Testing
When relying on OpenTelemetry for your Observability needs, it can be important to test that certain spans are created and attributes correctly set. For example, can you be sure that you attaching the right metadata to data that ultimately powers an SLO? This document covers an approach to that kind of validation.
Setup
Only the opentelemetry and opentelemetry_api libraries are required for
testing in Elixir/Erlang:
{deps, [{opentelemetry_api, "~> 1.2"},
{opentelemetry, "~> 1.3"}]}.
def deps do
[
{:opentelemetry_api, "~> 1.2"},
{:opentelemetry, "~> 1.3"}
]
end
Set your exporter to :none and the span processor to
:otel_simple_processor. This ensure that your tests don’t actually export data
to a destination, and that spans can be analyzed after they are processed.
%% config/sys.config.src
{opentelemetry,
[{traces_exporter, none},
{processors,
[{otel_simple_processor, #{}}]}]}
# config/test.exs
import Config
config :opentelemetry,
traces_exporter: :none
config :opentelemetry, :processors, [
{:otel_simple_processor, %{}}
]
A modified version of the hello function from the
Getting Started guide will serve as
our test case:
%% apps/otel_getting_started/src/otel_getting_started.erl
-module(otel_getting_started).
-export([hello/0]).
-include_lib("opentelemetry_api/include/otel_tracer.hrl").
hello() ->
%% start an active span and run a local function
?with_span(<<"operation">>, #{}, fun nice_operation/1).
nice_operation(_SpanCtx) ->
?set_attributes([{a_key, <<"a value">>}]),
world
# lib/otel_getting_started.ex
defmodule OtelGettingStarted do
require OpenTelemetry.Tracer, as: Tracer
def hello do
Tracer.with_span "operation" do
Tracer.set_attributes([{:a_key, "a value"}])
:world
end
end
end
Testing
-module(otel_getting_started_SUITE).
-compile(export_all).
-include_lib("stdlib/include/assert.hrl").
-include_lib("common_test/include/ct.hrl").
-include_lib("opentelemetry/include/otel_span.hrl").
-define(assertReceive(SpanName),
receive
{span, Span=#span{name=SpanName}} ->
Span
after
1000 ->
ct:fail("Did not receive the span after 1s")
end).
all() ->
[greets_the_world].
init_per_suite(Config) ->
application:load(opentelemetry),
application:set_env(opentelemetry, processors, [{otel_simple_processor, #{}}]),
{ok, _} = application:ensure_all_started(opentelemetry),
Config.
end_per_suite(_Config) ->
_ = application:stop(opentelemetry),
_ = application:unload(opentelemetry),
ok.
init_per_testcase(greets_the_world, Config) ->
otel_simple_processor:set_exporter(otel_exporter_pid, self()),
Config.
end_per_testcase(greets_the_world, _Config) ->
otel_simple_processor:set_exporter(none),
ok.
greets_the_world(_Config) ->
otel_getting_started:hello(),
ExpectedAttributes = otel_attributes:new(#{a_key => <<"a_value">>}, 128, infinity),
#span{attributes=ReceivedAttributes} = ?assertReceive(<<"operation">>),
%% use an assertMatch instead of matching in the `receive'
%% so we get a nice error message if it fails
?assertMatch(ReceivedAttributes, ExpectedAttributes),
ok.
defmodule OtelGettingStartedTest do
use ExUnit.Case
# Use Record module to extract fields of the Span record from the opentelemetry dependency.
require Record
@fields Record.extract(:span, from: "deps/opentelemetry/include/otel_span.hrl")
# Define macros for `Span`.
Record.defrecordp(:span, @fields)
test "greets the world" do
# Set exporter to :otel_exporter_pid, which sends spans
# to the given process - in this case self() - in the format {:span, span}
:otel_simple_processor.set_exporter(:otel_exporter_pid, self())
# Call the function to be tested.
OtelGettingStarted.hello()
# Use Erlang's `:otel_attributes` module to create attributes to match against.
# See the `:otel_events` module for testing events.
attributes = :otel_attributes.new([a_key: "a value"], 128, :infinity)
# Assert that the span emitted by OtelGettingStarted.hello/0 was received and contains the desired attributes.
assert_receive {:span,
span(
name: "operation",
attributes: ^attributes
)}
end
end
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