Monitoring & Tracing

The individual components of an Eclipse Hono™ installation need to work together in order to provide their functionality to devices and applications. Under normal circumstances these interactions work flawlessly. However, due to the nature of distributed systems, any one (or more) of the components may crash or become otherwise unavailable due to arbitrary reasons. This page describes how Hono supports operations teams by providing insights into the individual service components and their interactions with each other by means of reporting metrics and tracing the processing of individual messages through the system.

When a device uploads telemetry data to the HTTP adapter, the adapter invokes operations on the Device Registration, Credentials and the Tenant services in order to authenticate and authorize the device before sending the telemetry data downstream to the messaging infrastructure. The overall success of this process and the latency involved before the message reaches the consumer is determined by the individual interactions between the service components.


In a production environment, an operations team will usually want to keep track of some key performance indicators (KPI) which allow the team to determine the overall health of the system, e.g. memory and CPU consumption etc. Hono supports the tracking of such KPIs by means of metrics it can be configured to report. The metrics are usually collected in a time series database like InfluxDB or Prometheus and then visualized on a monitoring dash-board built using frameworks like Grafana. Such a dash-board can usually also be configured to send alarms when certain thresholds are exceeded.

Metrics usually provide insights into the past and current status of an individual component. The values can be aggregated to provide a picture of the overall system’s status. As such, metrics provide a great way to monitor system health and, in particular, to anticipate resource shortages and use such knowledge to pro-actively prevent system failure.

Enabling Collection of Metrics

The Hono build supports configuration of the Prometheus metrics back end by means of the metrics-prometheus Maven profile.

The profile is not active by default and needs to be activated explicitly:

mvn clean install -Pmetrics-prometheus ...

Most of the metrics back ends expect a component to actively push metrics to an endpoint provided by the back end. However, Prometheus is different in that it polls (or scrapes) all components periodically for new metrics data. For this to work, the Prometheus server needs to be configured with the host names/IP addresses of the components to monitor.

The components themselves need to expose a corresponding HTTP endpoint that the Prometheus server can connect to for scraping the meter data. All Hono components that report metrics can be configured to expose such an endpoint via their Health Check server which already exposes endpoints for determining the component’s readiness and liveness status.

Health Check Server Configuration

All of Hono’s service components and protocol adapters contain a Health Check server which can be configured to expose several HTTP endpoints for determining the component’s status. Under the hood, Hono uses Quarkus’ SmallRye Health extension to implement the health check server.

In particular, the server exposes a /started, a /readiness, a /liveness and an optional /prometheus URI endpoint.

The former three endpoints are supposed to be used by container orchestration platforms like Kubernetes to monitor the runtime status of the containers that it manages. These endpoints are always exposed when the health check server is started.

The /prometheus endpoint can be used by a Prometheus server to retrieve collected meter data from the component. It is only exposed if Prometheus has been configured as the metrics back end as described above.

Please refer to the Quarkus SmallRye Health extension documentation for details regarding configuration. The table below provides an overview of the configuration properties that have a Hono specific default value that differs from the Quarkus default:

OS Environment Variable
Java System Property


By default, all Hono components will log to the system console and can be configured using environment variables with the QUARKUS_LOG_CONSOLE_ prefix or using corresponding system properties. Logging to a file and syslog is also available. See the configuration reference for details.

Log formatting can be configured by either setting QUARKUS_LOG_CONSOLE_FORMAT, or, to enable logging in JSON format, by setting QUARKUS_LOG_CONSOLE_JSON to true. Follow the Quarkus documentation for more details.

Additionally, the Hono components contain support for sending log messages to a centralized log management system like Graylog, Logstash or Fluentd. This is done by means of the quarkus-logging-gelf extension, using TCP or UDP to send logs in the Graylog Extended Log Format (GELF). This can also be enabled using environment variables with the QUARKUS_LOG_HANDLER_GELF_ prefix or using corresponding system properties. See the configuration reference for details.


