doc/cookbook.md · mirrors_sourcegraph/lightstep-tracer-javascript

# LightStep JavaScript Tracing Cookbook

Creating a useful trace in LightStep takes only a few minutes. Once a single trace has been created in your system, the same logic can be applied to create more complex traces that will give you new insight into your running production system.

The cookbook recipes here assume you've already successfully installed the OpenTracing and LightStep NPM packages. If not, check out the [README](../README.md).

* Getting started
    * [Browser quick start](#browser-quick-start)
* Creating spans
    * [Spans in Promise-based code](#promises)
    * [Spans in callback-based code](#callbacks)
    * [Spans in code using "context" objects](#context-objects)
* Joining spans into traces
    * [Child spans in the same process](#parent-spans)
    * [Joining spans in different processes / services by "join:" tag](#join-by-tag)
    * [Joining spans in different processes / services by injection](#join-by-inject)

*Something missing from the Cookbook? [Log a GitHub issue](https://github.com/opentracing/opentracing-javascript/issues/new) and we'll get on it :)*

<a name='browser-quick-start'></a>
## Browser quick start

To try out LightStep in the browser with minimal changes, you can use hosted versions of the LightStep and OpenTracing scripts along with some auto-instrumentation options. The below will generate basic spans for page-load events in the browser as well as `XMLHttpRequest` operations.

While in production, you'll likely want more control than this, this is an easy option to try things out quickly!

**Steps**

1. Ensure you have created a valid project on app.lightstep.com and have your access token
2. Include the following scripts in your page HTML and replace `{your_access_token}` with, well, your access token value and replace `{name_to_use_for_your_component}` with a string identifier that'll be used in the LightStep UI to identify this component:

```html
<script src="PATH/TO/opentracing-browser.min.js"></script>
<script src="PATH/TO/lightstep-tracer.min.js"
        data-init_global_tracer="true"
        data-access_token="{your_access_token}"
        data-component_name="{name_to_use_for_your_component}"
        ></script>
```

The above will automatically create spans for the browser page load. You will be able to access the global OpenTracing `Tracer` via a static call to `opentracing.globalTracer()`.

<a name='promises'></a>
## Instrumenting Promise-based code

Instrumenting code that uses standard [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) objects is easy. There's also an [opentracing-tracedpromise package](https://www.npmjs.com/package/opentracing-tracedpromise) to automate the below:

1. Before initiating the request that returns the `Promise`, create a span
2. Assign your span a helpful name that describes the operation it is measuring
3. In handling the Promise completion, always call `finish()` on the span

*Note: if you're using a custom promise library or wrapper, it may be easier to make [spans part of the promise implementation itself](http://lightstep.com/blog/traced-promises/).*

```javascript
// Let's assume you have an application-specific object representing a request
// to a remote system (a database, an external API, etc.)
let req = ...;

// Before initiating the request, start a span: the name (here 'remote_request')
// will show up in the LightStep UI, so the more specific you can be, the
// better.  Setting a tag prefixed with "join:" will allow spans to be connected
// by LightStep across different services and systems (spans with the same
// key-value pairs are made part of the same trace).
let span = opentracing.globalTracer().startSpan('remote_request');
span.setTag('join:request_id', req.id);

// Initiate your normal request
yourApp.makeRemoteRequest(req)
    .then((response) => {
        // Log information that'll be useful when looking at this trace.
        // This particular span might be part of a larger anomalous trace
        // so logging details is useful even on 'local' success.
        span.logEvent('remote request ok', {
            response_size : response.contentLength,
            handler_id    : response.handlerID,            
        });

yourApp.doNormalWork(response);  // etc.
    })
    .catch((err) => {
        // The 'error' tag is a special tag letting LightStep know an error
        // occurred during this span        
        span.setTag('error', 'true');
        span.logEvent('remote request failed', {
            error   : err,
            request : req,
        });

yourApp.doNormalErrorHandling(err); // etc.
    })
    .then(() => {
        // Always finish the span: both in the case of success or an error.
        span.finish();
    });
```

<a name='callbacks'></a>
## Instrumenting callback-based code

Instrumenting code using asynchronous callbacks is simple:

1. Create the span before initiating the asynchronous operation
2. During the callback, whether an error has occurred or not, be sure to call `span.finish()` to denote the operation is done

```javascript
// Let's assume you have an application-specific object representing a request
// to a remote system (a database, an external API, etc.)
let req = ...;

// Initiate your normal request
yourApp.makeRemoteRequest(req, function (err, response) {
    if (err) {
        // The 'error' tag is a special tag letting LightStep know an error
        // occurred during this span
        span.setTag('error', 'true');
        span.logEvent('remote request failed', {
            error   : err,
            request : req,
        });

yourApp.doNormalErrorHandling(err); // etc.
    } else {
        // Log information that'll be useful when looking at this trace.
        // This particular span might be part of a larger anomalous trace
        // so logging details is useful even on 'local' success.
        span.logEvent('remote request ok', {
            response_size : response.contentLength,
            handler_id    : response.handlerID,            
        });

yourApp.doNormalWork(response);  // etc.
    }
    // Always finish the span: both in the case of success or an error.
    span.finish();
});
```

