การตรวจติดตามด้วย Telemetry และเมตริก

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.

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.

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.

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.

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.

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.

A Telemetry handler function must accept four arguments:

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

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

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.

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.

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

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!