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Plug for writing access logs.


To use the plug in your projects, edit your mix.exs file and add the project as a dependency:

defp deps do
    # ...
    {:plug_accesslog, "~> 0.15.0"},
    # ...


Add the plug to your plug pipeline/router:

defmodule AppRouter do
  use Plug.Router

  plug Plug.AccessLog,
    format: :clf,
    file: "/path/to/your/logs/access.log"

  plug :match
  plug :dispatch

  get "/hello" do
    send_resp(conn, 200, "world")

  match _ do
    send_resp(conn, 404, "oops")

Note: The usage examples apply to a usecase where your are using plug directly without any framework. Using the plug Plug.AccessLog line in a framework based on plug should be no problem. Please refer to your frameworks individual documentation or source to find a suitable place.

WAL Configuration

All log messages that will be written to a file are collected in a WAL process before actual writing. The messages will be fetched in a configurable interval to be written to the logfiles:

config :plug_accesslog,
    flush_interval: 100

The time is configured as "milliseconds between writing and flushing". The default value is 100 milliseconds.

Custom Formatters

If you want to extend the formatting capabilities or replace existing ones you can define a custom formatter pipeline to use:

defmodule CustomFormatter do
  @behaviour Plug.AccessLog.Formatter

  def format(format, conn) do
    # manipulate to your liking

defmodule Router do
  use Plug.Router

  plug Plug.AccessLog,
    format: :clf,
    formatters: [CustomFormatter, Plug.AccessLog.DefaultFormatter],
    file: "/path/to/your/logs/access.log"

If you do not configure a list of formatters only the DefaultFormatter will be used. If you define an empty list then no formatting will take place.

All formatters are called in the order they are defined in.

File Configuration

There are two ways to define the file you want log entries to be written to:

defmodule Router do
  use Plug.Router

  plug Plug.AccessLog, file: "/static/configuration.log"
  plug Plug.AccessLog, file: {:system, "SYS_ENV_VAR_WITH_FILE_PATH"}
  plug Plug.AccessLog, file: {:system, "SYS_ENV_VAR", "/path/to/default.log"}

Do Not Log Filter

To filter the requests before logging you can configure a "do not log" filter function:

defmodule LogFilter do
  def dontlog?(%{request_path: "/favicon.ico"}), do: true
  def dontlog?(_conn), do: false

defmodule Router do
  use Plug.Router

  plug Plug.AccessLog,
    dontlog: &LogFilter.dontlog?/1,
    format: :clf,
    file: "/path/to/your/logs/access.log"

If the function you pass to the plug returns true the request will not be logged.

Logging Functions

To have the parsed log message sent to a logging function instead of writing it to a file you can configure a logging function:

defmodule InfoLogger do
  def log(msg), do: Logger.log(:info, msg)

defmodule Router do
  use Plug.Router

  plug Plug.AccessLog,
    format: :clf,
    fun: &InfoLogger.log/1

If a logging function is configured the configured file (if any) will be ignored.

Log Format

The default format is CLF.

Available formats

Besides a self defined format you can use one of the predefined aliases:

> %{User-Agent}i
> curl/7.35.0

> %h %l %u %t "%r" %>s %b
> - - [10/Jan/2015:14:46:18 +0100] "GET / HTTP/1.1" 200 31337

> %v %h %l %u %t "%r" %>s %b
> - - [10/Jan/2015:14:46:18 +0100] "GET / HTTP/1.1" 200 31337

> %h %l %u %t "%r" %>s %b "%{Referer}i" "%{User-Agent}i"
> - - [22/Jan/2015:19:33:58 +0100] "GET / HTTP/1.1" 200 2 "" "curl/7.35.0"

> %v %h %l %u %t "%r" %>s %b "%{Referer}i" "%{User-Agent}i"
> - - [22/Jan/2015:19:33:58 +0100] "GET / HTTP/1.1" 200 2 "" "curl/7.35.0"

> %{Referer}i -> %U
> -> /

For more details about each formatting token and potential modifications please refer to the Plug.AccessLog.DefaultFormatter module.


A small utility script is provided to check how long it might take to process requests and write the log messages to your disk:

mix run utils/bench.exs

This call will send of a total of 10k requests and wait for them to be written to the disk.

Looking at the data written to utils/bench.log might give a hint at what overhead the log writing is introducing. As with all "benchmarks" of any kind: take the measurements with a pinch of salt and run them in dozens of different conditions yourself.


Apache License, Version 2.0