Existence

Asynchronous dependency health checks library.

Features

• User defined dependencies checks with flexible settings.
• Dependencies checks functions are executed asynchronously.
• Built-in Plug module providing customizable response for a http health-check endpoint.
• Support for multiple independent Existence instances and associated health-checks endpoints (example use case: separate Kubernetes readiness and liveness http probes).
• Checks states are stored and accessed using a dedicated ETS tables per Existence instance, which means practically unlimited requests per second processing capacity.

Installation

Add Existence library to your application dependencies:

def deps do
  [
    {:existence, "~> 0.1.1"}
  ]
end

Usage

Define dependencies checks functions MFA's and start Existence child with your application supervisor

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    health_checks = [
      check_1: %{mfa: {MyApp.Checks, :check_1, []}},
      check_2: %{mfa: {MyApp.Checks, :check_2, []}}
    ]

    children = [{Existence, checks: health_checks}]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Declare your dependencies checks functions:

defmodule MyApp.Checks do
  def check_1(), do: :ok
  def check_2(), do: :ok
end

Configure your Phoenix router to respond to the /healthcheck endpoint requests using for example Plug.Router.forward/2:

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  forward("/healthcheck", Existence.Plug)
end

Get current overall health-check state:

iex> Existence.get_state()
:ok

List individual dependencies checks current states:

iex> Existence.get_checks()
[check_1: :ok, check_2: :ok]

TODO

• add telemetry event emitted on an overall health-check state change.