Understanding Phoenix Logging with Telemetry and Plug.Telemetry

This article delves into the mechanisms of logging in the Phoenix framework, focusing on the processing of incoming HTTP requests and outgoing responses. The writer explains the role of the Erlang 'telemetry' package and the Elixir 'Plug.Telemetry' function in enabling this logging feature. By utilizing the Telemetry library, Phoenix can dynamically dispatch events related to request handling, which can then be used for metrics and instrumentation. A step-by-step guide is provided, showing how to attach handlers to events emitted by the application, using ':telemetry.attach/4' and ':telemetry.execute/2'. These principles are then applied to understand how 'Plug.Telemetry' emits events when processing requests. Central to logging in Phoenix is the 'Phoenix.Logger' module, which attaches to the ':phoenix, :endpoint, :start' and ':phoenix, :endpoint, :stop' telemetry events to log information about the HTTP method, path, status, and response times of the processed connections. The article concludes by providing insights into practical application and the convenience of leveraging Telemetry for event handling.