Create Service Server

Before examining service concepts, start the ROS master in MATLAB® and the sample ROS network. exampleHelperROSCreateSampleNetwork will create some service servers to simulate a realistic ROS network.

rosinit

Launching ROS Core...
...Done in 3.16 seconds.
Initializing ROS master on http://192.168.178.1:55438.
Initializing global node /matlab_global_node_55791 with NodeURI http://ah-csalzber:51640/

exampleHelperROSCreateSampleNetwork

Suppose you want to make a simple service server that displays "A service client is calling" when you call the service. Create the service using the rossvcserver command. Specify the service name and the service message type. Also define the callback function as exampleHelperROSEmptyCallback. Callback functions for service servers have a very specific signature. For details, see the documentation of rossvcserver.

For faster performance, use services with messages in structure format.

testserver = rossvcserver("/test","std_srvs/Empty",@exampleHelperROSEmptyCallback,"DataFormat","struct")

testserver = 
  ServiceServer with properties:

      ServiceType: 'std_srvs/Empty'
      ServiceName: '/test'
    NewRequestFcn: @exampleHelperROSEmptyCallback
       DataFormat: 'struct'

You can see your new service, /test, when you list all services in the ROS network.

rosservice list

/add
/matlab_global_node_55791/get_loggers
/matlab_global_node_55791/set_logger_level
/node_1/get_loggers
/node_1/set_logger_level
/node_2/get_loggers
/node_2/set_logger_level
/node_3/get_loggers
/node_3/set_logger_level
/reply
/test

You can get more information about your service using rosservice info. The global node is listed as node where the service server is reachable and you also see its std_srvs/Empty service type.

rosservice info /test

Node: /matlab_global_node_55791
URI: rosrpc://ah-csalzber:51639
Type: std_srvs/Empty
Args: MessageType

Create Service Client

Use service clients to request information from a ROS service server. To create a client, use rossvcclient with the service name as the argument.

Create a service client for the /test service that we just created.

testclient = rossvcclient("/test","DataFormat","struct")

testclient = 
  ServiceClient with properties:

    ServiceType: 'std_srvs/Empty'
    ServiceName: '/test'
     DataFormat: 'struct'

Create an empty request message for the service. Use the rosmessage function and pass the client as the first argument. This will create a service request function that has the message type and format that is specified by the service.

testreq = rosmessage(testclient)

testreq = struct with fields:
    MessageType: 'std_srvs/EmptyRequest'

Ensure that the service is connected to the client, waiting for them to connect if necessary.

waitForServer(testclient,"Timeout",3)

When you want to get a response from the server, use the call function, which calls the service server and returns a response. The service server you created before will return an empty response. In addition, it will call the exampleHelperROSEmptyCallback function and displays the string "A service client is calling". You can also define a Timeout parameter, which indicates how long the client should wait for a response.

testresp = call(testclient,testreq,"Timeout",3);

If the call function above fails, it will error. Instead of an error, if you would prefer to react to a call failure using conditionals, return the status and statustext outputs from the call function. The status output indicates if the call succeeded, while statustext provides additional information. Similar outputs can be returned from waitForServer.

numCallFailures = 0;
[testresp,status,statustext] = call(testclient,testreq,"Timeout",3);
if ~status
    numCallFailures = numCallFailues + 1;
    fprintf("Call failure number %d. Error cause: %s\n",numCallFailures,statustext)
else
    disp(testresp)
end

    MessageType: 'std_srvs/EmptyResponse'

Create a Service for Adding Two Numbers

Up to now, the service server has not done any meaningful work, but you can use services for computations and data manipulation. Create a service that adds two integers.

There is an existing service type, roscpp_tutorials/TwoInts, that we can use for this task. You can inspect the structure of the request and response messages by calling rosmsg show. The request contains two integers, A and B, and the response contains their addition in Sum.

rosmsg show roscpp_tutorials/TwoIntsRequest

int64 A
int64 B

rosmsg show roscpp_tutorials/TwoIntsResponse

int64 Sum

Create the service server with this message type and a callback function that calculates the addition. For your convenience, the exampleHelperROSSumCallback function already implements this calculation. Specify the function as a callback.

sumserver = rossvcserver("/sum","roscpp_tutorials/TwoInts",@exampleHelperROSSumCallback,"DataFormat","struct")

sumserver = 
  ServiceServer with properties:

      ServiceType: 'roscpp_tutorials/TwoInts'
      ServiceName: '/sum'
    NewRequestFcn: @exampleHelperROSSumCallback
       DataFormat: 'struct'

To call the service server, you have to create a service client. Note that this client can be created anywhere in the ROS network. For the purposes of this example, we will create a client for the /sum service in MATLAB.

sumclient = rossvcclient("/sum","DataFormat","struct")

sumclient = 
  ServiceClient with properties:

    ServiceType: 'roscpp_tutorials/TwoInts'
    ServiceName: '/sum'
     DataFormat: 'struct'

Create the request message. You can define the two integers, A and B, which are added together when you use the call command.

sumreq = rosmessage(sumclient);
sumreq.A = int64(2);
sumreq.B = int64(1)

sumreq = struct with fields:
    MessageType: 'roscpp_tutorials/TwoIntsRequest'
              A: 2
              B: 1

The expectation is that the sum of these two numbers will be 3. To call the service, use the following command. The service response message will contain a Sum property, which stores the addition of A and B. Ensure that the service server is available before making a call, and react appropriately if it is not.

if isServerAvailable(sumclient)
    sumresp = call(sumclient,sumreq,"Timeout",3)
else
    error("Service server not available on network")
end

sumresp = struct with fields:
    MessageType: 'roscpp_tutorials/TwoIntsResponse'
            Sum: 3

Shut Down ROS Network

Remove the sample nodes and service servers from the ROS network.

exampleHelperROSShutDownSampleNetwork

Shut down the ROS master and delete the global node.

rosshutdown

Shutting down global node /matlab_global_node_55791 with NodeURI http://ah-csalzber:51640/
Shutting down ROS master on http://192.168.178.1:55438.