Open-source documentation for users of Advantech's WebAccess/DMP software

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Device Desired and Reported states

To configure a device you need access to it, either to its web server or through SSH. When connected to a device all changes made are immediately applied and, if needed, services are restarted.

Remote device configuration on the other hand is a different business. It adds a second actor that can receive configuration changes and send them to the device. This raises new issues: Will this actor save a copy of all the properties or request them to the device everytime? What if the device is temporarily not available? These and other questions are addressed by the solution DMP implemented. Desired and Reported states is not a new idea, especially now that cloud management of IoT devices is increasingly popular. We took ideas from other solutions but aimed to make ours as simple and lightweight as possible.

If you are curious about the ideas behind, please visit the documentation for the following projects:

How it works

Desired Reported

DMP defines two sets of configuration:

In this model the Desired state can only be modified by the user, while the Reported only by the device. DMP is in charge of sending the difference between the two to the device, so eventually both Desired and Reported are the same. Devices are the source of truth, so the Desired state has to be updated at any change so DMP can take correct decisions.

Two things are implied in this explanation. The device needs a process capable of monitoring changes in order to report them and also be able to apply changes received from DMP. DMP also needs to be able to send changes to the device, possibly waiting for the device to connect. If the device doesn’t apply the changes DMP should be able to re-apply them.

DMP client

Routers allow extensibility through User Modules. We created the “WebAccess/DMP client” User Module with the purpose of being the service that updates DMP when the router changes and applies all changes sent by DMP, among other tasks. It mantains a connection with DMP using the MQTT protocol so changes in both sides can be reported to the other end.

Beacuse the router is the source of truth, when this client connects it reports all its configuration. This allows DMP to have a precise report of all the changes done when the device was disconnected.

Configuration on routers is naturally separated into sections, so we take advantage of it by sending to DMP all the properties of that section only. This allows for less information exchanged and is a practical difference with the other similar solutions.

The client knows were the changes need to be applied and what extra steps need to be taken. When finished, it reports back the changed configuration.

Applying configuration changes

Everytime the Desired state is changed DMP will determine the difference with the reported state and send it to the device. If the device is online it will receive the change immediately, but if it is offline all subsequent changes will be merged into the Desired state. When the device connects again it will receive only one change request per section modified. This behavior has a downside. For example, if a user wants to apply a set of changes, wait or request a reboot, and then apply some other changes to the same section while the device is offline, all changes will be merged into one request only. In this exeptional scenario, the user should wait for the device to reboot before applying the second set of changes.

Everything is an Application

DMP treats both Firmware and User Modules as applications. An application is a versioned set of properties grouped in sections. It can be replaced by a different version that may have a different set of properties. The Firmware is still a special application because it is related to the device type and cannot be uninstalled. This simplification allows to use Desired and Reported states in all applications.

How to use Desired and Reported states

DMP groups all properties into sections. The DMP UI reflects this on the left column of the configuration screen: Device configuration The desired value always takes priority in the UI. The reported is shown only if is different to the reported (like the “Name” property above, that was changed but not saved) or if there is no desired value at all. This simplified view allows to focus on what the user wants to change, showing details about the reported data only when is relevant.

The API on the other hand exposes all values to the user, that can then process and present it in the way it seems more appropiate. The API allows a user more control over the presentation as the way to do it is not imposed by DMP.

The following endpoints are the ones that allow us to interact with Desired and Reported states: Desired Reported endpoints

For example, the image of the UI above called the following endpoint to get the Desired and Reported states for the SNMP section : Device Desired and Reported through the API

Re-applying configuration with SyncEngine

When a configuration change is applied by a user DMP will send it to the device immediately, or as soon as the device connects. If the change is not applied by the device, DMP needs to re-apply the changes as configured previously by the user. This is the job of the SyncEngine.

SyncEngine is an active agent in DMP in charge of detecting not applied differences in all devices, and re-appling them following the policy set for the company the device belongs to.

This is how the SyncEngine can be configured: SyncEngine SyncEngine parameters

Sync Type “Never” disables the SyncEngine for the company. “Forever” will keep re-applying every “retry_interval” seconds until the device reports changes were applied. Custom mode will retry “retry_attempts” times.

When installing or removing applications SyncEngine also supervises the change following the company’s policy. SyncEngine will ensure an application is changed, retrying if needed.