Ping Simple

Most applications will require more than one operator. In this example, we will create two operators where one operator will produce and send data while the other operator will receive and print the data. The code in this example makes use of the built-in PingTxOp and PingRxOp operators that are defined in the holoscan::ops namespace.

In this example we’ll cover:

  • how to use built-in operators

  • how to use add_flow() to connect operators together

Note

The example source code and run instructions can be found in the examples directory on GitHub, or under /opt/nvidia/holoscan/examples in the NGC container and the debian package, alongside their executables.

Here is a example workflow involving two operators that are connected linearly.

%%{init: {"theme": "base", "themeVariables": { "fontSize": "16px"}} }%% classDiagram direction LR PingTxOp --|> PingRxOp : out...in class PingTxOp { out(out) int } class PingRxOp { [in]in : int }

Fig. 5 A linear workflow

In this example, the source operator PingTxOp produces integers from 1 to 10 and passes it to the sink operator PingRxOp which prints the integers to standard output.

We can connect two operators by calling add_flow() (C++/Python) in the application’s compose() method.

The add_flow() method (C++/Python) takes the source operator, the destination operator, and the optional port name pairs. The port name pair is used to connect the output port of the source operator to the input port of the destination operator. The first element of the pair is the output port name of the upstream operator and the second element is the input port name of the downstream operator. An empty port name (“”) can be used for specifying a port name if the operator has only one input/output port. If there is only one output port in the upstream operator and only one input port in the downstream operator, the port pairs can be omitted.

The following code shows how to define a linear workflow in the compose() method for our example. Note that when an operator appears in an add_flow() statement, it doesn’t need to be added into the workflow separately using add_operator().

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#include <holoscan/holoscan.hpp> #include <holoscan/operators/ping_tx/ping_tx.hpp> #include <holoscan/operators/ping_rx/ping_rx.hpp> class MyPingApp : public holoscan::Application { public: void compose() override { using namespace holoscan; // Create the tx and rx operators auto tx = make_operator<ops::PingTxOp>("tx", make_condition<CountCondition>(10)); auto rx = make_operator<ops::PingRxOp>("rx"); // Connect the operators into the workflow: tx -> rx add_flow(tx, rx); } }; int main(int argc, char** argv) { auto app = holoscan::make_application<MyPingApp>(); app->run(); return 0; }

  • The header files that define PingTxOp and PingRxOp are included on lines 2 and 3 respectively.

  • We create an instance of the PingTxOp using the make_operator() function (line 10) with the name “tx” and constrain it’s compute() method to execute 10 times.

  • We create an instance of the PingRxOp using the make_operator() function (line 11) with the name “rx”.

  • The tx and rx operators are connected using add_flow() (line 14)

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from holoscan.conditions import CountCondition from holoscan.core import Application from holoscan.operators import PingRxOp, PingTxOp class MyPingApp(Application): def compose(self): # Create the tx and rx operators tx = PingTxOp(self, CountCondition(self, 10), name="tx") rx = PingRxOp(self, name="rx") # Connect the operators into the workflow: tx -> rx self.add_flow(tx, rx) if __name__ == "__main__": app = MyPingApp() app.run()

  • The built-in holoscan operators, PingRxOp and PingTxOp, are imported on line 3.

  • We create an instance of the PingTxOp operator with the name “tx” and constrain it’s compute() method to execute 10 times (line 8).

  • We create an instance of the PingRxOp operator with the name “rx” (line 9).

  • The tx and rx operators are connected using add_flow() which defines this application’s workflow (line 12).

Running the application should give you the following output in your terminal:

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Rx message value: 1 Rx message value: 2 Rx message value: 3 Rx message value: 4 Rx message value: 5 Rx message value: 6 Rx message value: 7 Rx message value: 8 Rx message value: 9 Rx message value: 10

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