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# multi-threaded spinner

### Application Scenario

* Suppose that we have two callbacks (talker1 and talker2) running at 1Hz in the same ros node. The talker2 callback can be blocked if the talker1 callback takes more than 1s. Because `ros::spin()`  is single-threaded.
* Note: this issue only happens with roscpp. The rospy spinner is already multi-threaded, so you won’t be able to reproduce this issue if you translate the cpp subscriber node to Python.

### Solution One: MultiThreadedSpinner

* Blocking method. Simply just increase the number of threads to process callback queues.

```cpp
ros::MultiThreadedSpinner spinner(4); // Use 4 threads; or 0 to use all threads
spinner.spin(); // spin() will not return until the node has been shutdown
```

### Solution Two: AsyncSpinner

* Non-blocking method. Try to get each callback a thread if available.

```cpp
int main(int argc, char **argv) {
    ros::init(argc, argv, "talker_subscribers");
    ros::NodeHandle nh;
    ros::AsyncSpinner spinner(0); // 0 means max number of threads available
    spinner.start();  // start the AsyncSpinner asynchronously (non-blocking)
    ros::Subscriber counter1_sub = nh.subscribe("talker1", 10, callbackTalker1);
    ros::Subscriber counter2_sub = nh.subscribe("talker2", 10, callbackTalker2);
    ros::waitForShutdown(); // block the program; wait for the node to be killed
    // alternatively, you may have an infinite loop here to process some data
}
```

References: <http://wiki.ros.org/roscpp/Overview/Callbacks%20and%20Spinning>; <https://roboticsbackend.com/ros-asyncspinner-example>; <https://levelup.gitconnected.com/ros-spinning-threading-queuing-aac9c0a793f>


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