICENSE-2.0
'
' SPDX-License-Identifier: Apache-2.0
' *******************************************************************************

@startuml

left to right direction
skinparam linetype ortho

package "Proxy Side" as attribute_access_proxy_side {
component "Proxy/Skeleton Base" as attribute_access_proxy_skeleton_base
component "Request/Response" as attribute_access_request_response
component "Cluster Connection" as attribute_access_cluster_connection
}

component "Message Queue" as attribute_access_message_queue

attribute_access_proxy_skeleton_base --> attribute_access_request_response
attribute_access_proxy_skeleton_base --> attribute_access_cluster_connection
attribute_access_cluster_connection --> attribute_access_message_queue
attribute_access_cluster_connection --> attribute_access_proxy_skeleton_base

@enduml — Attribute access component view

Notes:

ProxyAttribute<T> provides a get(callback) method that sends a get request and keeps the callback until the getter response arrives.
SkeletonAttribute<T> stores the attribute value.
Getter responses use the Future mechanism (like method calls).
Change notifications use the event broadcast mechanism.
Attributes can have decorators based on the interface definition:
- Default (no decorators) — setter, getter and event subscriptions allowed
- readonly — getter and event subscriptions allowed
- noSubscriptions — setter and getter allowed
- readonly / noSubscriptions — getter allowed

Dynamic View

The dynamic view illustrates two attribute access patterns: asynchronous get and attribute notification.

Getter

Change Notification

Notes:

The skeleton can update attribute values at any time using attribute.set(newValue).
The application explicitly sends attribute notifications through the generated attribute send() operation. Updating the local value alone does not notify subscribed proxies.
Proxies must call attribute.setReceiveHandler(callback) to receive attribute notifications.
The proxy callback is invoked when the notification message is processed.