The place of Eclipse Hono™ is within an IoT cloud. The goal is simplified device connectivity for Business Applications (or Solutions). It abstracts away the multiple specifics of different device-side connection protocols from a backend application perspective. Hono defines a small set of APIs to communicate in a unified way with very different devices, whether they use, for example, connection-oriented protocols such as MQTT and AMQP 1.0 or stateless protocols such as HTTP. As a result, Hono frees developers of IoT backend applications from the need to know device protocols in detail and allows them to add or change connection protocols without having to modify their applications.
For each supported connection protocol, Hono contains a microservice called Protocol Adapter, which maps the connection protocol of the device to Hono’s APIs. Those APIs are offered via a central messaging system. Each of the messaging APIs has a so-called northbound API for the Business Applications. The southbound APIs are used by protocol adapters to forward messages from devices to the applications and vice versa. Messages flowing from devices to Business Applications are called downstream messages since this is the primary communication direction in practice. The other communication direction is called upstream. These are messages sent from a Business Application to a device via the Command & Control API.
The APIs defined by Hono for messaging are Telemetry, Event, and Command & Control. The Telemetry API is intended to send arbitrary data, e.g. from sensors, downstream. Events are downstream messages as well, intended less frequent but more important messages. Depending on the provided messaging network, events can be persisted and delivered later on if an application faces a limited downtime. The Command & Control API allows to send messages, like commands, from northbound applications upstream and optionally return a response from a device.
To learn the concepts more in-depth, start reading about the architecture and Hono’s concept of a Device Identity. The concept page Multi-tenancy explains how Hono allows isolating multiple sets of devices and data. Connecting Devices shows different ways to connect a device (directly or via a gateway), and Device Provisioning explains some options to register devices in Hono.