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isServerAvailable

Determine if ROS or ROS 2 service server is available

Since R2021b

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Syntax

status = isServerAvailable(client)

Description

status = isServerAvailable(client) determines whether a service server with the same service name as client is available and returns a status accordingly.

example

Examples

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Open Live Script

Connect to a ROS network.

rosinit
Launching ROS Core...
.Done in 1.2771 seconds.
Initializing ROS master on http://172.20.220.163:57900.
Initializing global node /matlab_global_node_01450 with NodeURI http://dcc821001glnxa64:32977/ and MasterURI http://localhost:57900.

Set up a service server. Use structures for the ROS message data format.

server = rossvcserver('/test', 'std_srvs/Empty', @exampleHelperROSEmptyCallback,...
                      'DataFormat','struct');
client = rossvcclient('/test','DataFormat','struct');

Check whether the service server is available. If it is, wait for the service client to connect to the server.

if(isServerAvailable(client))
    [connectionStatus,connectionStatustext] = waitForServer(client)
end
connectionStatus = logical
   1


connectionStatustext = 
'success'

Call service server with default message.

response = call(client)
response = struct with fields:
    MessageType: 'std_srvs/EmptyResponse'

If the call function above fails, it results in an 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.

numCallFailures = 0;
[response,status,statustext] = call(client,"Timeout",3);
if ~status
    numCallFailures = numCallFailues + 1;
    fprintf("Call failure number %d. Error cause: %s\n",numCallFailures,statustext)
else
    disp(response)
end
    MessageType: 'std_srvs/EmptyResponse'

Shut down the ROS network.

rosshutdown
Shutting down global node /matlab_global_node_01450 with NodeURI http://dcc821001glnxa64:32977/ and MasterURI http://localhost:57900.
Shutting down ROS master on http://172.20.220.163:57900.
Open Live Script

Create a sample ROS 2 network with two nodes.

node_1 = ros2node('node_1_service_client');
node_2 = ros2node('node_2_service_client');

Set up a service server and attach it to a ROS 2 node. Specify the callback function flipstring, which flips the input string. The callback function is defined at the end of this example.

server = ros2svcserver(node_1,'/test','test_msgs/BasicTypes',@flipString);

Set up a service client of the same service type and attach it to a different node.

client = ros2svcclient(node_2,'/test','test_msgs/BasicTypes');

Wait for the service client to connect to the server.

[connectionStatus,connectionStatustext] = waitForServer(client)
connectionStatus = logical
   1


connectionStatustext = 
'success'

Create a request message based on the client. Assign the string to the corresponding field in the message, string_value. 

request = ros2message(client);
request.string_value = 'hello world';

Check whether the service server is available. If it is, send a service request and wait for a response. Specify that the service waits 3 seconds for a response.

if(isServerAvailable(client))
    response = call(client,request,'Timeout',3);
end

The response is a flipped string from the request message which you see in the string_value field.

response.string_value
ans = 
'dlrow olleh'

If the call function above fails, it results in an 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.

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

The callback function used to flip the string is defined below. 

function resp = flipString(req,resp)
% FLIPSTRING Reverses the order of a string in REQ and returns it in RESP.
resp.string_value = fliplr(req.string_value);
end

Input Arguments

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ROS or ROS 2 service client, specified as a ros.ServiceClient or ros2serviceclient object handle, respectively. This service client enables you to send requests to the service server.

Output Arguments

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Status of service server availability, returned as a logical scalar. If a server of the same name and type as client is not available, status will be false.

Extended Capabilities

Version History

Introduced in R2021b

See Also

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