isolating_code_in_contexts.md

Isolating code in contexts

Based on my blog post

Agenda

• What?
• How?
• Why?
• Elixir specific stuff
  • Module naming
  • OTP
• Show-and-tell example code

What?

It is a pattern much similar to the Facade Pattern.

You probably already use it to some extend.

Diffs from Facade:

• Only isolates internal code
• Also isolates on file system level
• A Context covers a specific logical area of the application. A facade covers a specific technical area.

How?

Pretty simple:

Refactoring existing code:

• Find a group of files that logically belong together
• Put them in the same dir and namespace
• Minimize public API by creating the context module
  • Also minimize public types and structs

From scratch:

• After a few refactorings, it will become natural to isolate in contexts and benefits will be greater
• Profit

Why?

Immediate benefits of contexts:

• Smaller public API. Easier to call the code.
• Easier to understand a context as a logical chunk of the application.
• Easier to understand the entire codebase as a collection of contexts

Long term benefits:

• Explicit dependencies
• Explicit types and structs
• Forces developers to think about important stuff
• Easier to maintain and change the internal code or db structure.
• Easy to delete the entire context

Visualized:

VS

Elixir specific stuff

Module names

File structure

+ some_context
|-- some_context.ex
|-- some_thing.ex

Public/main module: MyApp.SomeContext Inner module: MyApp.SomeContext.SomeThing

OTP

Since Elixir 1.5, simple add main module to supervisor:

children = [
  SomeContext,
  SomeOtherContext
]

Supervisor.start_link(children, strategy: :one_for_one)

And then implement a child_spec in the main module:

def child_spec([]) do
  %{
    id: __MODULE__,
    start: {SomeWorker, :start_link, []}
  }
end

Or if starting a supervisor:

def child_spec([]) do
  %{
    id: __MODULE__,
    start: {SomeSupervisor, :start_link, []},
    type: :supervisor
  }
end

Example

Legolas.Geolocation

Further reading:

• Elixir 1.5 child spec docs: https://hexdocs.pm/elixir/Supervisor.html#module-child-specification
• Phoenix 1.3 release notes: http://phoenixframework.org/blog/phoenix-1-3-0-released
• My blog post on the topic: https://medium.com/@lasseebert/isolating-code-in-contexts-172ec93ce532
• Elixir forum post: https://elixirforum.com/t/how-would-you-explain-phoenix-contexts-to-a-newbie/5947/3