lesson01

Lesson 1 - Hello World

Objectives

Learn how to:

• Instantiate a Tracer
• Create a simple trace
• Annotate the trace

Walkthrough

A simple Hello-World program

Let's create a simple Node program lesson01/exercise/hello.js that takes an argument and prints "Hello, {arg}!".

mkdir -p lesson01/exercise
touch lesson01/exercise/hello.js

In lesson01/exercise/hello.js:

const assert = require("assert");

const sayHello = helloTo => {
  const helloStr = `Hello, ${helloTo}!`;
  console.log(helloStr);
};

assert(process.argv.length == 3, "Expecting one argument");
const helloTo = process.argv[2];
sayHello(helloTo);

Run it:

$ node lesson01/exercise/hello.js Bryan
Hello, Bryan!

Create a trace

A trace is a directed acyclic graph of spans. A span is a logical representation of some work done in your application. Each span has these minimum attributes: an operation name, a start time, and a finish time.

Let's create a trace that consists of just a single span. To do that we need an instance of the opentracing.Tracer. We can use a global instance returned by new opentracing.Tracer().

const opentracing = require("opentracing");

const tracer = new opentracing.Tracer();

const sayHello = helloTo => {
  const span = tracer.startSpan("say-hello");
  const helloStr = `Hello, ${helloTo}!`;
  console.log(helloStr);
  span.finish();
};

We are using the following basic features of the OpenTracing API:

• a tracer instance is used to start new spans via the startSpan function
• each span is given an operation name, "say-hello" in this case
• each span must be finished by calling its finish() function
• the start and end timestamps of the span will be captured automatically by the tracer implementation

If we run this program, we will see no difference, and no traces in the tracing UI. That's because the variable new opentracing.Tracer() points to a no-op tracer by default.

Initialize a real tracer

Let's create an instance of a real tracer, such as Jaeger (https://github.com/jaegertracing/jaeger-client-node).

const initJaegerTracer = require("jaeger-client").initTracer;

function initTracer(serviceName) {
  const config = {
    serviceName: serviceName,
    sampler: {
      type: "const",
      param: 1,
    },
    reporter: {
      logSpans: true,
    },
  };
  const options = {
    logger: {
      info(msg) {
        console.log("INFO ", msg);
      },
      error(msg) {
        console.log("ERROR", msg);
      },
    },
  };
  return initJaegerTracer(config, options);
}

To use this instance, let's replace new opentracing.Tracer() with initTracer(...):

const tracer = initTracer("hello-world");

Note that we are passing a string "hello-world" to the init method. It is used to mark all spans emitted by the tracer as originating from a hello-world service.

There's one more thing we need to do. Jaeger Tracer is primarily designed for long-running server processes, so it has an internal buffer of spans that is flushed by a background thread. Since our program exits immediately, it may not have time to flush the spans to Jaeger backend. Let's add the following to the end of hello.js:

tracer.close(() => process.exit());

If we run the program now, we should see a span logged:

$ node lesson01/exercise/hello.js Bryan
INFO  Initializing Jaeger Tracer with CompositeReporter and ConstSampler
Hello, Bryan!
INFO  Reporting span d42d649b3ba9f0f3:d42d649b3ba9f0f3:0:1

If you have Jaeger backend running, you should be able to see the trace in the UI.

Annotate the Trace with Tags and Logs

Right now the trace we created is very basic. If we call our program with argument Susan instead of Bryan, the resulting traces will be nearly identical. It would be nice if we could capture the program arguments in the traces to distinguish them.

One naive way is to use the string "Hello, Bryan!" as the operation name of the span, instead of "say-hello". However, such practice is highly discouraged in distributed tracing, because the operation name is meant to represent a class of spans, rather than a unique instance. For example, in Jaeger UI you can select the operation name from a dropdown when searching for traces. It would be very bad user experience if we ran the program to say hello to a 1000 people and the dropdown then contained 1000 entries. Another reason for choosing more general operation names is to allow the tracing systems to do aggregations. For example, Jaeger tracer has an option of emitting metrics for all the traffic going through the application. Having a unique operation name for each span would make the metrics useless.

The recommended solution is to annotate spans with tags or logs. A tag is a key-value pair that provides certain metadata about the span. A log is similar to a regular log statement, it contains a timestamp and some data, but it is associated with the span from which it was logged.

When should we use tags vs. logs? The tags are meant to describe attributes of the span that apply to the whole duration of the span. For example, if a span represents an HTTP request, then the URL of the request should be recorded as a tag because it does not make sense to think of the URL as something that's only relevant at different points in time on the span. On the other hand, if the server responded with a redirect URL, logging it would make more sense since there is a clear timestamp associated with such event. The OpenTracing Specification provides guidelines called Semantic Conventions for recommended tags and log fields.

Using Tags

In the case of hello Bryan, the string "Bryan" is a good candidate for a span tag, since it applies to the whole span and not to a particular moment in time. We can record it like this:

const span = tracer.startSpan("say-hello");
span.setTag("hello-to", helloTo);

Using Logs

Our hello program is so simple that it's difficult to find a relevant example of a log, but let's try. Right now we're formatting the helloStr and then printing it. Both of these operations take time, so we can log their completion:

const helloStr = `Hello, ${helloTo}!`;
span.log({
  event: "string-format",
  value: helloStr,
});

console.log(helloStr);
span.log({ event: "print-string" });

The log statements might look a bit strange if you have not previously worked with a structured logging API. Rather than formatting a log message into a single string that is easy for humans to read, structured logging APIs encourage you to separate bits and pieces of that message into key-value pairs that can be automatically processed by log aggregation systems. The idea comes from the realization that today most logs are processed by machines rather than humans. Just google "structured-logging" for many articles on this topic.

The OpenTracing API for JavaScript exposes a structured logging API method log that takes a dictionary, or hash, of key-value pairs.

The OpenTracing Specification also recommends all log statements to contain an event field that describes the overall event being logged, with other attributes of the event provided as additional fields.

If you run the program with these changes, then find the trace in the UI and expand its span (by clicking on it), you will be able to see the tags and logs.

Conclusion

The complete program can be found in the solution directory.

We moved the initTracer helper function into its own module so that we can reuse it in the other lessons with a require statement require("../../lib/tracing").

Next lesson: Context and Tracing Functions.