fixup! Describe the MQTT bus

docs/src/understand/mqtt-bus.md

@@ -1,7 +1,7 @@
 ---
 title: MQTT Bus
 tags: [Concept, MQTT]
-sidebar_position: 1
+sidebar_position: 3
 ---
 
 # Thin Edge MQTT bus
@@ -18,6 +18,23 @@ between the miscellaneous hardware and software components that make up a piece
 Collectively, the MQTT broker and the normalized MQTT topic schema form the __MQTT bus__.
 The local HTTP server and the normalized URLs schema are referred as the __file transfer service__.
 
+## Local Digital Twin
+
+The MQTT topic schema establishes the local representation of a piece of equipment, with its main device, child devices and services.
+This local representation has a one-to-one relationship with the remote digital twin.
+However, while the latter is a user interface the former is akin to a program interface, an API.
+
+- the main device as well as the child devices are each represented by a topic
+- each type of telemetry data, measurements, events, alarms, is given a specific sub-topic of the topic related to the source device
+- each type of operations is given a specific sub-topic of the topic related to the target device
+
+## Cloud Bridges
+
+The MQTT bus is extended to connect the cloud using an MQTT bridge
+that publishes back and forth messages from the local MQTT broker to the remoted MQTT endpoint.
+
+Such a bridge is by definition cloud specific - each kind of messages having to be sent or received from specific topics.
+
 ## Local File Transfer
 
 Using MQTT is appropriate to collect telemetry data from devices and services
@@ -27,19 +44,9 @@ However, it's more pertinent to use HTTP to transfer files from one device to an
 And this is not only because of payload size constraints, but also for *pull* versus *push* constraints.
 
 By combining MQTT with HTTP, a thin-edge service can *notify* a child-device that a file is *available* for local transfer
-and then let the child-device *download* the file when *ready*. 
+and then let the child-device *download* the file when *ready*.
 
 A notable use is to install on a child-device a new version of firmware, software package or configuration file.
 The device profile service will start the update operation by making the associated file locally available,
 before notifying the child-device of the update request
 and simply monitoring the progress of the operation once delegated to the child-device.
-
-## Local Digital Twin
-
-TODO: explain how the MQTT topic schema establishes the local representation of the device,
-with the child devices, the services, their hierarchy ...
-
-## Cloud Bridges
-
-TODO: explain that the MQTT bus is extended to connect the cloud.
-