Disco

Simple, opinionated yet flexible library to build CQRS/ES driven systems.

Disco has been extracted from work done to build apps following the CQRS/ES pattern. Several ideas come from other excellent examples such as commanded. However, the goal was to build something simpler and more flexible, because it's not always possible to follow 100% CQRS/ES pattern neither anywhere in your apps. Disco tries to solve this gap.

Production ready?

Not yet. Well, this approach and part of its code has been used to build several apps in production without problems, however before being really usable, it might need some polishing. Expect potential breaking changes until explicitly stated.

Installation and setup

The package is available in Hex, follow these steps to install:

1. Add disco to your list of dependencies in mix.exs in your app (if it's part of an umbrella project, add it where needed):

def deps do
  # Get from hex
  [{:disco, "~> 0.1.0"}]
  # Or use the latest from master
  [{:disco, github: "andreapavoni/disco"}]
end

2. Configure the event store only on the app where you want to run it. That means that if you're working on an umbrella project, you have to choose an app in particular. The best way is to have an app dedicated to work as event_store.

   • Generate migrations for the database

   mix do deps.get, compile, disco.generate_event_store_migrations

   • Configure event store repo

   # config/config.exs
   
   config :disco, otp_app: :my_app
   config :my_app, ecto_repos: [Disco.Repo]
   
   config :core, Disco.Repo,
     dadatabase: "my_app_database",
     username: System.get_env("POSTGRES_USER"),
     password: System.get_env("POSTGRES_PASSWORD"),
     hostname: System.get_env("POSTGRES_HOSTNAME")

3. Configure the event store client to the app(s) that need to interact with the event store

   • Configure the event store client

   # config/config.exs
   
   config :my_app, :event_store_client, MyApp.EventStoreClient

   • Create a wrapper for the event store client (so that it stays isolated)

   defmodule MyApp.EventStoreClient do
     use Disco.EventStore.Client
   end

Documentation

The documentation is available at https://hexdocs.pm/disco, it contains almost all the information needed to get started with Disco.

Usage

Quick and dirty console example, to show how it's supposed to work.

$ iex -S mix
# start Orchestrator process with the available aggregates
iex> Disco.Orchestrator.start_link [MyApp.Wallet]
{:ok, #PID<0.327.0>}

# get available commands
iex> Disco.Orchestrator.commands()
[:create_wallet]

# execute a command (sync)
iex> Disco.Orchestrator.dispatch(:create_wallet, %{user_id: UUID.uuid4(), balance: 100.0})
{:ok, %MyApp.Wallet.Aggregate{
  balance: 100.0,
  id: "4fd98a9e-8d6f-4e35-a8fc-aca5544596cb",
  user_id: "13bbece9-9bf3-4158-92b4-7e8a62d62361"
}}

# execute a command (async -> returns {:ok, aggregate_id})
iex> Disco.Orchestrator.dispatch(:create_wallet, %{user_id: UUID.uuid4(), balance: 100.0}, async: true)
{:ok, "ce998b0d-8d6f-4e35-a8fc-aca5544596cb"}

# ... under the hood an event consumer, if any, will work on the emitted events from command

# execute invalid command
iex> Disco.Orchestrator.dispatch(:create_wallet, %{balance: 100.0})
{:error, %{user_id: ["must be a valid UUID string"]}}

# get available queries
iex> Disco.Orchestrator.queries()
[:list_wallets]

# list user wallets
iex> Disco.Orchestrator.query(:list_wallets, %{user_id: "13bbece9-9bf3-4158-92b4-7e8a62d62361"})
[%{
  balance: 100.0,
  id: "4fd98a9e-8d6f-4e35-a8fc-aca5544596cb",
  user_id: "13bbece9-9bf3-4158-92b4-7e8a62d62361"
}]

# execute invalid query
iex> Disco.Orchestrator.query(:list_wallets, %{})
{:error, %{user_id: ["must be a valid UUID string"]}}

TODO / Short term roadmap

• [x] improve overall documentation
• [ ] consolidate Event to be a struct and/or protocol
• [ ] consolidate API for aggregates or event consumer (mostly based on feedback, if any)
• [ ] adopt an adapter-based approach for event store database

Contributing

Everyone is welcome to contribute to Disco and help tackling existing issues!

Use the issue tracker for bug reports or feature requests.

Please, do your best to follow the Elixir's Code of Conduct.

License

This source code is released under MIT License. Check LICENSE file for more information.