Event Sourcing - The Story Telling of Processes

Almost every application has to deal with state. No matter if you like relational databases or prefer to go NoSQL the result is the same: your application only has one state.

Unfortunately, business requirements are not that static. They change constantly and so do the underlying processes. So how can you prepare your application to quickly adapt to changing processes without constantly migrating your database?

The key to event sourcing is recording what happened and build relevant data models of these events.

In this session, I will guide you through an example project that has been designed with event sourcing and how it compares to a classical CRUD application.

Transcript

1. Event Sourcing
   The Story Telling
   of (Business) Processes
   

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3. Event Sourcing
   The Story Telling
   of (Business) Processes
   

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5. It’s a Tool
   

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6. Every single program can be reduced to
   Input
   Transformation
   Output
   * Former Boss
   

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7. Example Time
   

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9. Every team plays every other team twice
   306 Matches
   

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10. Home Team Away Team
    

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11. Home Team Away Team
    +3 +0
    

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12. Home Team Away Team
    +3 +0
    +1 +1
    

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15. Transformation
    Input
    Output
    

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16. Transformation
    Input
    Output
    

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17. Transformation
    Input
    Output
    

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18. Transformation
    Input
    Output
    ?
    

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19. https://www.kaggle.com/mkhvalchik/soccer/data
    

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20. Match
    Team1
    Team2
    League SoSo Soccer
    

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22. sum(m.away_team_points) as points
    from
    (select *,
    case
    when away_team_goal > home_team_goal then 3
    when home_team_goal = away_team_goal then 1
    else 0
    end as away_team_points,
    case
    when away_team_goal > home_team_goal then 1
    else 0
    end as away_team_wins,
    case
    when away_team_goal = home_team_goal then 1
    else 0
    end as away_team_draws,
    case
    when away_team_goal < home_team_goal then 1
    else 0
    end as away_team_losses
    from matches) m
    join leagues l on m.league_id = l.id
    group by season,
    away_team_api_id) t
    join leagues l on t.league_id = l.id
    join teams te on te.api_id = t.team_id
    group by (t.team_id,
    t.season)
    order by points desc,
    goal_difference desc,
    goals_for desc;
    

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23. sum(m.away_team_points) as points
    from
    (select *,
    case
    when away_team_goal > home_team_goal then 3
    when home_team_goal = away_team_goal then 1
    else 0
    end as away_team_points,
    case
    when away_team_goal > home_team_goal then 1
    else 0
    end as away_team_wins,
    case
    when away_team_goal = home_team_goal then 1
    else 0
    end as away_team_draws,
    case
    when away_team_goal < home_team_goal then 1
    else 0
    end as away_team_losses
    from matches) m
    join leagues l on m.league_id = l.id
    group by season,
    away_team_api_id) t
    join leagues l on t.league_id = l.id
    join teams te on te.api_id = t.team_id
    group by (t.team_id,
    t.season)
    order by points desc,
    goal_difference desc,
    goals_for desc;
    
    

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24. def standings(season, league_id)
    Repo.all(m in Match, where: m.season == ^season and m.league_id == ^league_id)
    |> Map.merge(%{row.home_team_api_id => home_team_values(row) }, &add_up/3)
    |> Map.merge(%{row.away_team_api_id => away_team_values(row) }, &add_up/3)
    |> Enum.sort_by(fn s ->
    {s.points, s.goal_difference, s.goals_for}
    end, &>=/2)
    end
    defp home_team_values(row) do
    %{
    games: 1,
    wins: Rules.wins(row.home_team_goal, row.away_team_goal),
    draws: Rules.draws(row.home_team_goal, row.away_team_goal),
    losses: Rules.losses(row.home_team_goal, row.away_team_goal),
    goals_for: row.home_team_goal,
    goals_against: row.away_team_goal,
    points: Rules.points(row.home_team_goal, row.away_team_goal)
    }
    end
    defp add_up(_k, nil, v2), do: v2
    defp add_up(_k, v1, v2) when is_integer(v1), do: v1 + v2
    defp add_up(_k, v1, v2) when is_map(v1), do: Map.merge(v1, v2, &add_up/3)
    

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25. Problem?
    

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26. Simplicity?
    

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27. Performance?
    

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28. Change!
    

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36. T1
    T2
    T3
    

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37. T1
    T2
    T3
    

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38. There are only two
    hard things in
    Computer Science
    …
    

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39. Rethink
    

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40. It’s a Tool
    

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41. It’s a Tool that
    helps you capture
    stuff that
    happened
    

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43. TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    

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44. TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamQualified
    TeamQualified
    TeamQualified
    

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45. TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamQualified
    TeamQualified
    TeamQualified
    SeasonStarted
    

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46. MatchStarted
    MatchStarted
    MatchStarted
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamQualified
    TeamQualified
    TeamQualified
    SeasonStarted
    

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47. MatchStarted
    MatchStarted
    MatchStarted
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamQualified
    TeamQualified
    TeamQualified
    MachEnded
    MachEnded
    MachEnded
    SeasonStarted
    

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48. MatchStarted
    MatchStarted
    MatchStarted
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamFounded
    TeamQualified
    TeamQualified
    TeamQualified
    MachEnded
    MachEnded
    MachEnded
    SeasonEnded
    SeasonStarted
    

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50. project %TeamQualifiedForSeason{} = qualified do
    Ecto.Multi.insert(multi, :standings, %Standing{
    season_id: qualified.season_id,
    team_api_id: qualified.team_api_id,
    team_long_name: qualified.team_long_name,
    league_id: qualified.league_id,
    sort_key: sort_key(0, 0, 0)
    })
    end
    project %MatchEnded{} = ended do
    season_id = id_from_stream_id(ended.season_id)
    home_team = Standing.by_team_and_season(ended.home_team_api_id, season_id)
    away_team = Standing.by_team_and_season(ended.away_team_api_id, season_id)
    home_team_diff = team_changeset(home_team, ended.home_team_goal, ended.away_team_goal)
    away_team_diff = team_changeset(away_team, ended.away_team_goal, ended.home_team_goal)
    multi |> Ecto.Multi.update(:home_team, home_team_changeset)
    |> Ecto.Multi.update(:away_team, away_team_changeset)
    end
    

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52. (Re)Tell
    (Re)Tell
    (Re)Tell
    Capture
    

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53. It’s a Tool that
    helps you capture
    events …
    

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54. … and tell a story
    from them
    

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55. Commands
    Aggregates
    Process
    Handlers
    

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56. Greg Young (yt/LDW0QWie21s)
    SoSo Soccer (gh/leifg/so_so_soccer)
    Commanded (gh/commanded)
    leif.io
    

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