# Fettle

Runs health-check functions periodically, and aggregates health status reports.

(AKA Elixir implementation of FT Health-Check standard).

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> **fettle**
*noun* - "his best players were in fine fettle": shape, trim, fitness, physical fitness, health, state of health; condition, form, repair, state of repair, state, order, working order, way; informal: kilter; British informal: nick.

This library implents an asynchronous periodic check mechanism, and defines a way of configuring checks, and getting reports in a particular format. It is intended for use with monitoring and dashboards.

Ships with the [FT Health Check V1](FTHealthcheckstandard.pdf) schema format for report generation, but the schema is configurable.

## See also 

* [`fettle_plug`]( - integration with [Plug]( to expose to HTTP.
* [`fettle_checks`]( - a small library of commonly useful checks.

## Getting Started

### Installation

Add a dependency in your `mix.exs` config:

For the bleeding edge:

def deps do
  [{:fettle, github: "Financial-Times/fettle"}]

Fettle is an OTP application, and in Elixir 1.4+, will be started with other applications through Elixir's auto-detection of OTP apps in dependencies.

### Defining checks

The [`fettle_checks`]( module provides pre-canned checks, but to write your own, implement a module with the `Fettle.Checker` `@behaviour` that runs a check, and returns a `Fettle.Check.Result` struct:

defmodule MyCheck do
  @behaviour Fettle.Checker

  def check(args) do
    case do_check(args) do  # assume do_check/1 does something useful!
      :ok -> 
        {Fettle.Check.Result.ok(), args}
      {:error, message} -> 
        {Fettle.Check.Result.error(message), args}

Then configure this check, with some required metadata, in the `:fettle` configuration:

config :fettle,
    system_code: "my_app", # required (for reports)
    checks: [
            name: "my-check",
            panic_guide_url: "https://...",
            business_impact: "...",
            technical_summary: "..."
            checker: MyCheck, # name of our check module
            args: "url_or_something" # arguments to the checker module (optional)

On application start-up, Fettle will start running your check, by default every 30 seconds, and you can retrieve the results using ``.

[`fettle_plug`]( can then be used to expose the results over an HTTP end-point.

See module docs for full details.