[953] tedge watchdog (#1026)

* [953] tedge-systemd-watchdog

* update service file to watchdog

* Closes #953 thin-edge watchdog

* health check for all the services

* update document

3 files changed, 62 insertions, 1 deletions

diff --git a/docs/src/SUMMARY.md b/docs/src/SUMMARY.md
index 3f504210..4f608c57 100644
--- a/docs/src/SUMMARY.md
+++ b/docs/src/SUMMARY.md
@@ -34,6 +34,7 @@
     - [How to use apama software management plugin](./howto-guides/017_apama_software_management_plugin.md)
     - [How to change temp path](./howto-guides/018_change_temp_path.md)
     - [How to use thin-edge.io with your preferred init system](./howto-guides/019_how_to_use_preferred_init_system.md)
+    - [How to enable watchdog using systemd?](./howto-guides/021_enable_tedge_watchdog_using_systemd.md)
 
 - [Developer Documentation](dev_doc.md)
   - [Architecture](architecture/README.md)
diff --git a/docs/src/howto-guides/021_enable_tedge_watchdog_using_systemd.md b/docs/src/howto-guides/021_enable_tedge_watchdog_using_systemd.md
new file mode 100644
index 00000000..aedaa335
--- /dev/null
+++ b/docs/src/howto-guides/021_enable_tedge_watchdog_using_systemd.md
@@ -0,0 +1,59 @@
+# Watchdog feature using `systemd` in `thin-edge.io`
+
+## Introduction
+
+`Watchdog` feature is used to check the health of a service/process by constantly exchanging
+the message between the watchdog process and the service/process that is being monitored.
+When the process does not update its health status within a specified `time` period, then the watchdog process will
+restart the service that is being monitored.
+
+This document shows how `thin-edge.io` services are managed using the systemd, then one can use `systemd`
+feature to check the health of these services as well. This document provides
+information about how to use `systemd` for checking the health of the services.
+
+## Enabling the `watchdog` feature in `systemd`
+
+Enabling the `watchdog` feature in systemd for a `thin-edge.io` service (tedge_agent, tedge_mapper_c8y/az/collectd)
+using the `systemd` is a two-step process.
+
+### Step 1: Enable the `watchdog` feature in the `systemd` service file
+For example to enable the `watchdog` feature for `tedge-mapper-c8y` service, update systemd service file as shown below.
+
+Add `tedge_watchdog.service` in  `After` under `[Unit]` section.
+Add `WatchdogSec=5` under `[Service]` section.
+
+The sample service file after updating looks as below.
+
+```shell
+[Unit]
+Description=tedge-mapper-c8y converts Thin Edge JSON measurements to Cumulocity JSON format.
+After=syslog.target network.target mosquitto.service tedge_watchdog.service
+
+[Service]
+User=tedge-mapper
+ExecStart=/usr/bin/tedge_mapper c8y
+Restart=on-failure
+RestartPreventExitStatus=255
+WatchdogSec=5
+```
+
+> Note: The systemd service file usually present in `/lib/systemd/system/tedge-mapper-c8y.service`.
+
+### Step 2: Start the `tedge-watchdog` service
+
+Start the `watchdog` service as below.
+```shell
+systemctl start tedge-watchdog.service
+```
+
+Now the `tedge-watchdog` service will be keep sending health check messages for every `WatchdogSec/2` seconds.
+Once the response is received from the particular service, the `watchdog` service will send the notification
+to the systemd on behalf of the service.
+
+## Debugging
+One can observe the message exchange between the `service` and the `watchdog` by subscribing to `tedge/health/#` and `tedge/health-check/#` topics.
+For more info check [here](./020_monitor_tedge_health)
+
+> Note: If the watchdog service did not send the notification to the systemd within `WatchdogSec`, then the systemd will kill the existing service process and restarts it.
+
+> Note: [Here](https://www.medo64.com/2019/01/systemd-watchdog-for-any-service/) is an example about using `systemd watchdog` feature.
\ No newline at end of file
diff --git a/docs/src/howto-guides/README.md b/docs/src/howto-guides/README.md
index bcc3ce42..e8c6b8b4 100644
--- a/docs/src/howto-guides/README.md
+++ b/docs/src/howto-guides/README.md
@@ -14,4 +14,5 @@
 12. [How to connect an external device?](./013_connect_external_device.md)
 13. [How to access the logs on the device?](./014_thin_edge_logs.md)
 14. [How to install thin-edge.io on any Linux OS (no deb support)?](./015_installation_without_deb_support.md)
-16. [How to manage apama software artefatcs with apama plugin?](./017_apama_software_management_plugin.md)
\ No newline at end of file
+16. [How to manage apama software artefatcs with apama plugin?](./017_apama_software_management_plugin.md)
+17. [How to enable watchdog using systemd?](./021_enable_tedge_watchdog_using_systemd.md)
\ No newline at end of file