WEBVTT 00:00.080 --> 00:04.370 Let's now create a service server so you can practice as soon as possible. 00:04.370 --> 00:06.140 And we will start with Python. 00:06.140 --> 00:13.460 We will create a service server that will take two numbers as a request and return the sum as a response. 00:13.460 --> 00:18.080 I'm using this example so we can use an existing interface that's already installed. 00:18.080 --> 00:22.160 And we will see how to create custom interfaces in the next section. 00:22.190 --> 00:22.400 Okay. 00:22.430 --> 00:26.090 So before we write any code let's focus on the interface. 00:26.090 --> 00:32.180 Because a service is defined by a name and an interface, the interface we will use. 00:32.180 --> 00:39.500 So you can do Ros2 interface show is also in the example interfaces package. 00:39.500 --> 00:45.770 But then so for topics you had MSG for services you have serve. 00:46.040 --> 00:48.050 And then we have Add. 00:48.050 --> 00:50.870 So you can find the add to interface. 00:50.870 --> 00:53.600 That's the service interface that we will be using. 00:53.600 --> 00:55.130 And let's press enter. 00:55.130 --> 01:00.880 And this is the interface the first big important thing that you can notice. 01:00.910 --> 01:01.810 And that's the most important. 01:01.840 --> 01:04.960 Here you can see we have three dashes here. 01:04.960 --> 01:06.490 Why do we have three dashes? 01:06.490 --> 01:11.170 Because as I told you previously, a service will contain two messages. 01:11.170 --> 01:13.060 It will contain a request. 01:13.060 --> 01:16.210 So that's what's going from the client to the server. 01:16.210 --> 01:19.150 And then it will contain a response. 01:19.150 --> 01:22.210 And that's the response from the server to the client. 01:22.240 --> 01:29.080 This here those three dashes is the separation between the request and the response. 01:29.080 --> 01:35.110 So you can see that as basically two different messages together with a separation here. 01:35.110 --> 01:37.570 And what do we have in the request. 01:37.600 --> 01:39.670 We have two fields here. 01:39.670 --> 01:46.150 So you see that this time we have two fields two integer numbers named a and b. 01:46.150 --> 01:49.450 So we need to send a and b as a request from the client. 01:49.450 --> 01:52.900 And then the response is a integer number. 01:52.900 --> 01:54.670 Which name is some. 01:54.700 --> 01:55.000 All right. 01:55.030 --> 01:57.130 So from the client you need to send a and b. 01:57.130 --> 02:03.180 And then you receive that in the server you process the service and then you return the response with 02:03.180 --> 02:05.100 a sum field. 02:05.130 --> 02:05.730 All right. 02:05.730 --> 02:09.840 And now let's see how we can actually use that inside our code. 02:09.870 --> 02:12.840 Let's go back to our workspace. 02:12.840 --> 02:18.990 And I'm still going to go to my Python package that was already set up in this. 02:19.020 --> 02:22.290 We already have all the previous nodes from the previous sections. 02:22.290 --> 02:26.730 I'm going to add a new file for the server. 02:26.730 --> 02:29.250 So the name of the interface is add to int. 02:29.250 --> 02:35.460 I'm just going to name it add to int server dot pi. 02:35.460 --> 02:39.900 And later on we're going to also create the add to int client.py. 02:39.900 --> 02:41.640 So like this. 02:41.640 --> 02:45.090 Let's make it executable okay. 02:45.090 --> 02:47.100 And we have the file right here. 02:47.130 --> 02:59.620 Now let's open vs code from the source folder of the workspace with code dot okay let's go back to This 02:59.620 --> 03:01.330 package and the file. 03:01.330 --> 03:05.470 And no surprise here we will start with the template. 03:05.500 --> 03:05.740 Okay. 03:05.770 --> 03:07.480 Because we're creating a new node. 03:07.480 --> 03:13.480 So I'm just going to take the code from here and add it to add to in server. 03:13.990 --> 03:24.010 And I'm going to name this add to int seven node for example like that. 03:24.010 --> 03:31.780 And let's name the node add to int um with with server. 03:33.370 --> 03:38.260 Let's remove that and that and that one. 03:38.260 --> 03:38.770 Great. 03:38.770 --> 03:42.550 And now how to import a service interface. 03:42.550 --> 03:45.460 Well that's the same thing as we did for topics. 03:45.790 --> 03:47.500 We're going to just import that. 03:47.500 --> 03:51.790 But you see the difference here is that we don't have an MSG folder. 