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Chapter 2. Related Work/Fundamentals 25

Services are for communication between two nodes, whereas topics are for many-to-many com-

munication. Nodes can host a service server under a string name and a client can send a request

and wait for the response. Services are composed of a name and a request and response message.

They are synchronous and therefore do not abstract sender and receiver.

Node A Node B

event

handler

Node A Node B

request

handler

response

wait

(a) Asynchronous event (b) Synchronous call

Figure 2.15.: Diﬀerences between a topic (a) and a service (b). Adopted image from [28]

Messages are strictly typed data structures. They can be composed of primitive data types, such

as int, ﬂoat, boolean, etc. as well of other messages and arbitrarily deep nested arrays of messages.

They are speciﬁed in text ﬁles, which are generated into headers for all supported programming

languages during compilation. An example of two diﬀerent messages is given in ﬁgure 2.16. To

ensure, that both sender and receiver have the same message ﬁles and not diﬀerent versions, each

message contains a MD5 checksum, which has to match for a successful transmission.

bool sender

int id

string name

myMessage data_struct

bool sender

int id

---

myMessage data_struct

device_has_been_seen.msg get_device_data.srv

Request

Response

Figure 2.16.: Two example messages: One topic message (left) and one service message (right).

Request and response are saved in the same ﬁle, but are separated by “- - -“. MyMes-

sage is a custom message from another .msg ﬁle.

The ROS-Master is a central declaration and registration service, which keeps track of all nodes,

topics and services and makes it possible for nodes to ﬁnd each other and exchange data. The

Master uses XML-RPC, providing all information encapsulated in XML ﬁles. When starting a new

node or creating a new topic or service, registration data is send to the Master and saved in a

database. Clients can request this information by sending a service or topic name and they will

get a list of subscribers or the address of the service server. The Master is not involved in the

data ﬂow; it just exchanges information, allowing the node to establish peer-to-peer connections.

Image of ros network generated with rxgraph.

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