Crow

Overview

Crow provides a Munin node implementation that deals with speaking the Munin data exchange protocol between the master and nodes. Crow plugins are implemented as modules following the Crow.Plugin behaviour.

What is Munin?

Quoting the official website:

Munin is a networked resource monitoring tool that can help analyze resource trends and "what just happened to kill our performance?" problems. It is designed to be very plug and play.

Munin follows a master-node architecture. The Munin master connects to nodes which run "plugins" on demand.

In the default Munin installation, a plugin is an executable (this can be a shell script, Python script, binary...) providing two invocation forms which print out metadata or graph values. In crow, we use modules instead of executables. See Writing plugins and the Crow.Plugin behaviour for more information.

A great overview about the architecture is available at the Munin's Architecture

Writing plugins

Crow plugins are modules implementing the Crow.Plugin behaviour.

The following plugin would display a graph of the current process count running in the BEAM:

defmodule MyApp.CrowPlugins.Processes do
  def name(_options) do
    'beam_process_count'
  end

  def config(_options) do
    [
      'graph_title Total processes',
      'graph_category BEAM',
      'graph_vlabel processes',
      'processes.label processes'
    ]
  end

  def values(_options) do
    [
      'processes.value #{length(:erlang.processes())}'
    ]
  end
end

The Crow.Plugin behaviour contains in-depth documentation of how to write your plugins. A full reference of plugin output can be found in the Plugin reference document. Additionally, example plugins can be found in the crow_plugins repository.

Configuration options

The following keys can be set under the :crow application key to configure crow:

:plugins - A list of modules implementing the Crow.Plugin behaviour which are listed to peers. This value can be changed at runtime and workers will pick up the change right away. For example, if you have crow_plugins installed and want the node to provide information about the BEAM, you could set the following:
```
# config/config.exs
config :crow,
  plugins: [
    CrowPlugins.BEAM.Memory,
    CrowPlugins.BEAM.SystemInfo
  ]
```
Plugins can also take options. For example, a plugin that receives mnesia table names to be shown in statistics could be configured like this:
```
# config/config.exs
config :crow,
  plugins: [
    {App.CrowPlugins.MnesiaTableSize, tables: [:users, :groups]}
  ]
```
:ip - The IP address to listen on. This is passed as the {:ip, ip} option to :gen_tcp.listen/2. By default, the node will listen on all network interfaces. For example, to only listen on localhost, you could use {127, 0, 0, 1}. This parameter can only be set at application start.
:port - The port to listen on. The default of 4949 matches the munin default, but you can change this as you like. This parameter can only be set at application start.