Elixir & Phoenix: Scalable Backend Development · Lesson

Monitoring with Telemetry and Metrics

Integrate Telemetry into your application to emit metrics and gain insights into its runtime behavior.

Monitoring with Telemetry and Metrics is a free Elixir & Phoenix: Scalable Backend Development lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Elixir & Phoenix: Scalable Backend Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What is Elixir Telemetry?

In Elixir, Telemetry is a powerful, low-level event dispatching library. It helps you understand what's happening inside your running application.

It's built into Elixir/OTP, making it a standard way to observe systems.
Think of it as a central nervous system for your app's events.
It enables you to collect metrics, trace operations, and monitor performance.

It's crucial for building observable and maintainable applications.

Events: The Core of Telemetry

Telemetry works by emitting events. An event is a signal that something notable has happened in your application.

Each event has a unique name (a list of atoms, e.g., [:my_app, :user, :login]).
Events can carry measurements (numeric values like duration) and metadata (additional context like user ID).
You don't emit events directly; instead, you use functions that wrap your code.

Measuring Duration with Telemetry.span

One common use of Telemetry is to measure how long an operation takes. This is done using :telemetry.span/3.

It executes a given function and measures its execution time.
It emits two events: one when the span starts and one when it ends.
The end event includes the duration as a measurement.

This is great for profiling function calls or database queries.

Emitting Discrete Events with Telemetry.execute

Sometimes, you just want to signal that an action occurred, without necessarily measuring its duration. For this, use :telemetry.execute/3.

It emits a single event with specified measurements and metadata.
Useful for tracking things like cache hits/misses, background job completions, or specific API calls.
It's a simpler, more direct way to emit a single point-in-time event.

Code: Emitting a Telemetry Span

Let's see :telemetry.span/3 in action. This example simulates a 'processing' task and measures its duration.

We define a simple module and call a function that wraps its work in a span.

defmodule MyApp.Worker do
  require Logger

  def process_data(data) do
    Logger.info("Starting data processing...")
    :telemetry.span(
      [:my_app, :worker, :process_data],
      %{input_length: byte_size(data)},
      fn ->
        # Simulate some work
        :timer.sleep(100)
        result = String.upcase(data)
        {:ok, result}
      end
    )
  end
end

# --- Main execution for CoddyKit ---
Logger.info("Calling MyApp.Worker.process_data...")
{:ok, result} = MyApp.Worker.process_data("hello elixir")
IO.puts("Processed result: #{result}")
IO.puts("\n(No Telemetry output yet - we need a handler!)")

Attaching Telemetry Handlers

Emitting events is only half the story! To do something useful with them, you need to attach handlers.

A handler is a function that gets called whenever a specific Telemetry event is emitted.
You attach a handler using :telemetry.attach/4, specifying the event name and your handler function.
Handlers allow you to react to events: log them, send them to a metrics system, or trigger other actions.

They are the listeners of your application's internal signals.

Handler Function Structure

A Telemetry handler function must accept four arguments:

event_name: The name of the event (e.g., [:my_app, :user, :login]).
measurements: A map of numeric values associated with the event (e.g., %{duration: 123456}).
metadata: A map of additional context (e.g., %{user_id: 123}).
config: Any custom configuration passed during attachment.

Your handler logic will use these arguments to process the event.

Code: Attaching and Handling Events

Now let's add a handler to our previous example. This handler will simply log the event details.

Notice how :telemetry.attach/4 links our handler function to the specific event name.

defmodule MyApp.WorkerWithHandler do
  require Logger

  # Define our simple handler function
  def handle_event(event_name, measurements, metadata, _config) do
    Logger.info("\n--- Telemetry Event Received ---")
    Logger.info("Event: #{inspect(event_name)}")
    Logger.info("Measurements: #{inspect(measurements)}")
    Logger.info("Metadata: #{inspect(metadata)}")
    Logger.info("------------------------------")
  end

  # Function that emits a Telemetry event
  def process_data(data) do
    Logger.info("Starting data processing...")
    :telemetry.span(
      [:my_app, :worker, :process_data],
      %{input_length: byte_size(data)},
      fn ->
        :timer.sleep(100)
        result = String.upcase(data)
        {:ok, result}
      end
    )
  end
end

# --- Main execution for CoddyKit ---
# Attach the handler *before* emitting events
:telemetry.attach(
  "my-worker-handler",
  [:my_app, :worker, :process_data], # Event name to listen for
  &MyApp.WorkerWithHandler.handle_event/4, # Our handler function
  nil # Optional config
)
Logger.info("Telemetry handler 'my-worker-handler' attached.")

# Perform some work, which will now trigger the handler
{:ok, result} = MyApp.WorkerWithHandler.process_data("hello world")
IO.puts("Processed result: #{result}")

# Clean up: detach the handler (good practice in tests/scripts)
:telemetry.detach("my-worker-handler")
Logger.info("Telemetry handler 'my-worker-handler' detached.")

Telemetry as a Metrics Source

Telemetry is not a metrics system itself, but it's an excellent source for them. By attaching handlers, you can feed events into dedicated metrics libraries.

Libraries like telemetry_metrics can aggregate Telemetry events into counters, gauges, and histograms.
These aggregated metrics are then often exported to monitoring systems like Prometheus or Datadog.
This separation keeps Telemetry lightweight and flexible, allowing you to choose your preferred metrics backend.

Quick Check: Telemetry Handlers

You've learned about emitting and handling Telemetry events. Let's test your understanding.

Recap: Telemetry for Observability

In this lesson, we explored Elixir's Telemetry library:

Telemetry is an event dispatching system for internal application observability.
It uses events with unique names, measurements, and metadata.
:telemetry.span/3 measures code execution duration.
:telemetry.execute/3 emits discrete, point-in-time events.
Handlers are functions attached via :telemetry.attach/4 to process events.
Telemetry events are a prime source for building application metrics.

Mastering Telemetry is key to understanding and monitoring your Elixir applications effectively!

Frequently asked questions

Is the “Monitoring with Telemetry and Metrics” lesson free?

Yes — the full text of “Monitoring with Telemetry and Metrics” is free to read here on the web, and the Elixir & Phoenix: Scalable Backend Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Elixir & Phoenix: Scalable Backend Development course, upgrade to CoddyKit PRO.

What will I learn in “Monitoring with Telemetry and Metrics”?

Integrate Telemetry into your application to emit metrics and gain insights into its runtime behavior. You practise Elixir & Phoenix: Scalable Backend Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Elixir & Phoenix: Scalable Backend Development?

No prior experience is required. Elixir & Phoenix: Scalable Backend Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Monitoring with Telemetry and Metrics” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Elixir & Phoenix: Scalable Backend Development lesson?

Yes. Every Elixir & Phoenix: Scalable Backend Development lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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