WEBVTT 00:00.110 --> 00:02.300 What is a ros2 node? 00:02.330 --> 00:08.360 A node is a subpart of your application that you will code, for example, with Python or C plus plus. 00:08.390 --> 00:10.460 A node should have a single purpose. 00:10.460 --> 00:16.460 Your application will contain many nodes which will be put into packages, and nodes will then communicate 00:16.460 --> 00:17.210 with each other. 00:17.210 --> 00:22.670 But let's just see with a real life example, I will show you a simplified example so you can easily 00:22.670 --> 00:26.150 understand what a node is and why it is useful. 00:26.150 --> 00:30.410 So let's go back to the previous slide where I explained to you what packages were. 00:30.440 --> 00:34.370 A package is an independent unit inside your application. 00:34.370 --> 00:37.520 Now a package which is empty will not do anything. 00:37.520 --> 00:40.760 So we will create nodes inside the packages. 00:40.760 --> 00:43.160 Let's start with the camera package. 00:43.160 --> 00:46.700 This package will handle a camera as an independent unit. 00:46.700 --> 00:48.830 So what should we put inside? 00:48.830 --> 00:54.200 First we need a driver for the camera to be able to program it and get frames from it. 00:54.230 --> 00:59.510 We also need a program that will take those frames and do some image processing work. 00:59.510 --> 01:05.530 We could also add any other program related to the camera that we are using all those programs in blue 01:05.560 --> 01:06.850 are nodes. 01:06.880 --> 01:09.400 Each node can be launched separately. 01:09.400 --> 01:13.840 So first we will launch the driver and then the image processing node. 01:13.870 --> 01:19.900 Now the nodes will communicate between each other using Ros to communication functionalities which we 01:19.900 --> 01:21.460 will see later in this course. 01:21.490 --> 01:22.000 All right. 01:22.000 --> 01:26.080 We have our camera package filled with all the nodes that we need. 01:26.110 --> 01:31.420 Note that sometimes it can be quite hard to know if you should put two nodes in the same package. 01:31.420 --> 01:37.930 For example, the image processing node could be part of another package which only handles image processing 01:37.930 --> 01:39.700 for any camera. 01:39.730 --> 01:45.790 We could then add many other processing nodes, and this new package would communicate with any other 01:45.790 --> 01:48.760 camera package, which includes a driver. 01:48.760 --> 01:54.640 But for this example, let's just say that the image processing is specific to the camera we are talking 01:54.640 --> 02:00.100 about, and both driver and image processing are using some common dependencies. 02:00.100 --> 02:02.770 We have two other packages in our application. 02:02.770 --> 02:05.560 Let's check the motion planning package. 02:05.560 --> 02:11.400 So in this package you can expect to have a motion planning node, which will compute motion planning 02:11.400 --> 02:12.510 for a robot. 02:12.540 --> 02:19.950 We can also add a path correction node whose role is to modify the motion planning due to external factors. 02:19.950 --> 02:20.730 So great! 02:20.730 --> 02:23.370 We have two packages filled with nodes. 02:23.370 --> 02:29.340 What we can do now is to make two nodes inside different packages communicate together. 02:29.340 --> 02:33.480 We link the image processing node to the path correction node. 02:33.480 --> 02:39.690 The image processing node will analyze frames coming from the camera, and will send an analysis of 02:39.690 --> 02:42.690 the environment to the path correction node. 02:42.690 --> 02:46.380 This node will then be able to notify the motion planning node. 02:46.380 --> 02:50.730 And we finished with our third package which is the hardware control. 02:50.730 --> 02:57.630 This package has an independent unit will control the hardware of the robot that can be wheels, robotic 02:57.630 --> 02:59.910 arm joints, or anything else. 03:00.300 --> 03:05.010 In this package, we will find some drivers to control the motors with a control loop. 03:05.010 --> 03:10.260 And let's say that the position feedback, which is coming back from the motor encoders, is also sent 03:10.260 --> 03:15.300 back to the control loop for precise control and published by a state publisher node. 03:15.300 --> 03:21.810 So we have two nodes in this package, and now the motion planning node from the motion planning package 03:21.810 --> 03:29.670 will send computed trajectories to the driver inside the hardware control package to complete the architecture. 