03:51.790 --> 03:52.900 It's an SV. 03:53.020 --> 04:06.900 So we are going to do from example interfaces Dot and you see we have s r v and then import add to ints. 04:06.930 --> 04:07.170 Okay. 04:07.200 --> 04:08.790 You can see it's found here. 04:08.790 --> 04:10.800 And as you can see we're going to use that later. 04:10.800 --> 04:15.210 We have added to INTs request and add to ints response. 04:15.390 --> 04:17.550 So let's import add to it. 04:17.580 --> 04:21.240 Now we are using the example interfaces package. 04:21.240 --> 04:26.850 We've already set the dependency in the package dot XML, but make sure that it's here. 04:26.970 --> 04:28.800 Okay so nothing more to do with that. 04:28.800 --> 04:31.470 And let's create a service server. 04:31.470 --> 04:33.450 Where do we create service server. 04:33.450 --> 04:36.000 We're going to do that in the constructor of the node. 04:36.120 --> 04:36.270 Okay. 04:36.300 --> 04:41.220 So just like we did for publishers and subscribers you're going to create your service servers inside 04:41.220 --> 04:42.120 the constructor. 04:42.120 --> 04:44.850 And so let's do self server. 04:44.880 --> 04:46.230 Let's name it server. 04:46.620 --> 04:47.220 Okay. 04:47.250 --> 04:49.800 We're going to save it as an attribute of the class. 04:49.800 --> 04:51.510 And how to create a server. 04:51.510 --> 04:56.490 Well it's quite easy you do self so you get access to the node functionalities. 04:56.490 --> 04:58.850 Create Service. 04:59.120 --> 05:03.140 When you do create service, it's going to create a service server. 05:03.170 --> 05:07.190 The first thing you need to provide is the service type which is that one. 05:07.190 --> 05:09.830 So add two ints. 05:09.860 --> 05:12.830 The second thing you need to provide is the name. 05:12.860 --> 05:16.550 Okay, so that's very similar to what we did with publishers and subscribers. 05:16.550 --> 05:19.190 You need to provide a type and a name. 05:19.190 --> 05:20.210 What is the name. 05:20.210 --> 05:23.300 So the name you choose it when you create the service. 05:23.300 --> 05:28.670 So you choose it here I'm going to use add to int okay. 05:28.700 --> 05:32.840 Because we are adding two ints and one very important thing. 05:32.870 --> 05:34.280 It's quite an important tip here. 05:34.280 --> 05:39.050 I would recommend to use a verb okay for a service name. 05:39.050 --> 05:39.500 Why? 05:39.530 --> 05:44.780 Because service will be used for mainly two things when you need to do some actions. 05:44.780 --> 05:46.610 So do something. 05:46.610 --> 05:53.450 Enable motor, disable motor, turn on LED okay that kind of stuff or a computation. 05:53.450 --> 05:56.450 So add to ints compute area. 05:56.480 --> 05:58.540 Compute inverse kinematics. 05:58.570 --> 05:58.930 Okay. 05:58.960 --> 06:01.750 Those are things that you can use services for. 06:01.750 --> 06:06.970 And because it's an action so action or computation you are going to use a verb. 06:07.000 --> 06:07.450 Great. 06:07.450 --> 06:12.250 So now we have the type name and we need to provide a callback. 06:12.250 --> 06:12.970 Why is that. 06:12.970 --> 06:17.560 Because well when you create a server the server is not going to do anything on its own. 06:17.560 --> 06:18.130 You need. 06:18.160 --> 06:21.820 Then you need a client to send a request to the server. 06:21.820 --> 06:25.270 So we are going to process the request as we receive them. 06:25.270 --> 06:27.490 And for that we need a callback. 06:27.490 --> 06:30.340 So I'm going to create a callback here. 06:30.340 --> 06:35.200 Let's call it callback add two ints. 06:35.200 --> 06:41.260 So what I do usually to name the callbacks for the service servers I use something similar as I do for 06:41.260 --> 06:46.360 subscribers is I start with callback, and then I put the name of the service and we have self. 06:46.360 --> 06:49.000 And then what do we receive as parameters? 06:49.000 --> 06:50.740 We are going to receive two parameters. 06:50.740 --> 06:54.670 We are going to receive the request and also the response. 06:54.670 --> 06:57.720 So we get a request object and a response object. 06:57.720 --> 07:01.020 From that we can get the request from the client. 07:01.020 --> 07:06.270 And then the response is well, already the response object that then we need to return. 07:06.270 --> 07:16.050 I'm going to make it a bit more explicit by saying that this is an add to insert dot request, and that 07:16.050 --> 07:21.570 one is an add to insert dot response. 