OpenTracing with NATS

Over the years, we've had periodic requests to support distributed tracing in NATS. While distributed tracing is valuable, philosophically we did not want to add external dependencies to NATS, internally or via API. Nor did we want to provide guidance that would make developers work in ways that didn't feel natural or aligned with the tenets of NATS. We left it to the application developers using NATS.

OpenTracing changes this and offers a way to implement distributed tracing with NATS that aligns with our goals and philosophy of simplicity and ease of use and does not require adding dependencies into NATS. This repository provides a reference to facilitate the use of OpenTracing with NATS enabled applications.

What is OpenTracing

OpenTracing provides a non-intrusive vendor-neutral API and instrumentation for distributed tracing, with wide language support. Because each use case is slightly different, we've decided not to provide specific implementations at this point. Instead we are providing a reference architecture with examples demonstrating easy usage of OpenTracing with NATS. In line with other NATS projects, these canonical examples are provided in Go, but this approach should port smoothly into many other languages. More language examples will be added soon.

How it works

OpenTracing is actually fairly simple to implement in an applicaton. A "Trace" is defined, and then sets up "spans" to represent an operation or event and log information, which is reported to the OpenTracing aggregator for reporting and display.

To propogate, span contexts are serialized into a NATS message using the binary format. We provide a not.TraceMsg which will do what is needed to inject span contexts into messages and to extract them on the other side.

Here's how to send a span context over NATS.

// A NATS OpenTracing Message.
var t not.TraceMsg

// Setup a span for the operation to publish a message.
pubSpan := tracer.StartSpan("Published Message", ext.SpanKindProducer)
ext.MessageBusDestination.Set(pubSpan, subj)
defer pubSpan.Finish()

// Inject span context into our traceMsg.
if err := tracer.Inject(pubSpan.Context(), opentracing.Binary, &t); err != nil {
    log.Fatalf("%v for Inject.", err)
}

// Add the payload.
t.Write(msg)

// Send the message over NATS.
nc.Publish(subj, t.Bytes())

Note that the payload is added after injection. This order is a requirement, but simplifies the API and feels natural.

Retrieving a span from an inbound message and associating with a new response span is straightforward as well.

// Create new TraceMsg from the NATS message.
t := not.NewTraceMsg(msg)

// Extract the span context from the request message.
sc, err := tracer.Extract(opentracing.Binary, t)
if err != nil {
    log.Printf("Extract error: %v", err)
}

// Setup a span referring to the span context of the incoming NATS message.
replySpan := tracer.StartSpan("Service Responder", ext.SpanKindRPCServer, ext.RPCServerOption(sc))
ext.MessageBusDestination.Set(replySpan, msg.Subject)
defer replySpan.Finish()

nc.Publish(msg.Reply, reply)
replySpan.LogEvent(fmt.Sprintf("Response msg: %s", reply))

Check out the examples for additional usage.

Setting up an OpenTracing Tracer

To run the the examples, we setup Jaeger as the OpenTracing tracer with its convenient "all-in-one" docker image. Jaeger is a CNCF open source, end-to-end distributed tracing project.

docker run -d --name jaeger \
  -e COLLECTOR_ZIPKIN_HTTP_PORT=9411 \
  -p 5775:5775/udp \
  -p 6831:6831/udp \
  -p 6832:6832/udp \
  -p 5778:5778 \
  -p 16686:16686 \
  -p 14268:14268 \
  -p 9411:9411 \
  jaegertracing/all-in-one:1.9

See Jaeger's getting started documentation for more information.

Request/Reply Examples

Open two terminals, in one terminal go to the reply example directory and run:

./reply foo "here's some help"

In the other terminal, go to the request example direcory and run:

./request foo help

Request Output

Initializing logging reporter
Published [foo] : 'help'
Received  [_INBOX.4fkjE3Kld4s26FoJxnfxNy.dyOD78y3] : 'here's some help'
Reporting span 3007527f6bf0a38e:3007527f6bf0a38e:0:1

Reply Output

Initializing logging reporter
Listening on [foo]
Received request: help
Reporting span 3007527f6bf0a38e:66628b457927103d:3007527f6bf0a38e:1

Viewing the Request/Reply output in the Jaeger UI

Navigate with a browser to http://localhost:16686. Find the NATS Requestor service in the services list and click the Find Traces button. Click on the NATS Requestor service and you will see a screen similar to the following:

You can see the entire span of the request and the associated replier span.

Publish/Subscribe Examples

Open three terminals, in the first two terminals go to the subscribe example directory and run:

go build
./subscribe foo

and in the second terminal:

./subscribe foo

And finally in the third terminal go to the publish example directory:

go build
./publish foo hello

Navigate with a browser to http://localhost:16686. Find the NATS Publisher service in the services list and click the Find Traces button. Click on the NATS Publisher service and you will see a screen to the following:

You can see the publish span and the two associated subscriber spans. The gap the middle includes the NATS client library publishing the message, the NATS server routing and fanning out the message, and the subscriber NATS clients receiving the messages and passing them to application code where the subscriber span is reported.

Subscriber Output

Initializing logging reporter
Listening on [foo]
Received msg: "hello"
Reporting span 2b78c114fc32bcad:132b0c35588f3c16:2b78c114fc32bcad:1

Publisher Output

Initializing logging reporter
Published [foo] : 'hello'
Reporting span 2b78c114fc32bcad:2b78c114fc32bcad:0:1

Our sponsor for this project

Many thanks to MasterCard for sponsoring this project. We appreciate MasterCard's support of NATS, CNCF, and the OSS community.

License

nats-io/not.go

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