Plug for writing access logs
Failed to load latest commit information.
.gitignore bootstraps plug Jan 10, 2015


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, "~> 0.14" } ]

You should also update your applications to start the plug:

def application do
  [ applications: [ :plug_accesslog ] ]


The easiest way to use the plug is to add it to your existing 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" }

Do Not Log Filter

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

defmodule LogFilter do
  def dontlog?(conn), do: "/favicon.ico" == full_path(conn)

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
> -> /

Formatting directives

The following formatting directives are available:

  • %% - Percentage sign
  • %a - Remote IP-address
  • %b - Size of response in bytes. Outputs "-" when no bytes are sent.
  • %B - Size of response in bytes. Outputs "0" when no bytes are sent.
  • %{VARNAME}C - Cookie sent by the client
  • %D - Time taken to serve the request (microseconds)
  • %{VARNAME}e - Environment variable contents
  • %h - Remote hostname
  • %{VARNAME}i - Header line sent by the client
  • %l - Remote logname
  • %m - Request method
  • %M - Time taken to serve the request (milliseconds)
  • %{VARNAME}o - Header line sent by the server
  • %P - The process ID that serviced the request
  • %q - Query string (prepended with "?" or empty string)
  • %r - First line of HTTP request
  • %>s - Response status code
  • %t - Time the request was received in the format [10/Jan/2015:14:46:18 +0100]
  • %T - Time taken to serve the request (full seconds)
  • %{UNIT}T - Time taken to serve the request in the given UNIT
  • %u - Remote user
  • %U - URL path requested (without query string)
  • %v - Server name
  • %V - Server name (canonical)

Note for %b and %B: To determine the size of the response the "Content-Length" will be inspected and, if available, returned unverified. If the header is not present the response body will be inspected using byte_size/1.

Note for %h: The hostname will always be the ip of the client (same as %a).

Note for %l: Always a dash ("-").

Note for %r: For now the http version is always logged as "HTTP/1.1", regardless of the true http version.

Note for %T: Rounding happens, so "0.6 seconds" will be reported as "1 second".

Note for %{UNIT}T: Available units are s for seconds (same as %T), ms for milliseconds (same as M) and us for microseconds (same as %D).

Note for %V: Alias for %v.


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