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defmodule UniqueUsername do
  @behaviour Commanded.Middleware

  alias Commanded.Middleware.Pipeline
  
  def before_dispatch(%Pipeline{command: %RegisterUser{} = command} = pipeline) do
    %RegisterUser{username: username} = command

    case Repo.insert(%Username{username: username}) do
      {:ok, _} -> 

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Big-O Time Complexities for Elixir data structures

Map [1]

Operation	Time Complexity
Access	O(log n)
Search	O(log n)
Insertion	O(n) for < 32 elements, O(log n) for >= 32 elements [2]
Deletion	O(n) for < 32 elements, O(log n) for >= 32 elements

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docker pull postgres:12-alpine

docker run --rm \
  --name postgres10 \
  --tmpfs=/pgtmpfs \
  -e PGDATA=/pgtmpfs \
  -e POSTGRES_PASSWORD=postgres \
  -e POSTGRES_USER=postgres \
  -p 5432:5432 \
  postgres:10-alpine

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def wait_until(timeout \\ 1_000, fun)

def wait_until(0, fun), do: fun.()

def wait_until(timeout, fun) do
  fun.()
rescue
  ExUnit.AssertionError ->
    :timer.sleep(10)

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defmodule EventStore.CategoryStreamLinker do
  @moduledoc """
    Links streams from aggregate instances to their respective category streams.

    example: events from stream_uuid of `contractors_contract-07c52787-da0c-444f-9783-5d380f7093f9` will be
    linked to stream_uuid of `contractors_contract`.
  """

  use Commanded.Event.Handler,
    application: My.App,

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defmodule InvalidEvent do
  @doc """
  Module to be used when an event cannot be deserialized from the event store.
  
  The payload field will be populated with the source event data if 
  deserialization is possible.
  
  Receiving this event usually indicates an event has been removed or renamed in
  the source code between releases. Deserializing to this invalid event module
  allows the application to continue running, otherwise it would terminate as

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Expectations

What I expect from some system managing storage of streams of events, intended to be used in an event-sourced system.

• ability to create streams
• ability to delete streams
• ability to use optimistic locking on stream level
• ability to write a batch of events in one stream atomically and durably
• ability to read events from one stream
• ability to read all events in the store ( the so called "$all" stream)

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defmodule Default.Behaviour do
  @moduledoc """
  Creates a behaviour that carries its own default implementation.

  When used into a behaviour module, when that module in turn is used, all functions
  defined on it are given to the using module.
  
  This allows you to have concrete implementations of the behaviour's default functionality
  for testing, unlike cramming them all into a __using__ macro.
  

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Elixir GenServer with a handle_info/2 callback will crash when receiving an unexpected message

Define a GenServer with a handle_info callback function:

defmodule Echo do
  use GenServer

  def start_link(reply_to) do
    GenServer.start_link(__MODULE__, reply_to)

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defmodule JsonSerializer do
  alias Commanded.EventStore.TypeProvider
  alias Commanded.Serialization.JsonDecoder

  @doc """
  Serialize given term to JSON binary data.
  """
  def serialize(term) do
    Jason.encode!(term)
  end