Command & Control
Business applications can send commands to devices by means of the Command & Control API. This concept page describes how this API is used by applications to send commands and describes how Hono’s protocol adapters process the commands so that they reach their target device.
Commands can be sent following a request/response or a one-way pattern. For Request/Response commands, there is always a response expected from the device.
In order for devices to be able to receive commands, they first have to connect to a Hono protocol adapter and indicate
their availability to receive commands. For devices communicating via AMQP or MQTT, this means connecting to an adapter
and explicitly subscribing for commands. For devices sending messages via HTTP or CoAP, this means using the
(time till disconnect) parameter when sending an event or telemetry message, thereby indicating for how long the device
will wait for a command message.
The protocol adapter will then forward a notification to downstream business applications about the device being able to receive commands and for how long.
An application can send a command to such a device via the used messaging infrastructure. Hono will receive the command by means of the Command Router component and will forward it to the protocol adapter instance that the device is connected to. The protocol adapter will then send the command to the device. In case of a request/response command, the device is expected to send back a command response message.
When the device explicitly ends the command subscription, the protocol adapter will send a corresponding notification to downstream applications.
Command & Control involving a gateway
Hono has special support for sending commands to devices that are connected to Hono via a gateway device. The gateways that a device may use to connect to Hono’s protocol adapters need to be configured in Hono’s device registry as part of the device provisioning process.
When sending commands, applications do not need to know to which gateway the command target device is connected to. An application sends the command with the device address and Hono will direct the command to a gateway that has subscribed for such commands and that is configured to act on behalf of the command target device. If there are multiple matching gateways, the one that the command target device was last connected to is chosen. The information about which gateways are subscribed and which gateway a device has last communicated by is managed via the Command Router API.
Message flow using AMQP messaging network
The following sections and the contained sequence diagrams provide an overview of a device indicating its availability for receiving commands, of an application sending a command to the device and of the device sending back a command response.
Device indicating availability to receive commands
When the Device subscribes for commands (1), the Protocol Adapter registers the command consumer with the Command Router service, associating the device with its protocol adapter instance identifier (2). The Command Router service creates a receiver link scoped to the device’s tenant (3) if it doesn’t exist yet. Following that, the notification about the device subscription is sent to the Application via the AMQP messaging network (4).
Business application sending a command to the device
Upon receiving the notification, the Application prepares sender and command response receiver links (1,2) and sends
the command message to the AMQP messaging network. The message is received by the Command Router service component (3),
which will determine the Protocol Adapter instance #1 that is able to handle the command message. The command then gets
forwarded to the AMQP messaging network on the address for adapter instance #1 (4). The Protocol Adapter instance #1
receives the message (5) and forwards it to the Device (6). As the last step, an
accepted disposition will be sent
back to the Application (7).
Device sending a command response message
The command response message is sent back to the Application from the Protocol Adapter via the AMQP messaging network. Note that the Command Router is not involved in this transfer.