Telemetry API Specification

The Telemetry API is used by Protocol Adapters to send telemetry data downstream. Business Applications and other consumers use the API to receive data published by devices belonging to a particular tenant.

The Telemetry API is defined by means of AMQP 1.0 message exchanges, i.e. a client needs to connect to Hono using AMQP 1.0 in order to invoke operations of the API as described in the following sections. Throughout the remainder of this page we will simply use AMQP when referring to AMQP 1.0.

Southbound Operations

The following operations can be used by Protocol Adapters to forward telemetry data received from devices to downstream consumers like Business Applications.

Forward Telemetry Data

Preconditions

  1. Adapter has established an AMQP connection with the AMQP Messaging Network.
  2. Adapter has established an AMQP link in role sender with the AMQP Messaging Network using target address telemetry/${tenant_id} where ${tenant_id} is the ID of the tenant that the client wants to upload telemetry data for.
  3. The device for which the adapter wants to send telemetry data has been registered (see Device Registration API).

The adapter indicates its preferred message delivery mode by means of the snd-settle-mode and rcv-settle-mode fields of its attach frame during link establishment.

snd-settle-mode rcv-settle-mode Delivery semantics
unsettled, mixed first Using unsettled for the snd-settle-mode allows for adapters to implement both AT LEAST ONCE or AT MOST ONCE delivery semantics, depending on whether the adapter waits for and considers the disposition frames it receives from the AMQP Messaging Network or not. This is the recommended mode for forwarding telemetry data.
settled first Using settled for the snd-settle-mode allows for adapters to implement AT MOST ONCE delivery semantics only. This is the fastest mode of delivery but has the drawback of less reliable end-to-end flow control and potential loss of messages without notice.

All other combinations are not supported by Hono and may result in the termination of the link or connection (depending on the configuration of the AMQP Messaging Network).

Message Flow

As indicated above, it is up to the discretion of the protocol adapter whether it wants to use AT LEAST ONCE or AT MOST ONCE delivery semantics.

Hono’s HTTP adapter allows devices to indicate, which delivery semantics they want to use when uploading telemetry data. The HTTP adapter always forwards messages unsettled and either ignores the outcome of the message transfer (AT MOST ONCE) or waits for the downstream peer to accept the message (AT LEAST ONCE) before acknowledging the reception of the message to the device.

The following sequence diagram illustrates the flow of messages involved in the HTTP Adapter forwarding an unsettled telemetry data message to the downstream AMQP Messaging Network implementing AT MOST ONCE delivery semantics.

Forward telemetry data flow (AT MOST ONCE)

  1. Device 4711 PUTs telemetry data to the HTTP Adapter
    1. HTTP Adapter transfers telemetry data to AMQP 1.0 Messaging Network.
    2. HTTP Adapter acknowledges the reception of the data to the Device.
  2. AMQP 1.0 Messaging Network acknowledges reception of the message which is ignored by the HTTP Adapter.
Info

In the example above the HTTP adapter does not wait for the outcome of the transfer of the message to the AMQP Messaging Network before sending back the HTTP response to the device. If the messaging network had sent a disposition frame with the rejected instead of the accepted outcome, the HTTP adapter would still have signaled a 202 status code back to the device. In this case the data would have been lost without the device noticing.

The following sequence diagram illustrates the flow of messages involved in the HTTP Adapter forwarding an unsettled telemetry data message to the downstream AMQP Messaging Network implementing AT LEAST ONCE delivery semantics.

Forward telemetry data flow (AT LEAST ONCE)

  1. Device 4711 PUTs telemetry data to the HTTP Adapter, indicating QoS Level 1.
    1. HTTP Adapter transfers telemetry data to AMQP 1.0 Messaging Network.
    2. AMQP 1.0 Messaging Network acknowledges reception of the message.
    3. HTTP Adapter acknowledges the reception of the data to the Device.

