docs: Document commands to de-register entities #2700

Author: albinsuresh

docs/src/operate/registration/deregister.md

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+---
+title: Deregister entities
+tags: [Child-Device, Registration]
+sidebar_position: 1
+description: Deregister child devices and services from %%te%%
+---
+
+# Deregister entities
+
+All the entities (devices and services) registered with %%te%% are stored with the MQTT broker as retained messages,
+with their metadata spread across multiple topics.
+For example, for a child device `child01`, the registration message is stored on the `te/device/child01//` topic,
+its twin data is stored across several `te/device/child01///twin/<twin-data-type>` topics,
+its command metadata stored across several `te/device/child01///cmd/<cmd-type>` topics and so on.
+
+On top of that, when a device has services associated with it, or has nested child devices,
+they all have their own respective registration topics and other metadata topics.
+So, deregistering a device involves deregistering itself, its metadata and
+the complete entity hierarchy that is associated with it.
+
+Even though %%te%% doesn't provide a direct API for this yet, this can be easily be done using third-party tools
+like `mosquitto_sub` that supports clearing multiple retained messages together using the `--remove-retained` option.
+
+## Deregister a child device and its services
+
+When the topic ids of the devices and services follow the default topic scheme,
+which maintains the service topic ids (`te/device/<device-id>/service/<service-id>`)
+directly under the device topic id (`te/device/<device-id>//`) in the hierarchy,
+a child device along with all of its services can be deleted as follows:
+
+```sh
+mosquitto_sub --remove-retained -W 3 -t "te/device/child01/+/+/#"
+```
+
+* The topic filter `te/device/child01/+/+` covers the registration messages of the device and all of its services,
+  and the trailing `#` covers their their metadata messages as well.
+* The `--remove-retained` clears all the retained messages matching that topic filter.
+  Running the same command without the `remove-retained` option shows the list of messages
+  that would be cleared with this command, which can be used for dry runs before the entities are really removed.
+* The `-W 3` option is used to stop the command after a timeout period of 3 seconds, without which the command will not exit.
+  Adjust this timeout value based on the number of entities in your deployment.
+
+:::note
+The entities in the cloud must also be removed separately as deregistrations on the device
+are not propagated to the cloud.
+The Cumulocity mapper must also be restarted if the same entity is to be recreated after deregistration.
+:::
+
+A single service of a device (main or child), can be deregistered as follows:
+
+```sh
+mosquitto_sub --remove-retained -W 3 -t "te/device/main/service/service01/#"
+```
+
+All the services associated to a device can be deregistered together, as follows:
+
+```sh
+mosquitto_sub --remove-retained -W 3 -t "te/device/child01/service/+/#"
+```
+
+:::note
+De-registering parent and child entities together with a single command only works
+when the parent and children are hierarchically linked via the topics, which can be queried with a single topic filter,
+as in the case of services while following the default topic scheme.
+While using custom topic schemes, that do not maintain the same topic prefix for a device and its services,
+querying and deregistering them may not be a on-liner as in the examples listed above.
+:::
+
+## Deregister all child devices and services
+
+All the child devices and services can be deregistered using a wildcard topic filter that covers all the entities
+combined with a topic exclusion filter for the main device, as follows:
+
+```sh
+mosquitto_sub -v -t "te/device/+/#" -T "te/device/main/#"
+```
+
+## Unsupported cases
+
+While using the default topic scheme, the following use-cases are not supported as it's not possible to define 
+wildcard topic filters for the same, as the topic scheme does not capture the parent-child relationship of devices:
+
+* Deregister all child devices of a given device
+* Deregister the entire nested child device hierarchy of a device
+
+## Custom topic schemes
+
+Most of the commands listed above are applicable only when the default topic scheme is used.
+The same rules, especially the ones involving multiple entities, do not apply to custom topic schemes,
+as different entities would be linked differently based on the topic scheme that's used.
+The filtering capabilities would also very from scheme to scheme.
+As long as you can define topic filters to select individual entities or a set of entities,
+the same can be used for deregistering as well.

docs/src/operate/registration/index.md

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+---
+title: Registration
+tags: [Registration, Deregistration]
+sidebar_position: 11
+---
+
+import DocCardList from '@theme/DocCardList';
+
+Managing registration and deregistration of child devices and services on %%te%%.
+
+<DocCardList />