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 AMQP 1.0 Messaging Network. 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.

Configuring a Metrics Back End

Hono uses Micrometer for providing metrics. It is possible to drop in any Micrometer compatible back end. Hono also uses the Micrometer integration with Spring Boot and Vert.x.

Please refer to the Micrometer documentation for details regarding the configuration of a specific Micrometer back end. In most cases, you only need to add the back end specific jar files to the class path and add back end specific configuration properties to the application.yml file.

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

Note that the profile is not active by default, i.e. you need to explicitly activate it when starting the build using Maven’s -P command line parameter:

mvn clean install -Pmetrics-prometheus ...

Using Prometheus

Most of the metrics back ends have data being pushed to from the components reporting the metrics. 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 IP addresses of the components to monitor. In the example deployment that comes with Hono, the Prometheus server is configured with the names of the Kubernetes services corresponding to the Hono components that it should scrape. 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. In particular, the server exposes a /readiness, a /liveness and an optional /prometheus URI endpoint.

The former two 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.

The health check server can be configured by means of the following environment variables:

OS Environment Variable
Java System Property
Default Value Description
hono.healthCheck.bindAddress The IP address of the network interface that the health check server’s secure port should be bound to. The server will only be started if this property is set to some other than the default value and corresponding key material has been configured using the HONO_HEALTHCHECK_KEYPATH and HONO_HEALTHCHECK_CERTPATH variables.
- The absolute path to the PEM file containing the certificate that the secure server should use for authenticating to clients. This option must be used in conjunction with HONO_HEALTHCHECK_KEYPATH.
Alternatively, the HONO_HEALTHCHECK_KEYSTOREPATH option can be used to configure a key store containing both the key as well as the certificate.
hono.healthCheck.insecurePortBindAddress The IP address of the network interface that the health check server’s insecure port should be bound to. The server will only be started if this property is set to some other than the default value.
8088 The port that the insecure server should listen on.
- The absolute path to the (PKCS8) PEM file containing the private key that the secure server should use for authenticating to clients. This option must be used in conjunction with HONO_HEALTHCHECK_CERTPATH. Alternatively, the HONO_HEALTHCHECK_KEYSTOREPATH option can be used to configure a key store containing both the key as well as the certificate.
8088 The port that the secure server should listen on.
- The password required to read the contents of the key store.
- The absolute path to the Java key store containing the private key and certificate that the secure server should use for authenticating to clients. Either this option or the HONO_HEALTHCHECK_KEYPATH and HONO_HEALTHCHECK_CERTPATH options need to be set in order to enable TLS secured connections with clients. The key store format can be either JKS or PKCS12 indicated by a .jks or .p12 file suffix respectively. The HONO_HEALTHCHECK_KEYSTOREPASSWORD variable can be used to set the password required for reading the key store.

The component/service will fail to start if neither the secure not the insecure server have been configured properly.


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 using OpenTracing. OpenTracing provides Vendor-neutral APIs and instrumentation for distributed tracing. The OpenTracing web page provides a list of supported tracer implementations from which users can choose in order to collect (and examine) the tracing information generated by Hono’s individual components.

Configuring a Tracer

Hint: The description in this chapter applies to any compatible OpenTracing implementation. For an easier approach to configure usage of Jaeger tracing, see the next chapter.

Hono’s components use the OpenTracing Tracer Resolver mechanism to find and initialize a concrete OpenTracing implementation during startup of the component. The discovery mechanism is using Java’s ServiceLoader and as such relies on the required resources to be available on the class path.

When starting up any of Hono’s Docker images as a container, the JVM will look for additional jar files to include in its class path in the container’s /opt/hono/extensions folder. Thus, using a specific implementation of OpenTracing is just a matter of configuring the container to mount a volume or binding a host folder at that location and putting the implementation’s jar files and resources into the corresponding volume or host folder.


This also means that (currently) only Tracer implementations can be used with Hono that also implement the Tracer Resolver mechanism.

Configuring usage of Jaeger tracing (included in Docker images)

In case Jaeger tracing shall be used, there is an alternative to putting the jar files in the container’s /opt/hono/extensions folder as described above. This is to have the Jaeger tracing jar files be included in the Hono Docker images by using the jaeger Maven profile when building Hono. For example, building the HTTP adapter image with the Jaeger client included:

# in directory: hono/adapters/http-vertx/
mvn clean install -Pbuild-docker-image,jaeger

Note that when running the created docker image, the environment variables for configuring the Jaeger client still need to be set. Please refer to the Jaeger documentation for details.

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 backend, 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.