Step 4: Deploy and test the component on a core device

In this section, you deploy the component to the core device to test its functionality. On the core device, you create the virtual battery level file to imitate a real battery. Then, you create additional deployments and observe the component log files on the core device to see the component defer and acknowledge updates.

To deploy and test the Hello World component that defers updates

Use a text editor to create a virtual battery level file. This file imitates a real battery.
- On Linux core devices, create a file named /home/ggc_user/virtual_battery.json. Run the text editor with sudo permissions.
- On Windows core devices, create a file named C:\Users\ggc_user\virtual_battery.json. Run the text editor as an administrator.
For example, on a Linux-based system, you can run the following command to use GNU nano to create the file.
```
sudo nano /home/ggc_user/virtual_battery.json
```
Copy the following JSON into the file.
```
{
  "battery_level": 50
}
```
Deploy the Hello World component to the core device. Do the following:
1. In the Amazon IoT Greengrass console navigation menu, choose Components.
2. On the Components page, choose the My components tab, and then choose com.example.BatteryAwareHelloWorld.
3. On the com.example.BatteryAwareHelloWorld page, choose Deploy.
4. From Add to deployment, choose an existing deployment to revise, or choose to create a new deployment, and then choose Next.
5. If you chose to create a new deployment, choose the target core device or thing group for the deployment. On the Specify target page, under Deployment target, choose a core device or thing group, and then choose Next.
6. On the Select components page, verify that the com.example.BatteryAwareHelloWorld component is selected, choose Next.
7. On the Configure components page, select com.example.BatteryAwareHelloWorld, and then do the following:
  1. Choose Configure component.
  2. In the Configure com.example.BatteryAwareHelloWorld modal, under Configuration update, in Configuration to merge, enter the following configuration update.
```
{
  "BatteryThreshold": 70
}
```
  3. Choose Confirm to close the modal, and then choose Next.
8. On the Confirm advanced settings page, in the Deployment policies section, under Component update policy, confirm that Notify components is selected. Notify components is selected by default when you create a new deployment.
9. On the Review page, choose Deploy.
  
  The deployment can take up to a minute to complete.
The Amazon IoT Greengrass Core software saves stdout from component processes to log files in the logs folder. Run the following command to verify that the Hello World component runs and prints status messages.
Linux or Unix
```
sudo tail -f /greengrass/v2/logs/com.example.BatteryAwareHelloWorld.log
```
Windows Command Prompt (CMD)
```
type C:\greengrass\v2\logs\com.example.BatteryAwareHelloWorld.log
```
PowerShell
```
gc C:\greengrass\v2\logs\com.example.BatteryAwareHelloWorld.log -Tail 10 -Wait
```
You should see messages similar to the following example.
```
Hello, World! Battery level (50) is below threshold (70), so the component will defer updates.
```
Note
If the file doesn't exist, the deployment may not be complete yet. If the file doesn't exist within 30 seconds, the deployment likely failed. This can occur if the core device doesn't have permission to download the component's artifacts from the S3 bucket, for example. Run the following command to view the Amazon IoT Greengrass Core software log file. This file includes logs from the Greengrass core device's deployment service.
Linux or Unix

sudo tail -f /greengrass/v2/logs/greengrass.log

Windows Command Prompt (CMD)

type C:\greengrass\v2\logs\greengrass.log

The type command writes the file's contents to the terminal. Run this command multiple times to observe changes in the file.

PowerShell

gc C:\greengrass\v2\logs\greengrass.log -Tail 10 -Wait
Create a new deployment to the core device to verify that the component defers the update. Do the following:
1. In the Amazon IoT Greengrass console navigation menu, choose Deployments.
2. Choose the deployment that you created or revised earlier.
3. On the deployment page, choose Revise.
4. In the Revise deployment modal, choose Revise deployment.
5. Choose Next at each step, and then choose Deploy.
Run the following command to view the component's logs again, and verify that it defers the update.
Linux or Unix
```
sudo tail -f /greengrass/v2/logs/com.example.BatteryAwareHelloWorld.log
```
Windows Command Prompt (CMD)
```
type C:\greengrass\v2\logs\com.example.BatteryAwareHelloWorld.log
```
PowerShell
```
gc C:\greengrass\v2\logs\com.example.BatteryAwareHelloWorld.log -Tail 10 -Wait
```
You should see messages similar to the following example. The component defers the update for 30 seconds, so the component prints this message repeatedly.
```
Deferred update for deployment 50722a95-a05f-4e2a-9414-da80103269aa.
```
Use a text editor to edit the virtual battery level file and change the battery level to a value above the threshold, so the deployment can proceed.
- On Linux core devices, edit the file named /home/ggc_user/virtual_battery.json. Run the text editor with sudo permissions.
- On Windows core devices, edit the file named C:\Users\ggc_user\virtual_battery.json. Run the text editor as an administrator.
For example, on a Linux-based system, you can run the following command to use GNU nano to create the file.
```
sudo nano /home/ggc_user/virtual_battery.json
```
Change the battery level to 80.
```
{
  "battery_level": 80
}
```

Run the following command to view the component's logs again, and verify that it acknowledges the update.

You should see messages similar to the following examples.


Hello, World! Battery level (80) is above threshold (70), so the component will acknowledge updates.
Acknowledged update for deployment f9499eb2-4a40-40a7-86c1-c89887d859f1.

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Step 3: Publish the component to the Amazon IoT Greengrass service

Interact with local IoT devices over MQTT

Step 4: Deploy and test the component on a core device

To deploy and test the Hello World component that defers updates

Note