Use quantum core for update scheduling

Rather than requiring a traditional crontab install, the app now
leverages quantum-core (link below) to schedule the instance update/sync
task every 5 minutes. Some updates as a result:

  - The new job is scheduled at runtime in server.ex.
  - The update.exs script was refactored to be compiled along with the
    rest of the app as instances.ex.
  - Scheduler and Server modules were added for creating and executing
    the new update task
  - All shell scripts were removed, as they are no longer needed

https://github.com/quantum-elixir/quantum-core

lib/farside/application.ex

@@ -7,7 +7,8 @@ defmodule Farside.Application do
 
   @impl true
   def start(_type, _args) do
-    plug_children = [
+
+    plug_children = System.get_env("FARSIDE_NO_ROUTER") && [] || [
       Plug.Cowboy.child_spec(
         scheme: :http,
         plug: Farside.Router,
@@ -19,9 +20,10 @@ defmodule Farside.Application do
     ]
 
     children = [
-      {Redix, {@redis_conn, [name: :redix]}} |
-      System.get_env("FARSIDE_NO_ROUTER") && [] || plug_children
-    ]
+      {Redix, {@redis_conn, [name: :redix]}},
+      Farside.Scheduler,
+      Farside.Server
+    ] ++ plug_children
 
     opts = [strategy: :one_for_one, name: Farside.Supervisor]
     Supervisor.start_link(children, opts)

lib/farside/instances.ex

@@ -0,0 +1,86 @@
+defmodule Farside.Instances do
+  @fallback_suffix Application.fetch_env!(:farside, :fallback_suffix)
+  @update_file Application.fetch_env!(:farside, :update_file)
+  @services_json Application.fetch_env!(:farside, :services_json)
+  @service_prefix Application.fetch_env!(:farside, :service_prefix)
+
+  def sync() do
+    File.rename(@update_file, "#{@update_file}-prev")
+    update()
+
+    # Add UTC time of last update
+    Redix.command(:redix, [
+      "SET",
+      "last_updated",
+      Calendar.strftime(DateTime.utc_now(), "%c")
+    ])
+  end
+
+  def request(url) do
+    cond do
+      System.get_env("FARSIDE_TEST") ->
+        :good
+      true ->
+        case HTTPoison.get(url) do
+          {:ok, %HTTPoison.Response{status_code: 200}} ->
+            # TODO: Add validation of results, not just status code
+            :good
+          _ ->
+            :bad
+        end
+    end
+  end
+
+  def update() do
+    {:ok, file} = File.read(@services_json)
+    {:ok, json} = Poison.decode(file, as: [%Service{}])
+
+    # Loop through all instances and check each for availability
+    for service <- json do
+      IO.puts "======== " <> service.type
+      result = Enum.filter(service.instances, fn(instance_url) ->
+        IO.puts "         " <> instance_url
+        request(instance_url <> service.test_url) == :good
+      end)
+
+      add_to_redis(service, result)
+      log_results(service.type, result)
+    end
+  end
+
+  def add_to_redis(service, instances) do
+    # Remove previous list of instances
+    Redix.command(:redix, [
+      "DEL",
+      "#{@service_prefix}#{service.type}"
+    ])
+
+    # Update with new list of available instances
+    Redix.command(:redix, [
+      "LPUSH",
+      "#{@service_prefix}#{service.type}"
+    ] ++ instances)
+
+    # Set fallback to one of the available instances,
+    # or the default instance if all are "down"
+    if Enum.count(instances) > 0 do
+      Redix.command(:redix, [
+        "SET",
+        "#{service.type}#{@fallback_suffix}",
+        Enum.random(instances)
+      ])
+    else
+      Redix.command(:redix, [
+        "SET",
+        "#{service.type}#{@fallback_suffix}",
+        service.fallback
+      ])
+    end
+  end
+
+  def log_results(service_name, results) do
+    {:ok, file} = File.open(@update_file, [:append, {:delayed_write, 100, 20}])
+    IO.write(file, "#{service_name}: #{inspect(results)}\n")
+    File.close(file)
+  end
+end

lib/farside/scheduler.ex

@@ -0,0 +1,3 @@
+defmodule Farside.Scheduler do
+  use Quantum, otp_app: :farside
+end

lib/farside/server.ex

@@ -0,0 +1,22 @@
+defmodule Farside.Server do
+  use GenServer
+  import Crontab.CronExpression
+
+  def init(init_arg) do
+    {:ok, init_arg}
+  end
+
+  def start_link(arg) do
+    if System.get_env("FARSIDE_TEST") do
+      IO.puts("Skipping sync job setup...")
+    else
+      Farside.Scheduler.new_job()
+      |> Quantum.Job.set_name(:sync)
+      |> Quantum.Job.set_schedule(~e[*/5 * * * *])
+      |> Quantum.Job.set_task(fn -> Farside.Instances.sync end)
+      |> Farside.Scheduler.add_job()
+    end
+
+    GenServer.start_link(__MODULE__, arg)
+  end
+end