Dx enables you to write Elixir code as if all your Ecto data is already (pre)loaded.

Example

defmodule MyApp.Core.Authorization do
  import Dx.Defd

  defd visible_lists(user) do
    if admin?(user) do
      Enum.filter(Schema.List, &(&1.title == "Main list"))
    else
      user.lists
    end
  end

  defd admin?(user) do
    user.role.name == "Admin"
  end
end

This can be called using

Dx.Defd.load!(MyApp.Core.Authorization.visible_lists(user))

Dx.Defd.load!/2 loads all required data automatically: The association role, and either all Schema.List matching the filter (translated to a single SQL query) or the user's associated lists, depending on whether it's an admin.

These function can just as well be called for many users, and Dx will load data efficiently (with batching and concurrently).

Background

Most server backends for web and mobile applications are split between the actual application and at least one database. In their day-to-day programming, most Elixir developers have to keep that in mind and think about how to store data in the database, and when and how to load it. It's so deeply engrained that we often take this problem for granted, having integrated it in how we think about code and code architecture. For example, Phoenix (the most popular web framework for Elixir) has API contexts that suggest structuring apps into modules that act as a boundary (or interface) to the rest of the code. Within these, data is loaded and returned. Since it's a generic interface, the simplest approach is to load all data that's possibly needed, and return it. However, as the app grows in functionality and thus complexity, this may become a lot of data. And it's still necessary to think about what to return, where it's needed, and how to slice it.

Imagine this problem would not exist. Enter Dx.

With Dx, Elixir developers don't have to think about loading data from the database at all. You just write Elixir code, as if all data is already loaded and readily available.

How it works

When working with data in the database, you define Elixir functions using defd instead of def (the regular Elixir function definition). The defd function must be imported from the Dx.Defd module. Within defd functions, you can write regular Elixir code, accessing all fields and associations as if they're already loaded. You can also call other defd functions and structure your code in modules as usual.

When the app is compiled, Dx translates your defd code into multiple versions with different ways to load data:

• Data loading: Any data that might need to be loaded is wrapped in a check that either returns the already loaded data, or returns a "data requirement". Dx runs the code at the entry point (the first function that's a defd function) and either receives the result, or receives a number of data requirements. These are loaded, using the dataloader library under the hood. Then the code is run again, this time either returning the result, or more data requirements, and so on.
• Data querying: Parts of the code may be translated to "data scopes", which are used to generate database queries out of your code. For example, using the standard library function Enum.filter in a defd function will try to translate the condition (the anonymous function passed as second argument) into a database query. When successful, the data will not be loaded and then filtered in Elixir, but will already be filtered in the database.

All this happens automatically in the background. Parts of the work are done when compiling your code. Other parts are done when running it.

Caveats

Dx is designed with great care for developer experience. You can just start using it, and will get warnings with explanations if something should or must be done differently. It still helps to understand the main limitations:

Pure functions

Dx translates your code into different other versions of it. The translated versions may then be run any number of times, more or less often than the original would have been run. Thus, that any code defined using defd should be functionally pure. This means, it should not have any side effects.

• When the same code is run with the same arguments, it must always return the same result. Examples for non-pure code are using date and time, or random numbers.
• defd functions should also not modify any external state, such as modifying data in the database, or printing text to the console. Except if it's fine that the modification is applied multiple times.

Calling non-defd functions

You can call non-defd functions from within defd functions. However, Dx can't "look into" them. No data inside them will be loaded, and they can never be translated to database queries. They will also be run any number of times, so they should be pure functions as well.

Dx will ask you to wrap the call in a non_dx/1 function call. This is just to make clear that the called function is not defined using defd when reading the code.

Finding good entry points

Any time a defd function is called from a regular Elixir function, that's an entry point. That's where any needed data will be loaded.

Dx will ask you to wrap the call in a Dx.Defd.load!/1 function call. This is just to make clear that the called function is an entry point to defd land and data may be loaded here.

It may help to create dedicated modules for all defd functions. They are usually the core of the application, with much of the (business) logic. Any code calling into them - the entry points - in contrast, are outside these modules, for example in a API function, a Phoenix controller, or an Oban worker. This is where the data is loaded, whereas the defd modules consist only of pure functions with (business) logic.

Filter conditions in Elixir vs. SQL

Conditions can behave quite differently in SQL vs. Elixir. In the future, Dx will fully translate all nuances correctly, but for now, you have to keep that in mind yourself.

• NULL never matches anything in SQL, but it does in Elixir. For example, title != "TODO" when title = nil will match in Elixir, but not match in SQL. Thus, nil cases must be handled individually: is_nil(title) or title != "TODO"
• Dx joins has_one and belongs_to associations using LEFT JOIN in SQL. This means, you can happily access association chains, even if interim assocation parts do not exist. This would crash in Elixir, but in SQL, all fields just appear as NULL. Thus, the presence of associations should be checked individually: not is_nil (list.creator) and is_nil(list.creator.deleted_at)

Currently supported

Syntax

All syntax except for and with is supported in defd and (private) defdp functions.

Standard library

• All Enum functions
• All Kernel functions except:
  • apply/2, apply/3
  • spawn/1, spawn_link/1, spawn_monitor/1
  • tap/2
  • then/2

Translatable to database queries

Functions

• Enum.count/1
• Enum.filter/2
• Enum.find/2 (not Enum.find/3!)

will be translated to database queries, if both

• the first argument is either
  • a schema module, f.ex. Enum.filter(Todo.Task, fn task -> task.priority == "high" end)
  • the result of another function listed above
• the function passed as second argument (if any) consists only of functions listed above or:
  • ==, <, >
  • and, or, &&
  • Enum.any?/2, Enum.all?/2
  • DateTime.compare/2