Incident

Event Sourcing and CQRS in Elixir made simple.

Goals

• incentivize the usage of Event Sourcing and CQRS as good choice for domains that can leverage the main benefits of this design pattern;
• serve as guidance when using Event Sourcing in your system;
• leverage functions and reducers for executing commands and applying events;
• allow customization for fine-grained needs without compromising the principles;

Event Sourcing and CQRS

In a nutshell, Event Sourcing ensures that all changes to application state are stored as a sequence of events. But if you are new to Event Sourcing and CQRS I highly recommend watch Greg Young's presentation at Code on the Beach 2014 before moving forward.

Main Components

• Command is a data structure with basic validation;
• Event is a data structure;
• Aggregate only defines its business logic to execute a command and apply an event, but not its state;
• Aggregate State is defined by playing all the events through Aggregate business logic;
• Event Store and Projection Store are swappable persistence layers to allow different technologies over time;
• Projection can be rebuilt based on the persisted events and on the same Aggregate business logic;

Roadmap

Next Steps

• use Ecto.Schema for command and event data structure;
• publish version 0.1.0 to Hex and make repository public;
• add Postgres as an option for event and projection storage via a built-in Ecto Adapter;

Done

• create a Proof of Concept that can exercise all library components, implementation and confirm goals;
• extract library components from the POC, remove implementation and document the implementation in the README;
• add example application within the library as a reference for newcomers;

Installation

If available in Hex, the package can be installed by adding incident to your list of dependencies in mix.exs:

def deps do
  [
    {:incident, "~> 0.1.0"}
  ]
end

Configuration

Event Store and Projection Store Setup

Configure incidentEvent Store and Projection Store adapters and if desired, some options. The options will be passed in during the adapter initialization.

config :incident, :event_store, adapter: Incident.EventStore.InMemoryAdapter,
  options: [
    initial_state: []
]

config :incident, :projection_store, adapter: Incident.ProjectionStore.InMemoryAdapter,
  options: [
    initial_state: %{}
]

Getting Started

There is an example application that implements a Bank application for reference, including all the details and usage in IEx as well.

Documentation

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/incident.