Building Event Sourced Apps

Designing your software around a story!

What is Event Sourcing?

Instead of defining a global data model that fits all of your use cases think about the things your system does. Record the events that occur and then build multiple data models that fit your individual use cases.

Sounds very abstract? In this talk, Leif will elaborate on some side projects including use cases with the hope to encourage you to switch to more event-centric thinking.

Transcript

1. Building Event Sourced Apps
4. My Example
5. Every team plays every other team twice 306 Matches
6. Home Team Away Team +3 +0 +1 +1
9. Transformation Input Output ?
10. https://www.kaggle.com/mkhvalchik/soccer/data
11. Match Team1 Team2 League SoSo Soccer
12. 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;
13. 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)
14. Problem?
15. Simplicity?
16. Performance?
17. Change!
18. T1 T2 T3
19. There are only two hard things in Computer Science …
20. Rethink
21. It’s a Tool
22. It’s a Tool that helps you capture stuff that happened
23. MatchStarted MatchStarted MatchStarted TeamFounded TeamFounded TeamFounded TeamFounded TeamFounded TeamQualified TeamQualified TeamQualified MachEnded MachEnded MachEnded SeasonEnded SeasonStarted
25. 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
27. (Re)Tell (Re)Tell (Re)Tell Capture https://github.com/leifg/so_so_soccer
28. It’s a Tool that helps you capture events …
29. … and tell a story from them
30. “But I don’t like Football”
31. Let’s get some Calories
32. Demo Time
33. TrackMeal Meal MealTracked Command Aggregate Event ?
34. /api/do_stuff Command
35. /meals TrackMeal Frontend MealTracked Phx Channel
36. Hacker News Vanity
37. HnStory •StorySubmitted •StoryVotedOn •StoryHitFrontPage •StoryFrontPagePositionChanged
41. Offer Price: £1000 Buyer: E-Corp Status: “accepted” SELECT SUM(price) FROM offers WHERE status = ‘accepted’ GROUP BY buyer_id; Offer Price: £1000 Buyer: E-Corp Status: “paid” SELECT SUM(price) FROM offers WHERE status in (‘accepted’, ‘paid’) GROUP BY buyer_id;
42. Offer Price: £1000 Buyer: E-Corp Status: “accepted” SELECT SUM(price) FROM offers WHERE status = ‘accepted’ GROUP BY buyer_id; Offer Price: £1000 Buyer: E-Corp Status: “rejected”
43. Event Sourcing is Great for Side Projects It’s also Great for Real World Applications leif.io If you know the business bit.ly/buildingconduit