In normal operation the vast majority of messages should be flowing through the system without any noteworthy delays or problems. In fact, that is the whole purpose of Hono. However, that doesn’t mean that nothing can go wrong. For example, when a tenant’s device administrator changes the credentials of a device in the Credentials service but has not yet updated the credentials on the device yet, then the device will start to fail in uploading any data to the protocol adapter it connects to. After a while, a back end application’s administrator might notice, that there hasn’t been any data being received from that particular device for quite some time. The application administrator therefore calls up the Hono operations team and complains about the data being lost somewhere.

The operations team will have a hard time determining what is happening, because it will need to figure out which components have been involved in the processing of the device and why the data hasn’t been processed as usual. The metrics alone usually do not help much here because metrics are usually not scoped to individual devices. The logs written by the individual components, on the other hand, might contain enough information to correlate individual entries in the log with each other and thus trace the processing of the message throughout the system. However, this is usually a very tedious (and error prone) process and the relevant information is often only logged at a level (e.g. DEBUG) that is not used in production (often INFO or above).

In order to address this problem, Hono’s service components are instrumented to produce tracing data via OpenTelemetry (with the instrumentation in the code using OpenTracing as described below). OpenTelemetry provides an end-to-end implementation to generate, emit, collect, process and export tracing data. The Hono container images all can be configured to send tracing data to an OpenTelemetry collector from which data can be exported to a back-end like Jaeger. The decision, whether a trace should be sampled and exported, controlling the amount of tracing data to be collected, is configured by means of a Sampler configuration.

Hono supports configuration of the trace exporter by means of the following configuration properties:

OS Environment Variable
Java System Property
Type Default Value Description
string - The OTLP endpoint of the OpenTelemetry Collector to connect to. The endpoint must start with either http:// or https://.

Please refer to the Quarkus OpenTelemetry documentation for further configuration options for the tracer exporter.

The sampler can be configured using the following OpenTelemetry SDK configuration properties.

OS Environment Variable
Java System Property
Type Default Value Description
string parentbased_always_on The sampler to use for determining if a tracing span should be reported. Please refer to the OpenTelemetry documentation for supported values.
In addition to the standard samplers, a rate-limiting sampler can be configured by setting this property to rate_limiting or rate-limiting and using the OTEL_TRACES_SAMPLER_ARG property to set the maximum number of traces to report per second.
string - A value to be used as the sampler argument. The specified value will only be used if OTEL_TRACES_SAMPLER is set. Each sampler type defines its own expected input, if any. Please refer to the OpenTelemetry documentation for details.

The sampler being created will always be a parent based sampler. For example, configuring a sampler of type parentbased_traceidratio will yield the exact same result as configuring a sampler of type traceidratio.

Deprecated Sampler configuration properties

In previous versions of Hono, configuring the Sampler was also possible using the Quarkus specific configuration properties quarkus.opentelemetry.tracer.sampler and quarkus.opentelemetry.tracer.sampler.ratio. Usage of these properties for configuring an always on, always off or a ratio based sampler still works but is deprecated and their support will be removed completely in Hono 3.0.0. Please use the standard OpenTelemetry SDK properties instead.

In order to integrate traces from Hono with those from other applications, Hono supports reading and writing trace context information from and to messages exchanged with these applications. This is done according to the W3C Trace Context and the W3C Baggage specifications.

OpenTracing instrumentation

OpenTracing is a predecessor to OpenTelemetry. Hono components still use OpenTracing APIs, but use the OpenTracing shim from the OpenTelemetry SDK as OpenTracing implementation, letting traces be exported in the OpenTelemetry format.

Enforcing the recording of traces for a tenant

Typically, in production systems, the tracing components will be configured to not store all trace spans in the tracing back end, in order to reduce the performance impact. For debugging purposes it can however be beneficial to enforce the recording of certain traces. Hono allows this by providing a configuration option in the Tenant information with which all traces concerning the processing of telemetry, event and command messages for that specific tenant will be recorded. Furthermore, this enforced trace sampling can be restricted to only apply to messages sent in the context of a specific authentication identifier. Please refer to the description of the tracing object in the Tenant Information for details.