07:23.190 --> 07:23.520 Okay. 07:23.520 --> 07:26.010 So this way it's more explicit. 07:26.040 --> 07:27.360 We're going to make less mistakes. 07:27.360 --> 07:31.920 And we also have nice auto completion features here for this callback okay. 07:31.950 --> 07:36.810 You can see that the add to insert here you can clearly see it's not one message. 07:36.810 --> 07:38.790 It's a combination of two messages. 07:38.790 --> 07:42.120 And the first message is add to insert request. 07:42.150 --> 07:45.450 The second message is add to insert response. 07:45.450 --> 07:47.100 And so what do we want to do here. 07:47.100 --> 07:48.330 Well it's pretty simple. 07:48.330 --> 07:51.000 We want to compute the sum of. 07:51.030 --> 08:00.250 So if I go back here of the a and the b fields that we receive from the request so I can do some is 08:00.250 --> 08:08.050 equal to request dot a plus request dot b. 08:08.050 --> 08:12.400 And I have the autocompletion here because I have provided the type okay. 08:12.430 --> 08:25.000 And then I can put the some inside the response so I can actually do directly response dot some okay 08:25.030 --> 08:26.170 response dot some. 08:26.170 --> 08:28.420 Because that's what we have here. 08:29.560 --> 08:36.280 Response dot some is equal to request dot a plus request dot b. 08:36.280 --> 08:37.210 And that's it. 08:37.240 --> 08:42.850 Actually there is one more thing you need to do is to return the response object. 08:42.880 --> 08:43.060 Okay. 08:43.090 --> 08:50.200 So that's a quite common mistake with services in Python is that you fill the response correctly but 08:50.200 --> 08:52.210 then you forget to return something okay. 08:52.240 --> 08:54.220 This method needs to return. 08:54.220 --> 08:57.600 This callback needs to return the response if you don't do it. 08:57.630 --> 08:59.640 Well, your program is going to be installed. 08:59.640 --> 09:05.670 You can run your node, but then when you send a request, you will get an exception from that callback. 09:05.670 --> 09:07.380 So just in case no. 09:07.380 --> 09:11.400 Again, once again, I repeat that you should absolutely return the response. 09:11.400 --> 09:13.350 And I'm also going to add a log. 09:13.350 --> 09:17.430 So it's going to be better for us to visualize what's happening. 09:17.970 --> 09:26.220 So self get logger I'm going to say so info I'm just going to print the equation okay. 09:26.220 --> 09:33.480 So let's cast to string request dot a plus. 09:33.480 --> 09:35.490 So let's add a plus sign. 09:35.910 --> 09:43.410 And then let's just go back to a new line with request dot b. 09:44.490 --> 09:46.800 And then we add the equal sign. 09:47.520 --> 09:54.360 And then we add the response dot sum. 09:54.390 --> 09:55.140 All right. 09:55.140 --> 09:58.640 So we just print a plus B is equal to sum. 09:58.640 --> 10:01.250 So we can see what's happening. 10:01.280 --> 10:03.530 The callback is now finished. 10:03.530 --> 10:05.600 We also need to set it here. 10:05.630 --> 10:05.900 Okay. 10:05.930 --> 10:07.730 So we have the create service. 10:07.760 --> 10:11.990 We come back to this with the type the name and callback. 10:11.990 --> 10:16.430 So self dot callback add to ints. 10:16.430 --> 10:18.830 And I'm going to go back to a new line here. 10:19.250 --> 10:19.820 Okay. 10:19.820 --> 10:21.530 So we have three parameters here. 10:21.560 --> 10:23.390 And let's finish with a log. 10:23.390 --> 10:26.990 Get logger with info. 10:26.990 --> 10:34.910 And let's say add to in server has been started. 10:35.600 --> 10:35.990 All right. 10:35.990 --> 10:36.440 And that's it. 10:36.440 --> 10:38.480 So let's do a very quick recap here. 10:38.480 --> 10:40.880 You will first initialize Rosetta communications. 10:40.880 --> 10:45.410 And then you create your node which is going to run the code inside the constructor. 10:45.410 --> 10:47.930 So you have the super line. 10:47.930 --> 10:50.840 And then we create the service server. 10:50.870 --> 10:54.910 You provide the type and name and we also provide a Callback. 10:54.940 --> 10:56.500 Okay, so nothing's going to happen now. 10:56.500 --> 10:58.750 It's just going to register a callback. 10:58.750 --> 11:01.510 When the node spins, it's going to be kept alive. 11:01.510 --> 11:04.330 And when you receive a request okay. 11:04.360 --> 11:08.980 For the add to in server, then this callback is going to be called. 