Distributed Tracing

When an organization has applications that consists of tens, hundreds, or thousands of services running across many hosts, and sharing messages over one or more event meshes, it's no longer possible to diagnose problems simply by troubleshooting an error message or looking at a log. In this kind of system, you need a solution that can track an event all the way from the sending application, between event brokers, and to the receiving application. Distributed tracing provides this ability, allowing an administrator to trace the lifecycle of an event as it travels through the event mesh.

When distributed tracing is enabled, the event broker generates special guaranteed messages when certain operations happen. The publisher and subscriber APIs also generate spans when messages are sent and received. OpenTelemetry receivers, including a purpose-built client (an instance of the Solace OpenTelemetry Receiver), then consume these messages from a particular queue and convert them to a format that can be received, processed, and viewed using common Open Telemetry backends such as Jaeger and DataDog.

The distributed tracing functionality of Solace event brokers complies with the OpenTelemetry specification.

Which Event Broker Actions are Traced?

Spans are generated at the following points:

• event message sent by the event broker to the consumer (Send Span)
• event message acknowledged by the consumer (Send Span)
• event message received by the event broker (Receive Span)
• event message enqueued by the event broker (Receive Span)

Tracing in the Event Broker

The event broker generates spans at certain points in the lifecycle of an event message. A span represents a single operation on the event broker, such as receiving and sending an event message.

The event broker generates a span when it receives an incoming event message or sends an outgoing event message. The broker sends trace messages, which contain one or more spans, to the telemetry queue. A client binds to the queue and consumes the trace messages. This client, referred to as a receiver, is an instance of the Solace OpenTelemetry Receiver, which is a plugin for the Open Telemetry Collector project. The receiver transforms each span in the trace message to a generic internal format for the Open Telemetry Collector. The Collector then converts the internal format into the appropriate format to send to the backend (Jaeger, Splunk, Prometheus, Zipkin, DataDog, DynaTrace, and so on).

These components are illustrated in the following diagram:

For information about how spans are correlated to produce traces, see Context Propagation for Distributed Tracing.

For details about how to set up the event broker for distributed tracing, see Configuring Distributed Tracing.

For details about how to set up PubSub+ Cloudevent broker services for distributed tracing, see Enabling Distributed Tracing.

Tracing with PubSub+ Messaging APIs

To manipulate trace context in PubSub+ Messaging APIs, you use the Solace PubSub+ OpenTelemetry API Libraries. These libraries provide special getter and setter methods that allow your applications to inject trace context into or extract trace context from SMF messages. The PubSub+ OpenTelemetry API Libraries also include the OpenTelemetry API, which allows your publisher and subscriber applications to create spans that backend applications use to generate traces.

For more information, see Context Propagation with PubSub+ Messaging APIs

Receivers

A receiver is a client that binds to the telemetry queue and consumes trace messages. The Solace OpenTelemetry Receiver, which is a plugin for the OpenTelemetry Collector project, makes it easy for a client to consume trace messages and convert them to a standard format. The Solace Receiver consumes trace messages as Guaranteed Messages from the event broker and produces generic trace spans for the OpenTelemetry Collector.

For more information, see Solace OpenTelemetry Receiver.

Event Broker Performance

Because distributed tracing causes the event broker to generate additional guaranteed trace messages, there can be an impact on the event broker's performance (throughput of event messages). As more messages pass through the broker, this performance impact becomes more evident. You should take this into account when deciding which messages to trace.

The ratio of event brokers (or HA groups) to receivers is flexible—multiple event brokers can feed a single receiver, and multiple receivers can consume trace messages from a single broker. However, we recommend that you deploy one receiver per event broker (or HA group) until the performance of your overall architecture is understood.

Feature Interactions

Disaster Recovery (Replication)

Standby nodes do not generate trace spans. However, receivers can bind to the telemetry queue on a standby node to allow residual spans to be drained after a VPN switches from active to standby.

Transactions

Messages published as part of a local transaction do not generate trace messages.

Messages that succeed in a commit will appear to be enqueued, even if a subsequent message in the local transaction causes the commit to fail. To determine why a committed message was not delivered, you must search all spans for the same transaction ID on the same session to find failed messages. If at least one message in the transaction failed, none of the messages in the transaction were ever eligible for delivery.

Ingress Selectors

Messages are traced as they are received by the event broker, and they are enqueued before the broker evaluates ingress selectors. If the selector discards a message, there will be no traces indicating this has occurred.

Message VPN Bridges

Messages received from bridges are traced the same as regular client messages.

DMR Links

Messages arriving over DMR links are traced the same as regular client messages.

DMQ Message Traces

Messages that are admin-copied or moved to a DMQ get a new Replication Group Message ID. When send spans are generated for these messages, the parent span will be the original message's receive, and there will be no span generated that records the message's new Replication Group Message ID.

Partitioned Queues

Messages are traced through partitioned queues, and the partition within the partitioned queue traversed by the message is identified.

Product Key

A product key is required to use distributed tracing in production or for long-term use. Contact Solace to obtain a product key. To install a key on your event brokers, see Product Keys .

Demo Mode

You can try distributed tracing in demo mode (without a product key) for seven days by setting up a telemetry profile and enabling distributed tracing on that profile.

For details about demo mode, including the terms of use, see https://solace.com/dt-demo-mode.

Restarting Your Event Broker

If you need more time, you can extend your demo by restarting your event broker.

To restart your event broker, enter the following commands in the CLI:

solace> enable
solace# reload

On the software event broker, the reload command stops the container. In machine images, the container automatically restarts. For container images, you may need to manually restart the container, depending on the restart policy of your container runtime.

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