Automatic build time metadata generator for ELB ping endpoints and other sanity.

Reading time: about 4 minutes

One of the few problems that comes with a fully automated deployment pipeline is validating which version of code exists in production, and validating that version is in production at all deployment sites. A good automated deployment can be self documenting and provide hooks for api clients to understand what’s deployed as well.

Consider this meta-endpoint:

curl -i my.app.com/ping

HTTP1.1 200 OK
{
  "okay": true,
  "rails_env": "production",
  "signature": "AppName Backend (production) 0.0.16",
  "git_revision": "1234123451234512345123451234123451234512",
  "version": "0.0.16",
  "build_timestamp": "2019-09-02 13:13:16"
}

For devops purposes, the git_revision and build_timestamp are helpful.

For an api consumer, the signature and version are helpful.

For the application developer, the rails_env (or mix_env, etc) is a good sanity check. Some applications inspect the database connection, others check to make sure an external API is stable, etc. For ELB purposes, the okay: true is also mirrored in

And whatever self test sanity check desired can be wired up behind okay: true.

A simple set of tools is used to automatically generate this document at build time, and some light application hooks serve up the file at runtime.

scripts/build generates a JSON file which contains git_revision, version, and build_timestamp. This file is used by developers/CI to do a docker build, so any build time environment declarations or other docker build hijinks can be committed to version control here as well. The usage of trap ... EXIT ensures that the BUILD_DETAILS file is available to the docker build but doesn’t hang around after the build.

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#!/bin/bash

# scripts/build

set -euo pipefail

build_stats () {
  echo -n '{ "build_timestamp" : "'
  date "+%Y-%m-%d %H:%M:%S" | tr -d '\n'

  echo -n '", "git_revision" : "'
  git rev-parse HEAD | tr -d '\n'

  echo -n '", "version" : "'
  git tag -l \
    | grep release \
    | tr '-' ' ' \
    | awk '{ print $2 }' \
    | sort -V \
    | tail -n 1 \
    | tr -d '\n'

  echo '"}'
}

function cleanup {
  rm BUILD_DETAILS
}
trap cleanup EXIT
build_stats > BUILD_DETAILS

docker build . -t backend_server

For a rails server, config/application.rb runs once at application boot, and Rails.application.config provides a global scope to stash the metadata. Phoenix and other servers have a similar boot script and global scope config.

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module Backend
  class Application < Rails::Application
    config.git_revision = nil
    config.server_version = nil
    config.server_signature = "Backend (#{Rails.env})"
    config.build_timestamp = nil

    if File.exist? 'BUILD_DETAILS'
      build_details = JSON.parse File.read('BUILD_DETAILS')
      config.git_revision = build_details["git_revision"]
      config.server_version = build_details["version"]
      config.server_signature += " #{build_details["version"]}"
      config.build_timestamp = build_details["build_timestamp"]
    end
  end
end

Lastly, a rails StatusController which displays the expanded payload.

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class StatusController < ApplicationController
  # GET /ping
  # Targeted by load balancer to check for server health.
  def show
    status_hash = {
      okay: stable,
      rails_env: Rails.env,
      signature: Rails.application.config.server_signature
    }

    if revision = Rails.application.config.git_revision
      status_hash[:git_revision] = revision
    end

    if version = Rails.application.config.server_version
      status_hash[:version] = version
    end

    if timestamp = Rails.application.config.build_timestamp
      status_hash[:build_timestamp] = timestamp
    end

    http_code = :ok
    http_code = :not_acceptable unless stable

    render_json status_hash, status: http_code
  end

  private
  def stable
    # check stuff
    true
  end
end

Date: 2019-Sep-02
Tags: bash rails aws elb docker devops
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