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Instrumenter to handle logging of LiveView and LiveComponent life-cycle events.

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Overview

The Phoenix.LiveView.Logger module provides automatic logging for Phoenix LiveView and LiveComponent lifecycle events using Telemetry. It logs mount, handle_params, and handle_event callbacks with parameter filtering support.

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Installation

The logger is installed automatically when LiveView starts. By default, the log level is set to :debug.

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Configuration

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Module-Level Configuration

Override the log level for an individual LiveView module: 
defmodule MyAppWeb.UserLive do
  use Phoenix.LiveView, log: :debug
  
  # ...
end
Disable logging for a specific LiveView: 
defmodule MyAppWeb.PrivateLive do
  use Phoenix.LiveView, log: false
  
  # ...
end

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Available Log Levels

  • :debug (default) - Logs all events
  • :info - Logs at info level
  • :warn - Logs warnings only
  • :error - Logs errors only
  • false - Disables logging for the module

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Logged Events

The following Telemetry events are automatically logged:

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LiveView Events

  • [:phoenix, :live_view, :mount, :start] - LiveView mount begins
  • [:phoenix, :live_view, :mount, :stop] - LiveView mount completes
  • [:phoenix, :live_view, :handle_params, :start] - Handle params begins
  • [:phoenix, :live_view, :handle_params, :stop] - Handle params completes
  • [:phoenix, :live_view, :handle_event, :start] - Handle event begins
  • [:phoenix, :live_view, :handle_event, :stop] - Handle event completes

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LiveComponent Events

  • [:phoenix, :live_component, :handle_event, :start] - Component event begins
  • [:phoenix, :live_component, :handle_event, :stop] - Component event completes

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Log Format

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Mount Logging

Start: 
MOUNT MyAppWeb.UserLive
  Parameters: %{"id" => "123"}
  Session: %{"user_token" => "..."}
Stop: 
Replied in 45ms

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Handle Params Logging

Start: 
HANDLE PARAMS in MyAppWeb.UserLive
  Parameters: %{"id" => "123", "tab" => "profile"}
Stop: 
Replied in 12ms

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Handle Event Logging

LiveView Event Start: 
HANDLE EVENT "save" in MyAppWeb.FormLive
  Parameters: %{"user" => %{"name" => "Alice"}}
LiveComponent Event Start: 
HANDLE EVENT "delete" in MyAppWeb.PageLive
  Component: MyAppWeb.UserComponent
  Parameters: %{"id" => "123"}
Stop: 
Replied in 23ms

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Parameter Filtering

The logger automatically filters parameters based on Phoenix.Logger configuration.

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Configure Parameter Filtering

In your config.exs: 
config :phoenix, :filter_parameters, ["password", "token", "secret"]
Before filtering: 
Parameters: %{"user" => %{"email" => "[email protected]", "password" => "secret123"}}
After filtering: 
Parameters: %{"user" => %{"email" => "[email protected]", "password" => "[FILTERED]"}}

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Functions

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install/0

Installs telemetry handlers for LiveView logging. Called automatically on application start. 
@doc false
install()

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Handler Functions

These functions are called by Telemetry and are not intended for direct use:

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lv_mount_start/4

Logs the start of a LiveView mount. 
lv_mount_start(event, measurement, metadata, config)

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lv_mount_stop/4

Logs the completion of a LiveView mount with duration. 
lv_mount_stop(event, measurement, metadata, config)

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lv_handle_params_start/4

Logs the start of handle_params callback. 
lv_handle_params_start(event, measurement, metadata, config)

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lv_handle_params_stop/4

Logs the completion of handle_params callback. 
lv_handle_params_stop(event, measurement, metadata, config)

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lv_handle_event_start/4

Logs the start of a LiveView event handler. 
lv_handle_event_start(event, measurement, metadata, config)

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lv_handle_event_stop/4

Logs the completion of a LiveView event handler. 
lv_handle_event_stop(event, measurement, metadata, config)

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lc_handle_event_start/4

Logs the start of a LiveComponent event handler. 
lc_handle_event_start(event, measurement, metadata, config)

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lc_handle_event_stop/4

Logs the completion of a LiveComponent event handler. 
lc_handle_event_stop(event, measurement, metadata, config)

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Connected vs Disconnected

Mount and handle_params events are only logged for connected sockets. Initial HTTP render (disconnected mount) is not logged to reduce noise. Event handlers are logged for both connected and disconnected states since they only occur after connection.

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Example Output

[debug] MOUNT MyAppWeb.DashboardLive
  Parameters: %{}
  Session: %{"user_id" => 42}
[debug] Replied in 67ms

[debug] HANDLE PARAMS in MyAppWeb.DashboardLive
  Parameters: %{"tab" => "overview"}
[debug] Replied in 8ms

[debug] HANDLE EVENT "refresh" in MyAppWeb.DashboardLive
  Parameters: %{}
[debug] Replied in 145ms

[debug] HANDLE EVENT "delete_item" in MyAppWeb.DashboardLive
  Component: MyAppWeb.ItemComponent
  Parameters: %{"id" => "123"}
[debug] Replied in 34ms

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Telemetry Integration

For custom telemetry handling, see the Telemetry guide. You can attach your own handlers to these events: 
:telemetry.attach(
  "my-handler",
  [:phoenix, :live_view, :mount, :stop],
  &MyApp.Telemetry.handle_event/4,
  %{}
)

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Performance Monitoring

Use the duration information from stop events for performance monitoring: 
def handle_event(
  [:phoenix, :live_view, :mount, :stop],
  %{duration: duration},
  metadata,
  _config
) do
  if duration > 1_000_000_000 do # 1 second in nanoseconds
    Logger.warn("Slow mount detected: #{inspect(metadata.socket.view)}")
  end
end

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