A platform agnostic tracing library
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2.0 Upgrade Guide

This is Datadog-specific since that's currently the only adapter.

  • Include the adapter as a dependency (see below).
  • Replace any occurrences of Spandex.Adapters.Datadog with SpandexDatadog.Adapter in your code.
  • Replace any occurences of Spandex.Adapters.ApiSender with SpandexDatadog.ApiSender in your code.


  • Datadog
  • Thats it so far! If you want another adapter, it should be relatively easy to write! This library is in charge of handling the state management of spans, and the adapter is just in charge of generating certain values and ultimately sending the values to the service.


This library could use some work! I've become unexpectedly busy lately, so I haven't had the time I thought I would to work on it. Any contributions, to things like sampling, strict mode, different storage strategies and the like would be greatly appreciated.


def deps do
  [{:spandex, "~> 2.4.0"}]

Setup and Configuration

Define your tracer:

defmodule MyApp.Tracer do
  use Spandex.Tracer, otp_app: :mya_app

Configure it:

config :my_app, MyApp.Tracer,
  service: :my_api,
  adapter: SpandexDatadog.Adapter,
  disabled?: false,
  env: "PROD"

Or at runtime, by calling configure/1 (usually in your application's startup)

MyApp.Tracer.configure(disabled?: System.get_env("TRACE") != "true")

For more information on Tracer configuration, view the docs for Spandex.Tracer. There you will find the documentation for the opts schema. The entire configuration can also be passed into each function in your tracer to be overridden if desired. For example:

MyApp.Tracer.start_span("span_name", service: :some_special_service)

Your configuration and the configuration in your config files are merged together, to avoid needing to specify this config at all times.

To bypass the Tracer pattern entirely, you can call directly into the functions in Spandex, like Spandex.start_span("span_name", [adapter: Foo, service: :bar]). Note that in this case, you will need to specify all of the configuration options in each call, because the Tracer is not managing the defaults for you.

Adapter specific configuration

For adapter configuration, see the documentation for that adapter

Phoenix Plugs

There are 3 plugs provided for usage w/ Phoenix:

  • Spandex.Plug.StartTrace - See moduledocs for options. Goes as early in your pipeline as possible.
  • Spandex.Plug.AddContext - See moduledocs for options. Either after the router, or inside a pipeline in the router.
  • Spandex.Plug.EndTrace - Must go after your router.

Distributed Tracing

Individual adapters can support distributed tracing. See their documentation for more information.

Logger metadata

In general, you'll probably want the current span_id and trace_id in your logs, so that you can find them in your tracing service. Make sure to add span_id and trace_id to logger_metadata

config :logger, :console,
  metadata: [:request_id, :trace_id, :span_id]

General Usage

The nicest interface for working with spans is the span macro, illustrated in span_me_also below.

defmodule ManuallyTraced do
  require Spandex

  # Does not handle exceptions for you.
  def trace_me() do
    Tracer.start_trace("my_trace") #also opens a span
    Tracer.update_span(service: :my_app, type: :db)

    result = span_me()



  # Does not handle exceptions for you.
  def span_me() do
    Tracer.update_span(service: :my_app, type: :web)

    result = span_me_also()


  # Handles exception at the span level. Trace still must be reported.
  def span_me_also() do
    Tracer.span("span_me_also) do


See the module documentation for Spandex.Span as well as the documentation for the structs contained as keys for that struct. They illustrate the keys that are known to either be common keys or to have UI sugar with certain clients. Its hard to find any kind of list of these published anywhere, so let me know if you know of more!

For example:

  type: :db,
  http: [url: "/posts", status_code: 400],
  sql_query: [query: "SELECT * FROM posts", rows: 10]

Asynchronous Processes

The current trace_id and span_id can be retrieved and later used (for example, from another process) as follows:

trace_id = Tracer.current_trace_id()
span_id = Tracer.current_span_id()
span_context = %SpanContext{trace_id: trace_id, parent_id: span_id}
Tracer.continue_trace("new_trace", span_context)

New spans can then be logged from there and sent in a separate batch.


There is (currently and temporarily) only one storage strategy, which can be changed via the strategy option. See Tracer opt documentation for an example of setting it. To implement your own (ETS adapter should be on its way), simply implement the Spandex.Strategy behaviour. Keep in mind that the strategy is not an atomic pattern. It represents retrieving and wholesale replacing a trace, meaning that it is not safe to use across processes or concurrently. Each process should have its own store for its own generated spans. This should be fine because you can send multiple batches of spans for the same trace separately.


Because the decorator library can cause conflicts when it interacts with other dependencies in the same project, we support it as an optional dependency. This allows you to disable it if it causes problems for you, but it also means that you need to explicitly include some version of decorator in your application's dependency list:

# mix.exs

defp deps do
    {:decorator, "~> 1.2"}

Then, configure the Spandex decorator with your default tracer:

config :spandex, :decorators, tracer: MyApp.Tracer

Span function decorators take an optional argument which is the attributes to update the span with. One of those attributes can be the :tracer in case you want to override the default tracer (e.g., in case you want to use multiple tracers).

IMPORTANT If you define multiple clauses for a function, you'll have to decorate all of the ones you want to span.

defmodule TracedModule do
  use Spandex.Decorators

  @decorate trace(service: :my_app, type: :web)
  def trace_me() do

  @decorate span(name: "span_1")
  def span_1() do

  @decorate span()
  def inner_span_1() do
    _ = ThirdPartyApi.different_service_call()

  @decorate span(tracer: MyApp.OtherTracer)
  def inner_span_2() do
    "this produces a span stack to be reported by another tracer"

  # Multiple Clauses
  @decorate span()
  def divide(n, 0), do: {:error, :divide_by_zero}
  @decorate span()
  def divide(n, m), do: n / m

defmodule ThirdPartyApi do
  use Spandex.Decorators

  @decorate span(service: :third_party, type: :cache)
  def different_service_call() do

Note: Decorators don't magically do everything. It often makes a lot of sense to use Tracer.update_span from within your function to add details that are only available inside that same function.

Ecto Tracing

Check out spandex_ecto.

Phoenix Tracing

Check out spandex_phoenix.