Store state, as a properties structure, externally to a process.

A process in Elixir stores data in its state that other processes may need -- a current status flag for instance. The standard way of sharing this data is to provide an API that results in a GenServer call. But, what if the GenServer is busy working on a long-running job? You could break your long running jobs into pieces and allow the GenServer to check its message queue. Yuck, cooperative multitasking and added complexity. You could create an ETS table or other external database record. Yuck, verbose, complex, and high-friction solution to what should be a simple problem.

ExternalState helps solve this problem by providing a clean way of stashing some or all of your state in a data structure managed by a different process.

Caveat lector: this works beautifully for named workers but doesn't work well with simple 1-for-1 workers because the state is managed using the module name.

defmodule MyGenserver do
  use ExternalState, persist: false, props: [foo: true]

  def init(:ok) do
    init_ex_state() # external state is now at the defaults specified in use
  end

  # ...

  def do_foo do
    # ... something that sets foo to true ...
    merge_ex_state(foo: true)
  end

  def undo_foo do
    # ... something that sets foo to false ...
    merge_ex_state(foo: false)
    # or: merge_ex_state(%{foo: false})
  end

  def foo? do
    get_ex_state().foo
  end

end

The following are added to your module when you use ExternalState:

• @ex_state_struct An atom name for your external state structure
• default_ex_state/0 Get a state structure with default values from props
• init_ex_state/0 Initialize your external state; must call once, multiple calls are okay
• get_ex_state/0 Get the current external state or nil if no init yet
• put_ex_state/1 Set the external state, returns the state or nil if no init yet
• merge_ex_state/1 Update the external state with values from the parameter, which can be a keyword list of keys and values or a map. Returns the updated state or nil if no init yet.

If ExternalState is used with persist: true, then the external state will remain valid after the process that calls init_ex_state/0 exits. This is the default.

The package can be installed by adding external_state to your list of dependencies in mix.exs:

def deps do
  [
    {:external_state, "~> 1.0.6"}
  ]
end

The docs can be found at https://hexdocs.pm/external_state.