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, "~> 0.2.4"}
  ]
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 string 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: