This page describes how devices are represented and identified throughout Hono and its API’s.
The main purpose of Hono is to provide a uniform API for applications to interact with devices, regardless of the particular communication protocol the devices natively use. In order to do so, Hono uses a unique logical identifier to refer to each device individually.
Hono does not make any assumptions about the format of a device identifier (or device-id for short). It basically is a string which is defined at the time a device is registered. Once registered, the device can be referred to by this identifier when using Hono’s APIs until the device is unregistered.
Hono supports the logical partitioning of devices into groups called tenants. Each tenant has a unique identifier, a string called the tenant-id, and can be used to provide a logical grouping of devices belonging e.g. to the same application scope or organizational unit. Each device can thus be uniquely identified by the tuple (tenant-id, device-id). This tuple is broadly used throughout Hono’s APIs when addressing a particular device.
Hono components use the Device Registration API to access device registration information. The API defines a single mandatory to implement operation (assert) for retrieving a token asserting a device’s registration status. In addition to that, it defines optional CRUD operations to register, update and remove device registration information. These operations are optional because Hono components do not require them during runtime. From a Hono perspective, it is not important how devices have been registered or how they are managed.
In many real world scenarios there will already be a component in place which keeps track of devices and which supports the particular provisioning process being used to bring devices into life. In such cases it makes sense to simply implement the mandatory operation of Hono’s Device Registration API as a facade on top of the existing component.
For demonstration purposes, Hono comes with a simple default implementation of the Device Registration API which keeps all data in memory only. This component implements all mandatory and optional operations but is not supposed to be used in production scenarios.
Devices connect to protocol adapters in order to publish telemetry data or events. Downstream applications consuming this data often take particular actions based on the content of the messages. Such actions may include simply updating some statistics, e.g. tracking the average room temperature, but may also trigger more serious activities like shutting down a power plant. It is therefore important that applications can rely on the fact that the messages they process have in fact been produced by the device indicated by a message’s source address.
Hono relies on protocol adapters to establish a device’s identity before it is allowed to publish telemetry data or send events. Conceptually, Hono distinguishes between two identities
- an identity associated with the authentication credentials (termed the authentication identity or auth-id), and
- an identity to act as (the device identity or device-id).
A device therefore presents an auth-id as part of its credentials during the authentication process which is then resolved to a device identity by the protocol adapter on successful verification of the credentials.
In order to support the protocol adapters in the process of verifying credentials presented by a device, the Credentials API provides means to look up secrets on record for the device and use this information to verify the credentials.
The Credentials API supports registration of multiple sets of credentials for each device. A set of credentials consists of an auth-id and some sort of secret information. The particular type of secret determines the kind of information kept. Please refer to the Standard Credential Types defined in the Credentials API for details. Based on this approach, a device may be authenticated using different types of secrets, e.g. a hashed password or a pre-shared key, depending on the capabilities of the device and/or protocol adapter.
Once the protocol adapter has resolved the device-id for a device, it uses this identity when referring to the device in all subsequent API invocations, e.g. when forwarding telemetry messages downstream to the Hono Messaging component.