11:08.980 --> 11:13.690 In this callback, you can get the request from the request field here from the parameter. 11:13.690 --> 11:17.290 And then you need to well you need to process that request. 11:17.290 --> 11:19.450 So you may want to validate the data. 11:19.480 --> 11:21.880 You may want to do whatever you want okay. 11:21.910 --> 11:23.200 Depends on your application. 11:23.200 --> 11:28.630 And then you need to fill the response parameter plus return it. 11:28.660 --> 11:29.230 Great. 11:29.230 --> 11:30.760 So let's test that. 11:30.760 --> 11:43.180 Let's go to setup.py and let's add yet another executable add to it server that's going to be my pi 11:43.630 --> 11:48.220 pkg add to server. 11:48.220 --> 11:52.330 So that's the file name here and the main function. 11:52.510 --> 11:52.930 All right. 11:52.930 --> 11:54.600 So you can see it's always the same thing. 11:54.600 --> 12:00.720 We save all the files and let's go back to our roster workspace. 12:00.750 --> 12:04.470 Let's do a click on build packages. 12:04.470 --> 12:08.100 Select with my pi pkg. 12:08.130 --> 12:11.100 You can add the symlink install if you want to. 12:11.100 --> 12:12.570 I'm just going to build it like that. 12:13.200 --> 12:19.620 And now let's just open some new terminals or use the ones that you already have. 12:20.280 --> 12:28.470 And I'm going to run Ros to run my Pi pkg with add two ints server. 12:28.470 --> 12:33.210 And you see the node has been started and now it's waiting. 12:33.210 --> 12:37.230 And so here for example let's do Ros two node list. 12:37.560 --> 12:38.640 We have the node. 12:38.640 --> 12:40.590 Let's do Ros two node info. 12:40.620 --> 12:45.030 We have that and in the service server. 12:45.030 --> 12:47.850 So I'm going to come back to all of those later in this section. 12:47.850 --> 12:51.300 But you can see we have the add two ints okay. 12:51.330 --> 12:54.220 With example interfaces service add to it. 12:54.220 --> 12:55.450 That's the one we have created. 12:55.450 --> 12:56.860 So it's a service server. 12:56.890 --> 12:59.050 Now what we can do already. 12:59.050 --> 13:01.180 So we have the server. 13:01.180 --> 13:03.280 We will need to write a client to test it. 13:03.280 --> 13:07.540 But what we can do already is to test this from the terminal okay. 13:07.570 --> 13:10.900 So I'm going to write the command and I'm going to give more details about that. 13:10.900 --> 13:16.810 There's a dedicated lesson about the Ros two command line tool for services a bit later in this section. 13:16.810 --> 13:19.180 So here I'm just going to write the command to test it. 13:19.180 --> 13:21.370 And you'll get more details later. 13:21.370 --> 13:27.490 We can do Ros two service call and use the service name. 13:27.490 --> 13:30.880 So call add to it. 13:30.910 --> 13:34.120 Then we need to provide the interface. 13:34.120 --> 13:37.300 So the interface is example interfaces etc.. 13:37.300 --> 13:39.940 So just type the beginning and press tab. 13:39.940 --> 13:44.380 So Ros two service call with the service name then the interface. 13:44.380 --> 13:47.830 And then we can provide the request. 13:47.830 --> 13:52.390 So the request if you remember we have an A and a B field. 13:52.390 --> 13:53.760 So we only need to provide this. 13:53.790 --> 13:57.150 We don't provide the response because that's what we're going to receive. 13:57.240 --> 13:59.880 So let's use double quotes. 13:59.880 --> 14:05.370 And in curly brackets we can say a colon and then a space important. 14:05.400 --> 14:13.650 Let's say three and then comma B colon space let's say seven. 14:13.980 --> 14:15.300 Let's press enter. 14:16.110 --> 14:20.640 And you can see on the server side we have this log okay. 14:20.640 --> 14:27.960 And on the client side here on the terminal you see that we sent a request and we get a response with 14:27.960 --> 14:29.370 the sum ten. 14:29.550 --> 14:33.810 So with that we can already validate that the server is working. 14:33.810 --> 14:39.540 But just note that it's only going to work for very simple requests. 14:39.570 --> 14:41.130 Here we just have two fields. 14:41.130 --> 14:47.610 If you have arrays of many different fields, it's going to be very complicated to create a request 14:47.610 --> 14:48.300 from the terminal. 14:48.300 --> 14:50.220 So for that you will need to write some code. 14:50.220 --> 14:53.520 And that's exactly what we're going to do in the next lesson.