Offline first collaboration with event sourcing

Demo code at https://github.com/stevenchan/event-sourcing-todo-example

Transcript

1. Offline First Collaboration with Event Sourcing Steven Chan - @stevnchan
2. GoodNotes
3. Syncing is hard
4. What we have tried
5. Apple CloudKit • Remote database API • Need to implement syncing with local cache ourselves • Designing a correct algorithm to sync two object graphs is hard • Limited schema migration support • Undocumented limit for sharing records
6. Google Firebase • Real-time database • Mobile SDKs available • Offline mode: store updates in a queue • Performance become worse as the queue become long
7. Event sourcing to rescue
8. Event sourcing to rescue 1. Store all changes as events in a single queue 2. Only need to sync the event queue 3. Derive the app state from the event queue 4. Migration = rewrite the derivation function
9. Todo app example 1. {inserted, id123, “Buy milk”} 2. {completed, id123} 3. {deleted, id123}
10. Update app state 1. {inserted, id123, “Buy milk”} 2. {completed, id123} 3. {deleted, id123}
11. Update app state 1. {inserted, id123, “Buy milk”} 2. {completed, id123} 3. {deleted, id123}
12. Update app state 1. {inserted, id123, “Buy milk”} 2. {completed, id123} 3. {deleted, id123}
13. Update app state 1. {inserted, id123, “Buy milk”} 2. {completed, id123} 3. {deleted, id123}
14. Syncing • Server: maintain an event queue = the source of truth • Clients: upload local events to server, download unseen events, and then update the app state. • That’s it!
15. Syncing A B C A B C D E F Client Server F D E
16. Syncing A B C A B C D E F Client Server F D E
17. Syncing A B C A B C D E F Client Server F D E
18. Conflicts handling • Pick the latest value by update time • Pick the value with the largest update count • Use vector clocks to detect conflicts
19. Demo
20. Q&A