Agent Check

HAProxy Agent Check protocol implementation for Elixir/Phoenix apps. Allows for easy rolling restarts and dynamic backpressure to your loadbalancer.

Installation

If available in Hex, the package can be installed by adding agent_check to your list of dependencies in mix.exs:

def deps do
  [
    {:agent_check, "~> 0.2.8"}
  ]
end

  def application do
    [
      extra_applications: [:logger, :agent_check],
      mod: {Your.Application, []}
    ]
  end

Then the following needs to be added to your config:

  config :agent_check,
         port: System.get_env("AGENT_CHECK_PORT") || 6666,
         capacity_callback: {Your.Application, :calculate_capacity},
         maint_callback: {Your.Application, :close_all_open_connections}

The capacity callback must return a binary or integer between 1 and 100. Both callbacks are optional.

Next you need to configure Haproxy to check for your agent.

backend your_backend_web
  mode tcp
  balance roundrobin
  server websrv1 192.168.1.101:443 weight 255 check agent-check agent-port 6666 agent-addr 192.168.1.101 agent-send state\n
  server websrv2 192.168.1.102:443 weight 255 check agent-check agent-port 6666 agent-addr 192.168.1.102 agent-send state\n

Usage

To check if the agent is running correctly, connect to it via telnet:

$ telnet 192.168.0.101 6666 
Trying 192.168.0.101...
Connected to websrv1.
Escape character is '^]'.
state
ready 100
Connection closed by foreign host.

The following commands are available:

Command	Description
maint	Go to maintenance mode
ready	Return from maintenance mode to up state
state	Get server state and capacity information in HAProxy format
stats	Get server state and capacity information in raw format