03:29.700 --> 03:35.160 Let's say that the hardware status of the robot is published, and both the motion planning and path 03:35.160 --> 03:37.920 correction nodes are getting those messages. 03:37.920 --> 03:43.110 Well, that's maybe quite complicated for you to get everything right now, but I think you have a better 03:43.110 --> 03:46.440 idea of what a node is and how they are used. 03:46.470 --> 03:50.280 Feel free to come back to this lesson after you make some progress in this course. 03:50.280 --> 03:52.020 You will understand more things. 03:52.050 --> 03:54.930 And so you've seen the big picture with a real life example. 03:54.930 --> 03:57.840 Now let's go back to the node definition. 03:57.870 --> 04:03.420 A node is a subprogram of your application which is responsible for one thing. 04:03.420 --> 04:09.030 Just as when you write a class with object oriented programming, you know that the class only serves 04:09.030 --> 04:10.410 a single purpose. 04:10.410 --> 04:15.320 Well, if you have two different functionalities to implement, you will have two different classes 04:15.320 --> 04:17.060 that's similar for nodes. 04:17.090 --> 04:23.930 Nodes are combined into a graph and communicate between each other using for example, topics, services, 04:23.930 --> 04:28.430 parameters, etc. we will cover the communication tools in the next sections. 04:28.460 --> 04:32.210 Now what are the characteristics and benefits of nodes? 04:32.240 --> 04:35.390 First, they help you reduce code complexity. 04:35.630 --> 04:40.850 If you correctly separate your application into packages and nodes, then it will be much easier for 04:40.850 --> 04:42.530 you to scale your application. 04:42.560 --> 04:43.640 Believe me on that. 04:43.640 --> 04:49.520 If you write everything into one big block of code, then after some time you will spend more time fixing 04:49.520 --> 04:52.670 your code than actually developing new functionalities. 04:52.700 --> 04:55.700 Nodes also provide great fault tolerance. 04:55.700 --> 05:01.250 If you run your nodes in different processes, then they are not directly linked and they can still 05:01.250 --> 05:04.520 continue to communicate thanks to Ross two communications. 05:04.520 --> 05:08.840 If one node crashes, it will not make the other nodes crash first. 05:08.870 --> 05:10.220 That's great for debugging. 05:10.220 --> 05:15.230 And second, that's great if you have, let's say, a critical node running your hardware that is well 05:15.260 --> 05:19.520 tested and you just add another node in your program that you want to test. 05:19.550 --> 05:24.770 Even if this later node can crash, it will not affect the critical hardware node. 05:24.770 --> 05:27.890 And then Ros2 is language agnostic. 05:27.890 --> 05:34.820 It means that you can write one node in Python, another node in Cplusplus, and both nodes can communicate 05:34.820 --> 05:36.710 without any problem. 05:36.740 --> 05:42.500 Python and Cplusplus are the two most common languages for Ros, and with some other libraries you can 05:42.500 --> 05:44.480 also use other languages. 05:44.480 --> 05:46.700 This characteristic is really great. 05:46.700 --> 05:52.070 For example, you can choose to develop most of your application in Python, while some nodes will be 05:52.070 --> 05:56.090 written in Cplusplus because they need fast execution speed. 05:56.090 --> 05:58.850 And just a quick note, and we're going to come back to this later. 05:58.880 --> 06:01.400 Two nodes cannot have the same name. 06:01.400 --> 06:06.770 If you want to run multiple instances of the same node, you will have to change their name or add them 06:06.770 --> 06:08.870 into a different namespace for example. 06:08.900 --> 06:09.410 All right. 06:09.410 --> 06:15.440 So in the next lessons we will see how to actually create nodes in both Python and C plus. 06:15.440 --> 06:21.440 Plus how to use them with command line tools and then how to make them communicate between each other.