<a name='context-objects'></a>
## Instrumenting 'context object'-based code

Some applications already have a "context" object that is passed around as part of complex operations. In such systems, simple object composition provide good instrumentation data.  In other applications, this might be referred to as a `Transaction` object.  Regardless of the name, if you already have code that's bracketing important operations with "start" and "finish" calls, this is a good place to consider adding instrumentation.

```javascript
function Context() {
    this._yourNormalField1 = ...;
    this._yourNormalField2 = ...;

// Add a span object that will start and finish along with the Context
    // object
    this._span = ...;
}

Context.prototype.begin = function(appDomain, name, transactionID) {
    this._span = opentracing.globalTracer().startSpan(appDomain + '/' + name);
    this._span.setTag('join:transaction_id', transactionID);
    //
    // <Normal implementation code here>
    //
};

Context.prototype.end = function() {
    //
    // <Normal implementation code here>
    //
    this._span.finish();
}
```

<a name="parent-spans"></a>
## Child spans in the same process

In a single process, creating parent-child relationships between spans is trivial:

1. Be sure to pass the parent span object into the code creating the child
2. Call `startSpan()` with the additional `childOf` option

The additional parameter will let LightStep know about the relationship so traces are constructed and displayed accurately.

```javascript
function startSubOperation(parentSpan, param1, param2, param3) {
    let childSpan = opentracing.globalTracer().startSpan('my_child', { childOf : parentSpan });

//
    // Do normal work here...
    //

childSpan.finish();
}
```

<a name="join-by-tag"></a>
## Joining spans in different processes / services by tag

Joining spans from different processes or services by tag in LightStep is easy. LightStep recognizes a special `"join:"` prefix on tag keys (set via the `setTag()` OpenTracing API). Any spans with the same value for a given "join key" and overlapping in time will automatically be considered part of the same trace.

This is usually a very easy way to create cross-service traces in a system that is already maintaining a request ID or transaction ID.

Join tags will regardless of platform, making this a trivial way to initially connect spans across services without having to modify communication protocols, inter-service APIs, or even the programming language of the platform in question.

*Note: the "join:" prefix is a LightStep-specific feature and is not part of the OpenTracing specification.*

```javascript
// in parent_process.js
function startParallelWorkers() {

let transactionId = generateNewTransactionID();

let span = opentracing.globalTracer().startSpan('large_operation');
    span.setTag('join:transaction_id', transactionId);

// Launch a bunch of parallel child processes
    async.parallel(jobArray, function(job, next) {
        // Ensure the transaction ID is sent to the child
        job.transactionId = transactionId;
        let child = child_process.fork('./child_process.js', job);
        child.on('message', function (msg) {
            span.log('Received message', msg);
            next();
        });
    }, function() {
        span.finish();
    });
}

// in child_process.js
function doWork(job) {
    let span = opentracing.globalTracer().startSpan('worker_operation');
    span.setTag('join:transaction_id', job.transactionId);

asyncWork(function(result) {
        span.logEvent('Worker finished', result);
        span.finish();
    });
}
```

<a name="join-by-inject"></a>
## Joining spans in different processes / services using `inject`

Joining spans from different processes or services using the OpenTracing `inject` and `extract` methods is an alternative to using tags. While the tag approach is more concise, injection allows parent-child relationships to be conveyed unambiguously without relying on implicit timing information.

The injection approach also has the advantage of being part of all OpenTracing-compliant implementations. A disadvantage is that this approach requires all "links in the chain" of operations to be instrumented for a trace to be constructed between services (join tags, on the other hand, can be used to join between service A and service C without knowledge of an intermediary service B).

*Note: it's perfectly acceptable in LightStep to use a combination of "join tags" and `inject()`/`join()` calls.*

1. Before crossing the process boundary call `inject()` to transfer the span context information to a text or binary carrier object
2. Pass the carrier data along with the normal payload being sent to the out-of-process service
3. In the external service, use `extract()` to reconstruct the original span context
4. Call `startSpan(name, { childOf : context })`) to create a new span with the parent context

#### More information about the particulars of what and when to use `inject()` and `extract()` is available on [the OpenTracing site](http://opentracing.io/).

```javascript
// in parent_process.js
function startParallelWorkers() {

let transactionId = generateNewTransactionID();

let span = opentracing.globalTracer().startSpan('large_operation');

var carrier = {};
    opentracing.globalTracer().inject(span.context(), opentracing.FORMAT_TEXT_MAP, carrier);

// Launch a bunch of parallel child processes
    async.parallel(jobArray, function(job, next) {
        // Ensure the 'carrier' information is transmitted to the child
        job.spanCarrier = carrier;
        let child = child_process.fork('./child_process.js', job);
        child.on('message', function (msg) {
            span.log('Received message', msg);
            next();
        });
    }, function() {
        span.finish();
    });
}

// in child_process.js
function doWork(job) {
    let extractedContext = opentracing.globalTracer().extract(opentracing.FORMAT_TEXT_MAP, job.spanCarrier);
    let span = opentracing.globalTracer().startSpan('worker_operation', { childOf : extractedContext });

asyncWork(function(result) {
        span.logEvent('Worker finished', result);
        span.finish();
    });
}
```

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