Fastify OpenTelemetry

A Fastify plugin that uses the OpenTelemetry API to provide request tracing.

Usage

npm i @autotelic/fastify-opentelemetry

Examples

Plugin Usage

// index.js
// Load your OpenTelemetry/API configuration first. Now the configured SDK will be available
// to fastify-opentelemetry. (See the example configuration below.)
require('./openTelemetryConfig')
const openTelemetryPlugin = require('@autotelic/fastify-opentelemetry')
const fastify = require('fastify')();

(async () => {
  await fastify.register(openTelemetryPlugin, { wrapRoutes: true })

  fastify.get('/', async function (request, reply) {
    const {
      activeSpan,
      tracer,
      // context,
      // extract,
      // inject,
    } = request.openTelemetry()
    // Spans started in a wrapped route will automatically be children of the activeSpan.
    const childSpan = tracer.startSpan(`${activeSpan.name} - child process`)
    // doSomeWork()
    childSpan.end()
    return 'OK'
  })

  fastify.listen(3000, (err, address) => {
    if (err) {
      fastify.log.error(err)
      process.exit(1)
    }
  })
})()

OpenTelemetry API Configuration

// openTelemetryConfig.js
const {
  BatchSpanProcessor,
  ConsoleSpanExporter,
  TraceIdRatioBasedSampler
} = require('@opentelemetry/sdk-trace-base')

const { NodeTracerProvider } = require('@opentelemetry/sdk-trace-node')

// Configure a tracer provider.
const provider = new NodeTracerProvider({
  sampler: new TraceIdRatioBasedSampler(0.5)
})

// Add a span exporter.
provider.addSpanProcessor(
  new BatchSpanProcessor(new ConsoleSpanExporter())
)

// Register a global tracer provider.
provider.register()

// Note: the above is just a basic example. fastify-opentelemetry is compatible with any
// @opentelemetry/api configuration.

See /example for a working example app. To run the example app locally, first clone this repo and then npm i and run:

npm run dev

API

Configuration

This plugin leaves all tracer configuration to the OpenTelemetry API. The tracer and propagation method are pulled in from the global tracer provider and global propagator, respectively. This allows the config for the plugin itself to be minimal.

The plugin accepts the the following configuration properties:

• exposeApi : boolean - Used to prevent the plugin from decorating the request. By default the request will be decorated (i.e. defaults to true).

• formatSpanName : (FastifyRequest) => string - Custom formatter for the span name. The default format is `${req.method} ${req.routeOptions.url}`.

• formatSpanAttributes : object - Contains formatting functions for span attributes. Properties:

  • request: (FastifyRequest) => object - On request, the returned object will be added to the current span's attributes. The default request attributes are:

    { 'req.method': request.raw.method, 'req.url': request.raw.url }
    

  • reply: (FastifyReply) => object - On reply, the returned object will be added to the current span's attributes. The default reply attributes are:

    { 'reply.statusCode': reply.statusCode }
    

  • error: (Error) => object - On error, the returned object will be added to the current span's attributes. The default error attributes are:

    {
      'error.name': error.name,
      'error.message': error.message,
      'error.stack': error.stack
    }
    

• wrapRoutes : boolean | string[] - When true, all route handlers will be executed with an active context equal to request.openTelemetry().context. Also accepts an array containing all route paths that should be wrapped. Optional - disabled by default.

  • Route paths must have a leading / and no trailing / (eg. '/my/exact/route/path').
  • Wrapping a route allows for any span created within the scope of the route handler to automatically become children of the current request's activeSpan. This includes spans created by automated OpenTelemetry instrumentations.
  • A global context manager (eg. AsyncHooksContextManager) is required to provide an active context to the route handler.

• ignoreRoutes : string[] | (path: string, method: string) => boolean - Configure the hooks added by this plugin to ignore certain route paths (ie. no automated tracing for that route). Takes precedence over wrapRoutes.

  • Can be an array of Route paths (following the same format as wrapRoutes).

  • Can be a function that receives a route's path and method, and returns a boolean (return true to ignore). For example, to disable tracing on OPTIONS routes:

    fastify.register(openTelemetryPlugin, {
      ignoreRoutes: (path, method) => method === 'OPTIONS'
    })
    

  • The ignored routes will still have access to request.openTelemetry, but activeSpan will be undefined.

• propagateToReply : boolean - When true, the current span context will be injected into the reply headers using your registered propagators. Defaults to false.

• spanOptions : SpanOptions | (FastifyRequest) => SpanOptions - SpanOptions or a function that returns SpanOptions. These will be passed into tracer.startSpan on each request. Defaults to { kind: 1 }.

Request Decorator

This plugin decorates the request with an openTelemetry function that returns an object with the following properties:

• context: Context - Context containing the active span along with any extracted context.

• activeSpan: Span - The active span (while this is available via context, here we just provide a shortcut to it.).

• extract: Propagation.extract - Wraps the propagation API's extract method, and passes in the current request's context as the context argument. This returns a new context and will not affect the request's trace context. Accepts the following arguments:

  • carrier: object - Object containing the context to be extracted.
  • getter: TextMapGetter - Object containing get and keys methods. Used to extract values from carrier. Defaults to OpenTelemetry's defaultTextMapGetter.

• inject: Propagation.inject - Wraps the propagation API's inject method, and passes in the current request's context as the context argument. Accepts the following arguments:

  • carrier: object - Object the context will be injected into.
  • setter: TextMapSetter - Object containing set method. Used to inject values into the carrier. Defaults to OpenTelemetry's defaultTextMapSetter.

• tracer: Tracer - The tracer created and used by the plugin.

Hooks

This plugin registers the following Fastify hooks:

• onRequest: Start the span.

• onReply: Stop the span.

• onError: Add error info to span attributes.

• onRoute: Added only if wrapRoutes is enabled.

• onSend: Added only if propagateToReply is enabled.

OpenTelemetry Compatibility

As the OpenTelemetry API uses a variable on the global object to store the global API, care needs to be taken to ensure all modules are compatible.

Each version of the OpenTelemetry API will require a specific release of fastify-opentelemetry.