Existence

Asynchronous dependency health checks library.

Features

• User defined dependencies health-checks with flexible settings, including configurable health-check callback function timeout, startup delay and initial check state.
• Existence.Plug module providing customizable response for a http health-check endpoint.
• Support for multiple Existence instances and associated health-checks endpoints (example use case: separate Kubernetes readiness and liveness http probes).
• Dependencies health-checks states are cached and accessed using a dedicated ETS table per Existence instance, which means practically unlimited requests per second processing capacity without putting unnecessary load on a health-checked dependency.

Installation

Add Existence library to your application mix.exs file:

# mix.exs
def deps do
  [
    {:existence, "~> 0.3.1"}
  ]
end

Usage

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

# lib/my_app/application.ex
defmodule MyApp.Application do
  use Application

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

    children = [{Existence, checks: health_checks}]

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

Declare your dependencies checks callback functions, in our example case PostgreSQL health-check:

# lib/my_app/checks.ex
defmodule MyApp.Checks do
  def check_postgres() do
    "SELECT 1;"
    |> MyApp.Repo.query()
    |> case do
      {:ok, %Postgrex.Result{num_rows: 1, rows: [[1]]}} -> :ok
      _ -> :error
    end
  end
end

Dependency health-check function check_postgres/0 will be spawned asynchronously by default every 30 seconds, checking if PostgreSQL instance associated with MyApp.Repo is healthy. check_postgres/0 results will be cached in the ETS table used further to provide responses to user requests. If /healthcheck endpoint requests are issued thousands of times per second, they do not hit and overload PostgreSQL, instead cached results from ETS are returned.

Current overall health-check state can be examined with get_state/1:

iex> Existence.get_state()
:ok

Dependencies health-checks states can be examined with get_checks/1:

iex> Existence.get_checks()
[check_postgres: :ok]

Library provides Plug module generating responses to the /healthcheck endpoint requests. Example Existence.Plug usage with Plug.Router.forward/2:

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

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