Arcology.Application detangled

arcology_app.org

@@ -0,0 +1,109 @@
+:PROPERTIES:
+:ID:       20230303T100429.248407
+:END:
+#+TITLE: Arcology Elixir Backend Logic
+
+#+AUTO_TANGLE: t
+
+* Setting up the =Arcology= elixir module
+
+This is a module, the submodules are where the interesting things happen. The module doc says what it does:
+
+#+begin_src text :noweb-ref arcology.moduledoc
+Arcology keeps the contexts that define your domain
+and business logic.
+
+Contexts are also responsible for managing your data, regardless
+if it comes from the database, an external API or others.
+#+end_src
+
+#+begin_src elixir :tangle lib/arcology.ex :noweb yes
+defmodule Arcology do
+  @moduledoc """
+  <<arcology.moduledoc>>
+  """
+end
+#+end_src
+
+* =Arcology.Application= Elixir Application Startup
+
+#+begin_src elixir :tangle lib/arcology/application.ex :noweb yes
+defmodule Arcology.Application do
+  @moduledoc false
+
+  use Application
+
+  <<start_2>>
+
+  <<config_change_3>>
+end
+#+end_src
+
+So =Arcology.Application.start/2= is the entrypoint of the Phoenix [[https://hexdocs.pm/elixir/Application.html][OTP application]], it starts up a bunch of other *Supervised*[fn:1] Elixir processes and makes sure they keep running.
+
+** =Arcology.Application.start/2=
+
+We start the Ecto repository [[roam:Arcology.Repo]]:
+
+#+begin_src elixir :noweb-ref app-children
+Arcology.Repo
+#+end_src
+
+We start the PubSub messaging system:
+
+#+begin_src elixir :noweb-ref app-children
+{Phoenix.PubSub, name: Arcology.PubSub}
+#+end_src
+
+We start [[https://github.com/sneako/finch][Finch]], an HTTP client:
+
+#+begin_src elixir :noweb-ref app-children
+{Finch, name: Arcology.Finch}
+#+end_src
+
+We start the [[roam:ArcologyWeb.Endpoint]] which handles HTTP requests from the web.
+
+#+begin_src elixir :noweb-ref app-children
+ArcologyWeb.Endpoint
+#+end_src
+
+We will also start the =ArcologyWeb.Telemetry= server provided by Phoenix [n.b. this code block isn't directly included so that i can generate decent list-syntax without relying on metaprogramming or not being able to detangle this to my heart's content]
+
+This supervisor is configured to only restart a single process when the process dies; for other applications you may want it to restart *all* children, or to only restart "younger" processes which may rely on state within the dead process. This is pretty simple for now. Check out [[https://hexdocs.pm/elixir/Supervisor.html][the docs]] if you care.
+
+#+begin_src elixir :noweb-ref start_2 :noweb yes
+@impl true
+def start(_type, _args) do
+  children = [
+       ArcologyWeb.Telemetry
+     , <<app-children>>
+  ]
+
+  opts = [strategy: :one_for_one, name: Arcology.Supervisor]
+  Supervisor.start_link(children, opts)
+end
+#+end_src
+
+This =config_change= function is a stub that sends new configuration to the =ArcologyWeb.Endpoint= process when the appication is upgraded. I'll probably just restart the thing in place, but for highly-available OTP applications this sort of stuff is bread-and-butter.
+
+#+begin_src elixir :noweb-ref config_change_3
+@impl true
+def config_change(changed, _new, removed) do
+  ArcologyWeb.Endpoint.config_change(changed, removed)
+  :ok
+end
+#+end_src
+
+* =Arcology.Mailer=
+
+This is a stub for now -- maybe i'll send mails for new posts in the future but *shrug*:
+
+#+begin_src elixir :tangle lib/arcology/mailer.ex
+defmodule Arcology.Mailer do
+  use Swoosh.Mailer, otp_app: :arcology
+end
+#+end_src
+
+* Footnotes
+
+[fn:1] Elixir is built on Erlang, and Erlang has a process-based Actor model, you can send messages to processes whose IDs you have. The Supervisor is responsible for starting and managing the lifecycle of these processes. See [[https://www.erlang.org/doc/design_principles/sup_princ.html][the OTP Design Principles]] doc.

lib/arcology.ex

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+defmodule Arcology do
+  @moduledoc """
+  Arcology keeps the contexts that define your domain
+  and business logic.
+  
+  Contexts are also responsible for managing your data, regardless
+  if it comes from the database, an external API or others.
+  """
+end

lib/arcology/application.ex

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+defmodule Arcology.Application do
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+         ArcologyWeb.Telemetry
+       , Arcology.Repo
+       , {Phoenix.PubSub, name: Arcology.PubSub}
+       , {Finch, name: Arcology.Finch}
+       , ArcologyWeb.Endpoint
+    ]
+  
+    opts = [strategy: :one_for_one, name: Arcology.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+
+  @impl true
+  def config_change(changed, _new, removed) do
+    ArcologyWeb.Endpoint.config_change(changed, removed)
+    :ok
+  end
+end

lib/arcology/mailer.ex

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+defmodule Arcology.Mailer do
+  use Swoosh.Mailer, otp_app: :arcology
+end