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Chapter 4. Implementation 52

by the "switch sender/receiver"-services and topics. However, if the state was changed by

direct manipulation via the "add/remove/change"-services, no message is published.

Database changed: The "database changed"-publisher publishes a message, every time a

database entry is changed by the "add/remove/change"-services to enable automatic syn-

chronization of other device manager instances. The published message contains the action

that was performed and the full receiver or sender structure. The database changed sub-

scriber listens for those messages and applies them to the database. Messages from the

same node are ﬁltered out to prevent and endless loop.

Switch receiver/sender: These two topics can switch receivers on and oﬀ, similar to the "switch

receiver/sender"-services. The diﬀerence is the missing response message, therefore requests

via topics are performed faster. Nevertheless, an indirect acknowledgement can be obtained

by subscribing to the "device changed"- topic.

It is possible to start multiple instances of the device manager, as it provides services and topics for

synchronization, such as "set/get device list" and "database changed". However, the node cannot

start the synchronization by itself, it must be started externally. It is not recommended to run

multiple instances simultaneously, because there are no major advantages and the synchronization

can be problematic. Instead, the synchronization functions were designed to enable local copies

of the database.

device manager

rosnode

main

request

device

list

ros

device manager

thread 1 thread 2

XML

database

Figure 4.14.: The internal structure of the device manager node.

The internal structure of the database (ﬁgure 4.14) is composed of two main parts, the device

manager object and the rosnode class. The main function initialises both objects and starts the

Qt event system to enable the signal and slot mechanism. Similar to the gateway driver, the

device manager is divided in two threads, to separate the ROS from the rest of the program. The

communication between the two parts is mainly done via signals/slots but also via direct method

calls. Direct methods calls are not thread safe, therefore the calling-thread has to wait until the

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RFXCOM RFXtrx433 Especificaciones Pagina 59