Programming Robots with ROS

Chapter 4. Services

Services are another way to pass data between nodes in ROS. Services are just synchronous remote procedure calls; they allow one node to call a function that executes in another node. We define the inputs and outputs of this function similarly to the way we define new message types. The server (which provides the service) specifies a callback to deal with the service request, and advertises the service. The client (which calls the service) then accesses this service through a local proxy.

Service calls are well suited to things that you only need to do occasionally and that take a bounded amount of time to complete. Common computations, which you might want to distribute to other computers, are a good example. Discrete actions that the robot might do, such as turning on a sensor or taking a high-resolution picture with a camera, are also good candidates for a service-call implementation.

Although there are several services already defined by packages in ROS, we’ll start by looking at how to define and implement our own service, since this gives some insight into the underlying mechanisms of service calls. As a concrete example in this chapter, we’re going to show how to create a service that counts the number of words in a string.

Defining a Service

The first step in creating a new service is to define the service call inputs and outputs. This is done in a service-definition file, which has a similar structure to the message-definition files we’ve already ...