When the AMQP Messaging Network fails to settle the transfer of a telemetry message or settles the transfer with any other outcome than accepted, the protocol adapter MUST NOT try to re-send such rejected messages but SHOULD indicate the failed transfer to the device if the transport protocol provides means to do so.

Message Format

The following table provides an overview of the properties a client needs to set on a Forward Telemetry Data message.

Name Mandatory Location Type Description
content-type yes properties symbol A content type indicating the type and characteristics of the data contained in the payload, e.g. text/plain; charset="utf-8" for a text message or application/json etc. The value may be set to application/octet-stream if the message payload is to be considered opaque binary data.
creation-time yes properties timestamp The instant in time when the message has been created (see the AMQP 1.0 specification for details).
device_id yes application-properties string The identifier of the device that the data in the payload is originating from.
ttd no application-properties int The time ’til disconnect indicates the number of seconds that the device will remain connected to the protocol adapter. The value of this property must be interpreted relative to the message’s creation-time. A value of -1 is used to indicate that the device will remain connected until further notice, i.e. until another message indicates a ttd value of 0. In absence of this property, the connection status of the device is to be considered indeterminate. Backend Applications might use this information to determine a time window during which the device will be able to receive a command.

The body of the message MUST consist of a single AMQP Data section containing the telemetry data. The format and encoding of the data MUST be indicated by the content-type and (optional) content-encoding properties of the message.

Any additional properties set by the client in either the properties or application-properties sections are preserved by Hono, i.e. these properties will also be contained in the message delivered to consumers.

Northbound Operations

Receive Telemetry Data

Hono delivers messages containing telemetry data reported by a particular device in the same order that they have been received in (using the Forward Telemetry Data operation). Hono MAY drop telemetry messages that it cannot deliver to any consumers. Reasons for this include that there are no consumers connected to Hono or the existing consumers are not able to process the messages from Hono fast enough.

Hono supports multiple non-competing Business Application consumers of telemetry data for a given tenant. Hono allows each Business Application to have multiple competing consumers for telemetry data for a given tenant to share the load of processing the messages.

Preconditions

  1. Client has established an AMQP connection with Hono.
  2. Client has established an AMQP link in role receiver with Hono using source address telemetry/${tenant_id} where ${tenant_id} represents the ID of the tenant the client wants to retrieve telemetry data for.

Hono supports both AT MOST ONCE as well as AT LEAST ONCE delivery of telemetry messages. However, clients SHOULD use AT LEAST ONCE delivery in order to support end-to-end flow control and therefore SHOULD set the snd-settle-mode field to unsettled and the rcv-settle-mode field to first in their attach frame during link establishment.

A client MAY indicate to Hono during link establishment that it wants to distribute the telemetry messages received for a given tenant among multiple consumers by including a link property subscription-name whose value is shared by all other consumers of the tenant. Hono ensures that messages from a given device are delivered to the same consumer. Note that this also means that telemetry messages MAY not be evenly distributed among consumers, e.g. when only a single device sends data. NB This feature is not supported yet.

In addition a client MAY include a boolean link property ordering-required with value false during link establishment in order to indicate to Hono that it does not require messages being delivered strictly in order per device but instead allows for messages being distributed evenly among the consumers. NB This feature is not supported yet.

Message Flow

The following sequence diagram illustrates the flow of messages involved in a Business Application receiving a telemetry data message from Hono. The delivery mode used is AT LEAST ONCE.

Receive Telemetry Data

  1. AMQP 1.0 Messaging Network delivers telemetry message to Business Application.
    1. Business Application acknowledges reception of message.
Info

The Business Application can only consume telemetry messages that have been uploaded to Hono after the Business Application has established the link with the AMQP 1.0 Messaging Network. This is because telemetry messages are not durable, i.e. they are not persisted in Hono in order to be forwarded at a later time.

Message Format

The format of the messages containing the telemetry data is the same as for the Forward Telemetry Data operation.