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Chapter 2. Related Work/Fundamentals 26

There exist further useful tools and functions:

Logging and Playback: ROS supports two diﬀerent mechanisms for logging and playback of

data: the rosout topic and bag ﬁles. The ﬁrst mechanism is a topic called rosout. It will display

information sent from all nodes and save them in textual log ﬁles.

The second mechanism is called rosbags. In contrast to rosout, bags allow the storage of all

published messages, not only the output of speciﬁc logging functions. These messages can be

played back afterwards, making it possible to simulate real data and custom setups, even if the

hardware is not available. In addition, data can be played back in real time or sped up, which is

a useful feature for debugging. Bags can only store messages from topics, as service connections

are invisible.

The Parameter Server is a centralised database, designed for sharing data between all nodes.

Due to its relatively slow speed, it is mainly used for static or slowly changing data, such as

settings and constants. It is part of the ROS-Master and is implemented in XML-RPC.

Roslaunch and Namespaces: Roslaunch is a tool for easily launching multiple nodes locally

or over SSH with given parameters and settings. The conﬁguration is deﬁned in XML-based

launch-ﬁles, which cover options, such as to automatically respawn processes, once they have

died, and uploading parameters to the parameter server. ROS supports namespaces to support

the hierarchical arrangement of nodes, topics and services and to prevent name conﬂicts. There

are three diﬀerent namespace models, one for each node, topic and service. Each namespace

consists of one root element and various sub-elements, which are separated by "/", for example

"/building2/room1913/power-plug2".

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RFXCOM RFXtrx433 Spezifikationen Seite 33