hackney_telemetry

Telemetry adapter for Hackney metrics.

This module is a metrics handler for the Hackney HTTP client. It receives calls from Hackney to update metrics and generates Telemetry events.

Hackney supports storing metrics in Folsom or Exometer. Unfortunately, these libraries do not export data in a way that is useful for Telemetry, so we need to transform the metrics data before reporting it.

Telemetry metrics

The following metrics are exported by this library to telemetry.

Metric	Tags	Meaning
`hackney.nb_requests`	-	Current number of requests
`hackney.finished_requests`	-	Total number of finished requests
`hackney.total_requests`	-	Total number of requests
`hackney_pool.free_count`	pool	Number of free sockets in a connection pool
`hackney_pool.in_use_count`	pool	Number of busy sockets in a connection pool
`hackney_pool.no_socket`	pool	Count of new connections
`hackney_pool.queue_count`	pool	Number of requests waiting for a connection in a pool
`hackney_pool.take_rate`	pool	Rate at which a connection is retrieved from the pool

This module implements all the callbacks required by hackney_metrics but it does not support host metrics.

To use it, configure Hackney mod_metrics to use this module and make sure that the hackney_telemetry application starts before your application.

Hackney calls the module specified by mod_metrics to report instrumentation metrics. This module receives the data from Hackney and passes it to a hackney_telemetry_worker which keeps the current state of the metric and generates Telemetry events.

A worker process has two jobs:

Calculate metric values

Hackney does not keep the state of its metrics, but instead emits events to the metrics engine, like "increase this counter by 1", "set this gauge to X", "add Y to this histogram". The job of a metric worker is to process these events and keep up-to-date state representing the value of the tracked metric. State updates run in constant time (O(1)), important since a single request generates about nine metric updates.
Send metric values to Telemetry

If we send the metric value to Telemetry after every update, then telemetry processing may not be able to keep up, and Telemetry will apply backpressure.

Since the worker maintains the most up-to-date value, we can send the current value periodically. Gauge metrics may be less accurate, but it avoids overload.

Installation

Install it from Hex or Github.

Configuration

Configure set Hackney to send metrics to this module:

Erlang

{hackney, [{mod_metrics, hackney_telemetry}]}

Elixir

config :hackney, mod_metrics: :hackney_telemetry

Options

Report interval

By default, workers will report data to telemetry every 1000 milliseconds. If set to 0, events are generated after every update.

You can change that by setting the report_interval option:

Erlang

{hackney_telemetry, [{report_interval, 2000}]}

Elixir

config :hackney_telemetry, report_interval: 2_000

Usage

After installing the module, your application will receive Telemetry events. You can handle them in your application, or install a reporting module such as Telemetry.Metrics or prom_ex.

Elixir

defmodule YourApplcation.Telemetry do
  import Telemetry.Metrics

  def metrics do
  [
    # other metrics

    last_value("hackney.nb_requests"),
    last_value("hackney.finished_requests"),
    last_value("hackney.total_requests"),
    last_value("hackney_pool.free_count", tags: [:pool]),
    last_value("hackney_pool.in_use_count", tags: [:pool]),
    last_value("hackney_pool.no_socket", tags: [:pool]),
    last_value("hackney_pool.queue_count", tags: [:pool]),
    last_value("hackney_pool.take_rate", tags: [:pool])
  ]
  end
end

Building

To build the source code locally you'll need rebar3:

rebar3 compile

Test

rebar3 ct

Docs

rebar3 ex_doc

Format code

rebar3 steamroll

Code of Conduct

This project Contributor Covenant version 2.1. Check CODE_OF_CONDUCT.md file for more information.

License

hackney_telemetry source code is released under Apache License 2.0.

Check NOTICE and LICENSE files for more information.

jhonndabi/hackney_telemetry