Summary

Durable, globally unique processes (Entites) are created by controlling side effects.
(In short side effects are deferred until the state change of the entity has been committed to storage).

The purpose of an entity is to handle data that must be strongly consistent within a given execution context.

Example (as of 0.2.0)

Entity behaviour

defmodule MyApp.Counter do
  @behaviour Pachyderm.Entity

  alias MyApp.Counter.{Increment, ...}
  alias MyApp.Counter.{Increased, ...}

  def init() do
    %{count: 0}
  end

  def handle(%Increment{}, %{count: count}) do
    events = [%Increased{amount: 1}]

    if count == 9 do
      effects = [{MyApp.AdminMailer, %{threshold: 10}}]
      {:ok, {events, effects}}
    else
      {:ok, events}
    end
  end

  def update(%Increased{amount: amount}, state = %{count: current}) do
    %{state | count: current + amount}
  end
end

Calling Entities

type = MyApp.Counter
id = UUID.uuid4()
reference = {type, id}

{:ok, state} = Pachyderm.call(reference, %Increment{})
# => {:ok, %{count: 1}}

Design notes

A detailed discussion of why this API was chosen and further examples is available in the project README.

Why

The single global process is very easy to reason about, but has many pitfalls.
The dangers of the single global process.
Pachyderm tries to rescue this simple model by offering a standard way of managing some of the pitfalls.

Prior Art

The description of this project is quite difficult because there are lots of related but different approaches to this problem.

Other terminology includes:

• Single Global Processes
• The Virtual actor model Microsoft Orleans
• Resilient Actors
• An Actor Database Actor-Relational Database Systems: A Manifesto
• A database-less App framework
• Sticky Processes